Bart Walter De Pontieu

• Dalda, Alberto Sainz; Rodríguez, Jaime de la Cruz; Hansteen, Viggo Haraldson; Pontieu, Bart Walter De & Gošić, Milan (2026). The IRIS^2+ Inversion Tool: Recovering the Radiative Losses and the Thermodynamics in the Lower Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 997(2). doi: 10.3847/1538-4357/ae274c. Full text in Research Archive Show summary

  We introduce an improved and fast inversion tool that is able to provide the thermodynamics of the solar atmosphere from the photosphere to the top of the chromosphere, as well as the integrated radiative losses (IRLs) in the chromosphere for data observed by the Interface Region Imaging Spectrograph (IRIS). This NASA mission has been observing the Sun and providing, among other kinds of data, multiline spectral observations sensitive to changes in the lower solar atmosphere since 2013. In this paper, we explain the new inversion tool IRIS2+ based on the IRIS2+ database, which is based on 135,472 synthetic representative profiles (RPs), each of them consisting of six chromospheric lines and six photospheric lines, their corresponding representative model atmospheres (RMA), and the IRL associated with these atmospheres. A nearest neighbor (k-nn) model algorithm is trained with the synthetic representative profiles to predict the closest RP in the database to the one observed, at which point IRIS2+ assigns the RMA and the IRL to the location of that observed profile. We have compared the results obtained by IRIS2+ with results obtained from the state-of-the-art inversion code STiC, which is also used to build the IRIS2+ database. We find that the thermodynamics and the IRL obtained with both methods are comparable in most cases. Therefore, IRIS2+ is a fast and reliable inversion tool that provides approximate values of the thermodynamic state and the radiative losses in the lower solar atmosphere for a large variety of solar scenes observed with IRIS.

• Cozzo, G.; Testa, Paola; Sykora, Juan Martinez; Reale, F.; Pagano, P. & Rappazzo, F. [Show all 16 contributors for this article] (2026). 3D MHD Simulations of Coronal Loops Heated via Magnetic Braiding. I. Continuous Driving. The Astrophysical Journal (ApJ). ISSN 0004-637X. 998(1). doi: 10.3847/1538-4357/ae2d56. Full text in Research Archive Show summary

  The nature and detailed properties of the heating of the million-degree solar corona are important issues that are still largely unresolved. Nanoflare heating might be dominant in active regions and quiet Sun, although direct signatures of such small-scale events are difficult to observe in the highly conducting, faint corona. The aim of this work is to test the theory of coronal heating by nanoflares in braided magnetic field structures. We analyze a 3D MHD model of a multistrand flux tube in a stratified solar atmosphere, driven by twisting motions at the boundaries. We show how the magnetic structure is maintained at high temperature and for an indefinite time, by intermittent episodes of local magnetic energy release due to reconnection. We synthesize the optically thin emission with SDO/AIA and MUSE and compare the synthetic observations with the intrinsic coronal plasma properties, focusing on the response to impulsive coronal heating. Currents’ buildup and their impulsive dissipation into heat are also investigated through different runs. In this first paper, we describe the proliferation of heating from the dissipation of narrow current sheets in realistic simulations of braided coronal flux tubes at unprecedented high spatial resolutions.

• Tiwari, Sanjiv K.; Panesar, Navdeep K.; Moore, Ronald L.; Savage, Sabrina L.; Winebarger, Amy R. & Vigil, Genevieve D. [Show all 20 contributors for this article] (2025). A Sudden Fine-scale Bright Kernel Captured by Hi-C Flare in 11 MK Emission during an M1.6-class Solar Flare’s Postmaximum Phase. Astrophysical Journal Letters. ISSN 2041-8205. 995(2). doi: 10.3847/2041-8213/ae24e2. Full text in Research Archive Show summary

  On 2024 April 17, the third successful Hi-C sounding rocket flight, Hi-C Flare, recorded coronal images in Fe XXI 129 Å emission from 11 MK plasma during the postmaximum phase of an M1.6-class solar flare, achieving unprecedented spatial (∼300 km) and temporal (1.3 s) resolutions. The flare started at 21:55 UT, peaked at 22:08 UT, and lasted ∼40 minutes. Hi-C observed for over 5 minutes (22:15:45–22:21:25), starting roughly 8 minutes after flare maximum. A sudden compact bright burst—875 ± 25 km wide, lasting 90 ± 1.3 s, exhibiting a proper motion of ∼ 50 km s −1 , and splitting into two toward the end—occurs near the foot of some postflare loops. Its size and brightness are reminiscent of flare-ribbon kernels during a flare’s rapid rise phase, kernels marking sites of sudden heating and hot plasma upflow, making its occurrence during the late phase surprising. Such isolated brightenings in a flare’s postmaximum phase are rare and have not been previously reported. The kernel was detected in all Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly channels. Its 1600 Å light curve peaked ∼50 s earlier than its 131 Å light curve, similar to that of flare-ribbon kernels, albeit with a smaller delay of ∼25 s, during the impulsive phase of the flare. In SDO/Helioseismic Magnetic Imager magnetograms, the kernel sits in unipolar positive magnetic flux near an embedded clump of negative flux. Although localized magnetic reconnection within the kernel (a microflare) cannot be ruled out for its cause, the observations favor the localized brightening being an isolated, exceptionally late flare-ribbon kernel, resulting from an exceptionally late burst of the flare’s coronal reconnection.

• Alaimo, Edoardo; Todaro, Michela; Sciortino, Luisa; Cicero, Ugo Lo; Fiorentino, Federico & D’Anca, Fabio [Show all 30 contributors for this article] (2025). Optical blocking filters for the Multi-slit Solar Explorer mission: in-band extreme ultraviolet and soft X-ray transmittance spectroscopy measurements and analysis. Journal of Astronomical Telescopes, Instruments, and Systems (JATIS). ISSN 2329-4124. 11(4). doi: 10.1117/1.JATIS.11.4.048001. Full text in Research Archive Show summary

  Extreme ultraviolet (EUV) filters are critical components in solar space missions, ensuring the required wavelength selection and visible light attenuation. As part of the advancement for the NASA Multi-slit Solar Explorer (MUSE) mission, we present a detailed in-band EUV and out-of-band soft X-ray transmittance characterization of small samples of filter candidates representing two design choices for the mission: a baseline design based on heritage filters from past successful missions and an alternative design employing carbon nanotube (CNT) pellicles as substrates. Our aim is both to present the experimental EUV/soft X-ray transmittance measurements and, through proper data analysis, to retrieve the atomic areal densities and the inner layer thicknesses of each sample, including the quantification of the metal oxide layers for each different substrate. The measurements demonstrate the in-band transmittance performance of both heritage and CNT-based filters across EUV bands. These results provide useful input for the construction of the response functions for MUSE and similar EUV space missions. Furthermore, the methodology establishes a framework for the optimization and characterization of thin-film materials tailored as EUV entrance filters for space-based solar observations.

• Delgado, David Afonso; Alemán, Tanausú del Pino; Bueno, Javier Trujillo; Ishikawa, Ryohko; Ballester, Ernest Alsina & McKenzie, David E. [Show all 19 contributors for this article] (2025). Determining the Magnetic Field of Active Region Plages Using the Whole CLASP2/2.1 Spectral Window. The Astrophysical Journal (ApJ). ISSN 0004-637X. 991(2). doi: 10.3847/1538-4357/adfcd4. Full text in Research Archive Show summary

  The Chromospheric LAyer SpectroPolarimeter missions, CLASP2 and CLASP2.1, demonstrated that the near-UV spectral region between 279.30 and 280.68 nm is suitable for studying the magnetism of the solar chromosphere. In particular, the spectropolarimetric observations in the Mg ii h and k resonant doublet, Mn i 279.91 and 280.19 nm resonant lines, and Fe ii 279.79 and 280.66 nm lines acquired by these suborbital space experiments have been proven useful for inferring the magnetic field stratification in the solar chromosphere. However, several lines of the CLASP2/2.1 spectral region with significant circular polarization signals had remained unexplored. After identifying two Ni i (279.95 and 280.59 nm) lines, one Mn ii (280.62 nm) line, and one Fe i (280.53 nm) line, here we apply the weak-field approximation to the spectropolarimetric observations of active region plages by CLASP2 and CLASP2.1. By comparing the results with previous studies, we are able to estimate the formation heights of these CLASP2/2.1 additional spectral lines and to demonstrate their suitability to determine the magnetic field stratification from the photosphere to the upper chromosphere.

• Russell, Alexander J. B.; Polito, Vanessa; Testa, Paola; Pontieu, Bart Walter De & Belov, Sergey A. (2025). Solar Flare Ion Temperatures. Astrophysical Journal Letters. ISSN 2041-8205. 990(2). doi: 10.3847/2041-8213/adf74a. Full text in Research Archive Show summary

  This Letter proposes that the ion temperature is several times the local electron temperature in the hot onset phase and in the above-the-loop region of solar flares. The Letter considers: the evidence of spectral line Doppler widths (“nonthermal” broadening); evidence for “universal” ion and electron temperature increase scaling relations for magnetic reconnection in the solar wind, Earth’s magnetopause, Earth’s magnetotail, and numerical simulations; and thermal equilibration times for onset and above-the-loop densities, which are much longer than previous estimates based on soft X-ray flare loops. We conclude that the ion temperature is likely to reach 60 MK or greater and that it may represent a substantial part of spectral line widths, significantly contributing to solving the long-standing issue of the excess nonthermal broadening in flare lines.

• Ishikawa, Ryohko; Bueno, Javier Trujillo; McKenzie, David E.; Song, Donguk; Alemán, Tanausú del Pino & Ballester, Ernest Alsina [Show all 20 contributors for this article] (2025). Determining the Magnetic Field in the Atmosphere of a Solar Active Region Observed by the CLASP2.1 Sounding Rocket Experiment. The Astrophysical Journal (ApJ). ISSN 0004-637X. 990(2). doi: 10.3847/1538-4357/aded0e. Full text in Research Archive Show summary

  We determine magnetic fields from the photosphere to the upper chromosphere combining data from the Hinode satellite and the CLASP2.1 sounding rocket experiment. CLASP2.1 provided polarization profiles of the Mg ii h and k lines, as well as of the Mn i lines around 2800 Å, across various magnetic structures in an active region, containing a plage, a pore, and the edges of a sunspot penumbra. By applying the weak-field approximation to the circular polarization profiles of these spectral lines, we obtain a longitudinal magnetic field map at three different heights in the chromosphere (lower, middle, and upper). This is complemented by data from Hinode (photospheric magnetic field), the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory (high spatial resolution observations of the chromosphere and corona). We quantify the height expansion of the plage magnetic fields and find that the magnetic fields expand significantly in the middle chromosphere, shaping the moss observed above in the transition region and corona. We identified an area with polarity reversal at the upper chromosphere around the edge of the pore, suggesting the presence of a magnetic discontinuity in the upper chromosphere. Transient and recurrent jetlike events are observed in this region, likely driven by magnetic reconnection. Around the penumbral edge, we find large-scale magnetic fields corresponding to the superpenumbral fibrils seen in the upper chromosphere. In the superpenumbral fibrils, we find Zeeman-induced linear polarization signals, suggesting the presence of a significantly inclined magnetic field, as strong as 1000 G in the upper chromosphere.

• Weitz, Ayla; Tiwari, Sanjiv K.; Cauzzi, Gianna; Reardon, Kevin P. & Pontieu, Bart Walter De (2025). Bright Dots and Coronal Plume Formation in Sunspot Penumbra. The Astrophysical Journal (ApJ). ISSN 0004-637X. 988(1). doi: 10.3847/1538-4357/ade3c1. Full text in Research Archive Show summary

  Coronal plumes are narrow, collimated structures that are primarily viewed above the solar poles and in coronal holes in the extreme ultraviolet, but also in sunspots. Open questions remain about plume formation, including the role of small-scale transients and whether plumes embedded in different magnetic field configurations have similar formation mechanisms. We report on coordinated Solar Orbiter/Extreme Ultraviolet Imager (EUI), Interface Region Imaging Spectrograph, and Solar Dynamics Observatory observations of the formation of a plume in sunspot penumbra in 2022 March. During this observation, Solar Orbiter was positioned near the Earth–Sun line and EUI observed at a 5 s cadence with a spatial scale of 185 km pixel −1 in the solar corona. We observe fine-scale dots at various locations in the sunspot, but the brightest and highest density of dots is at the plume base. Space-time maps along the plume axis show parabolic and V-shaped patterns, and we conclude that some of these dots are possible signatures of magneto-acoustic shocks. Compared to other radial cuts around the sunspot, along the plume shows the longest periods (∼7 minutes) and the most distinct tracks. Bright dots at the plume base are mostly circular and do not show elongations from a fixed origin, in contrast to jetlets and previously reported penumbral dots. We do not find high-speed, repeated downflows along the plume, and the plume appears to brighten coherently along its length. Our analysis suggests that jetlets and downflows are not a necessary component of this plume’s formation, and that mechanisms for plume formation could be dependent on magnetic topology and the chromospheric wave field.

• Bose, Souvik; Joshi, Jayant; Testa, Paola & Pontieu, Bart Walter De (2025). On the Million-degree Signature of Spicules. Astrophysical Journal Letters. ISSN 2041-8205. 983(1). doi: 10.3847/2041-8213/adc30d. Full text in Research Archive Show summary

  Spicules have often been proposed as substantial contributors toward the mass and energy balance of the solar corona. While their transition region (TR) counterpart has unequivocally been established over the past decade, the observations concerning the coronal contribution of spicules have often been contested. This is mainly attributed to the lack of adequate coordinated observations, their small spatial scales, highly dynamic nature, and complex multithermal evolution, which are often observed at the limit of our current observational facilities. Therefore, it remains unclear how much heating occurs in association with spicules to coronal temperatures. In this study, we use coordinated high-resolution observations of the solar chromosphere, TR, and corona of a quiet-Sun region and a coronal hole with the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) to investigate the (lower) coronal (∼1 MK) emission associated with spicules. We perform differential emission measure analysis on the AIA passbands using basis pursuit and a newly developed technique based on Tikhonov regularization to probe the thermal structure of the spicular environment at coronal temperatures. We find that the emission measure (EM) maps at 1 MK reveal the presence of ubiquitous, small-scale jets with a clear spatiotemporal coherence with the spicules observed in the IRIS/TR passband. Detailed spacetime analysis of the chromospheric, TR, and EM maps show unambiguous evidence of rapidly outward-propagating spicules with strong emission (2–3 times higher than the background) at 1 MK. Our findings are consistent with previously reported MHD simulations that show heating to coronal temperatures associated with spicules.

• Barbera, Marco; D'Anca, Fabio; Todaro, Michela; Sciortino, Luisa; Cicero, Ugo Lo & Alaimo, Edoardo [Show all 30 contributors for this article] (2025). Metal coated carbon nanotube ultra-thin pellicles: a high-performance solution for the optical blocking filters of the NASA MIDEX solar mission MUSE. Proceedings of SPIE, the International Society for Optical Engineering. ISSN 0277-786X. 13699. doi: 10.1117/12.3075219. Full text in Research Archive Show summary

  The NASA MIDEX mission MUSE (MUlti-slit Solar Explorer), presently under development with a launch scheduled no earlier than 2027, will focus on high spectral and time resolution imaging of the solar corona by use of two separate instruments sensitive in the EUV wavelength range ~ 10 − 30 nm: a multi-slit spectrograph (SG) and a context imager (CI). Both instruments use back-illuminated CCD detectors and thus need optical blocking filters, mounted both on the telescope entrance pupil and on a filter wheel close to the detector plane, to strongly attenuate the UV/Vis band of the solar spectrum. As part of the Italian contribution to the MUSE mission, funded by the Italian Space Agency, we have proposed to use an innovative filter technology based on metal-coated Carbon Nanotubes (CNT) pellicles. We present the current design of the optical blocking filters for MUSE based on this innovative CNT technology, discuss potential advantages with respect to the baseline design, and report first results on the technology readiness assessment activity performed on manufactured small filter samples and fully representative filter models.

• Ashfield, William; Polito, Vanessa; Lörinčík, Juraj; Pontieu, Bart Walter De; Chintzoglou, Georgios & Bose, Souvik [Show all 10 contributors for this article] (2025). Spectroscopic observations of solar flare pulsations driven by oscillatory magnetic reconnection. Nature Astronomy. 10(1), p. 54–63. doi: 10.1038/s41550-025-02706-4. Full text in Research Archive Show summary

  Solar and stellar flares often exhibit oscillations, or quasi-periodic pulsations (QPPs), across the electromagnetic spectrum. While magnetic-field reconnection drives these events, it remains to be determined whether oscillatory reconnection causes the quasi-periodicity, or whether waves drive or mediate this process. Exploiting coordinated observations from NASA’s Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope, here we present spectroscopic observations of QPPs in a solar flare at high-temporal (<1 s) and high-spatial (~60 km) resolution. Downwards velocities in the flare ribbon show synchronized oscillations at different atmospheric layers with a period of ~32 s. These velocities correlate with hard X-ray emissions, indicating a modulated deposition of accelerated electrons in the chromosphere as the driver. By negating magnetohydrodynamic sausage modes as the modulator, we demonstrate that repeated reconnection drives the QPPs. The QPP–reconnection relationship established here provides observational benchmarks for reconnection models and diagnostics for probing energy release across astrophysical environments.

• Wargnier, Q. M.; Vilmart, G.; Martinez-Sykora, Juan; Hansteen, Viggo Haraldson & Pontieu, Bart Walter De (2025). Time-adaptive PIROCK method with error control for multi-fluid and single-fluid magnetohydrodynamics systems. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452351. Full text in Research Archive Show summary

  Context. The solar atmosphere is a complex environment characterized by numerous species with varying ionization states, which are particularly evident in the chromosphere, where the significant variations in ionization degree occur. This region transitions from highly collisional to weakly collisional states that exhibit diverse plasma state transitions influenced by varying magnetic strengths and collisional properties. The complexity of processes in the solar atmosphere introduces substantial numerical stiffness in multi-fluid models, leading to severe timestep restrictions in standard time integration methods. Aims. To address the computational challenges, new numerical methods are essential. These methods must effectively manage the diverse timescales associated with multi-fluid and multi-physics models, including convection, dissipative effects, and reactions. The widely used time operator splitting technique provides a straightforward approach but necessitates careful timestep management to prevent stability issues and errors. Despite studies on splitting errors, their impact on solar and stellar astrophysics has largely been overlooked. Methods. We focus on a multi-fluid multi-species model, which poses significant challenges for time integration. We propose a second-order Partitioned Implicit-Explicit Orthogonal Runge–Kutta (PIROCK) method. This method combines efficient explicit stabilized and implicit integration techniques while employing variable time-stepping with error control. Results. Compared to a standard third-order explicit time integration method and a first-order Lie splitting approach as considered recently, the PIROCK method demonstrates robust advantages in terms of accuracy, numerical stability, and computational efficiency. For the first time, our results reveal PIROCK’s capability to effectively solve multi-fluid problems with unprecedented efficiency. Preliminary results on chemical fractionation, combined with this efficient method, represent a significant step toward understanding the well-known first-ionization-potential effect in the solar atmosphere.

• Gošić, M.; Hansteen, Viggo Haraldson; Dalda, A. Sainz; Pontieu, Bart Walter De & Voort, Luc Rouppe van der (2025). Bifrost Models of the Quiet Sun. I. Comparison with Solar Observations. The Astrophysical Journal (ApJ). ISSN 0004-637X. 990(2). doi: 10.3847/1538-4357/adf4db. Full text in Research Archive Show summary

  Studying the emergence of magnetic fields is essential for understanding the physical mechanisms behind various phenomena in the solar atmosphere. Most importantly, the emerging fields offer valuable insights into how energy and mass are transferred to the upper solar atmosphere. As a result, they have garnered significant attention from both observational and theoretical perspectives. In this article, we present two models of quiet-Sun-like magnetic fields generated by the Bifrost code. We compare these models with observations from the Swedish 1 m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). By tracking the magnetic features in both the SST and Bifrost data, we determine the similarities and differences between the fields identified in the models and those observed. We conduct a quantitative comparison of various properties, such as flux content, flux densities, horizontal and line-of-sight velocities, lifetimes, sizes, and surface interactions. Additionally, we identify and analyze the properties of the largest emerging bipoles in the SST and Bifrost data. Our findings indicate that the magnetic bipoles in the Bifrost simulations are generally stronger than those observed with the SST. However, a qualitative comparison of the chromospheric and transition region responses to the emerging fields in the Bifrost models, SST, and IRIS observations shows similar heating processes occurring above and around the emerging fields. Finally, we outline our plans for future work aimed at studying the emergence of magnetic fields in the quiet Sun, with a particular focus on the chromosphere and upper atmospheric layers.

• Robinson, Rebecca Anne; Pontieu, Bart Walter De; Steel, Simon; Harman, Pamela; Polito, Vanessa & Jin, Meng (2025). A mission-embedded outreach program for NASA’s Multi-slit Solar Explorer (MUSE) mission: inspiring future generations of solar explorers beyond boundaries and backgrounds. Frontiers in Astronomy and Space Sciences. 12. doi: 10.3389/fspas.2025.1658638. Full text in Research Archive Show summary

  In the wake of the Decadal Survey for Solar and Space Physics, it is clear that the general knowledge of these subjects in the US is deficient in comparison to other subfields of astrophysics. This is concerning because we rely on the Sun to survive, and understanding our relationship with the Sun is crucial for understanding our immediate space environment. With that, it is our responsibility as scientists and educators to advocate for and share the knowledge of our Sun and space environments as accessibly as possible. One way to do so is by developing mission-specific outreach programs that tackle fundamental science questions and topics that are relevant to the Sun-Earth system. As a new and budding example, we introduce a mission-embedded outreach program for NASA’s Multi-slit Solar Explorer (MUSE) mission. This program officially began on 1 December 2024 and includes three main local partners: California Academy of Sciences, Chabot Space and Science Center, and the Boys and Girls Club of the Peninsula. Each partner’s scope of work is shaped by their resources and expertise, and each remains committed to creating reusable and versatile outreach products. This manuscript will summarize the scientific goals of the MUSE mission, detail the MUSE outreach program, highlight our collaborations with other heliophysics missions (e.g., PUNCH), and provide a preliminary assessment of our contribution to the future of heliophysics outreach.

• Cozzo, G.; Pagano, P.; Reale, F.; Testa, Paola; Petralia, A. & Martinez-Sykora, Juan [Show all 8 contributors for this article] (2025). Coronal energy release by MHD avalanches: III. Identification of a reconnection outflow from a nanoflare. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452426. Full text in Research Archive Show summary

  Context. Outflows perpendicular to the guide field are believed to be a possible signature of magnetic reconnection in the solar corona. Specifically, outflows can help detect the occurrence of ubiquitous small-angle magnetic reconnection. Aims. The aim of this work is to identify possible diagnostic techniques of such outflows in hot coronal loops with the Atmospheric Image Assembly (AIA) on board the Solar Dynamics Observatory and the forthcoming MUltislit Solar Explorer (MUSE), in a realistically dynamic coronal loop environment where a magnetohydrodynamic (MHD) avalanche is occurring. Methods. We considered a 3D MHD model of two magnetic flux tubes, including a stratified, radiative, and thermal-conducting atmosphere, twisted by footpoint rotation. The faster rotating flux tube becomes kink-unstable and soon involves the other one in the avalanche. The turbulent decay of this magnetic structure on a global scale leads to the formation, fragmentation, and dissipation of current sheets, driving impulsive heating akin to a nanoflare storm. We captured a clear outflow from a reconnection episode soon after the initial avalanche and synthesised its emission as detectable with AIA and MUSE. Results. The outflow has a maximum temperature around 8 MK, total energy of 1024 erg, velocity of a few hundred km/s, and duration of less than 1 min. We show the emission in the AIA 94 Å channel (Fe XVIII line) and in the MUSE 108 Å Fe XIX spectral line. Conclusions. This outflow shares many features with nanojets recently detected at lower temperatures. However, its low emission measure makes its detection difficult with AIA, while Doppler shifts can be measured with MUSE. Conditions become different in the later steady-state phase, when the flux tubes are filled with denser and relatively cooler plasma.

• Cho, Kyuhyoun; Pontieu, Bart Walter De & Testa, Paola (2024). On the Nature of Nonthermal Broadening of Spectral Lines Observed by IRIS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 975(1). doi: 10.3847/1538-4357/ad7586. Full text in Research Archive Show summary

  The origin of nonthermal broadening in solar spectra is one of the long-standing questions in solar physics. Various processes have been invoked—including unresolved flows, waves, and turbulent processes—but definitive answers are lacking. To investigate the physical processes responsible for nonthermal broadening, we examine its relation with the angle between the magnetic field and the line of sight in three different closed-field regions above plage regions at different locations on the solar disk. We obtained the nonthermal width of transition-region Si iv 1403 Å spectra observed in active regions by the Interface Region Imaging Spectrograph, after subtraction of the thermal and instrumental line broadening. To investigate the dependence of the measured broadening on the viewing angle between the line of sight and magnetic field direction, we determined the magnetic field direction at transition-region heights using nonlinear force-free extrapolations based on the observed photospheric vector magnetic field taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We found that the nonthermal broadening shows a correlation with downward motion (redshifts) and alignment between the magnetic field and the observer's line-of-sight direction. Based on the observed correlations, we suggest that velocity gradients within plasma flowing down along the magnetic field may lead to a significant portion of the observed nonthermal broadening of transition-region spectral lines in closed fields above plage regions.

• Bose, Souvik; Pontieu, Bart Walter De; Hansteen, Viggo Haraldson; Dalda, Alberto Sainz; Savage, Sabrina & Winebarger, Amy (2024). Chromospheric and coronal heating in an active region plage by dissipation of currents from braiding. Nature Astronomy. doi: 10.1038/s41550-024-02241-8. Full text in Research Archive Show summary

  Abstract The question of what heats the outer solar atmosphere remains one of the longstanding mysteries in astrophysics. Statistical studies of Sun-like stars reveal a correlation between global chromospheric and coronal emissions, constraining theoretical models of potential heating mechanisms. However, spatially resolved observations of the Sun have surprisingly failed to show a similar correlation on small spatial scales. Here we use unique coordinated observations of the chromosphere (from the IRIS satellite) and the low corona (from the Hi-C 2.1 sounding rocket), and machine-learning-based inversion techniques, to show a strong correlation on spatial scales of a few hundred kilometres between heating in the chromosphere and emission in the upper transition region in strong magnetic field regions (‘plage’). Our observations are compatible with an advanced three-dimensional magnetohydrodynamic simulation in which the dissipation of current sheets caused by magnetic field braiding is responsible for heating the plasma simultaneously to chromospheric and coronal temperatures. Our results provide deep insight into the nature of the heating mechanism in solar active regions.

• Gošić, M.; Pontieu, Bart Walter De & Dalda, A. Sainz (2024). Global Impact of Emerging Internetwork Fields on the Low Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 964(2). doi: 10.3847/1538-4357/ad2e03. Full text in Research Archive Show summary

  Abstract Small-scale, newly emerging internetwork (IN) magnetic fields are considered a viable source of energy and mass for the solar chromosphere and possibly the corona. Multiple studies show that single events of flux emergence can indeed locally heat the low solar atmosphere through interactions of the upward propagating magnetic loops and the preexisting ambient field lines. However, the global impact of the newly emerging IN fields on the solar atmosphere is still unknown. In this paper, we study the spatiotemporal evolution of IN bipolar flux features and analyze their impact on the energetics and dynamics of the quiet-Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph, Hinode, and the Solar Dynamics Observatory to identify emerging IN magnetic fields and follow their evolution. Our observational results suggest that only the largest IN bipoles are capable of heating locally the low solar atmosphere, while the global contribution of these bipoles appears to be marginal. However, the total number of bipoles detected and their impact estimated in this work is limited by the sensitivity level, spatial resolution, and duration of our observations. To detect smaller and weaker IN fields that would maintain the basal flux, and examine their contribution to the chromospheric heating, we will need higher resolution, higher sensitivity, and longer time series obtained with current and next-generation ground- and space-based telescopes.

• Dalda, Alberto Sainz; Agrawal, Aaryan; Pontieu, Bart Walter De & Gošić, Milan (2024). IRIS2+: A Comprehensive Database of Stratified Thermodynamic Models in the Low Solar Atmosphere. Astrophysical Journal Supplement Series. ISSN 0067-0049. 271(1). doi: 10.3847/1538-4365/ad1e55. Full text in Research Archive Show summary

  Abstract We present the most comprehensive collection of stratified-in-optical-depth thermodynamic models of the lower solar atmosphere. This database, named IRIS 2+ , consists of 40,320 synthetic representative profiles (RPs) and their corresponding representative model atmospheres (RMAs). The latter result from inverting the observed RPs, which are the result of clustering multiline spectral profiles observed by the Interface Region Imaging Spectrograph in 126 active regions (ARs). Each AR was clustered in 320 RPs considering five solar features: umbra, pore or pore-like, penumbra, plage, and quiet Sun. The multiline spectral profiles contain 12 lines with encoded physical information from the top of the chromosphere to the midphotosphere. These 12 spectral lines, six chromospheric and six photospheric, were simultaneously inverted with the Stockholm Inversion Code. Therefore, the stratified-in-optical-depth low solar atmosphere models obtained in this study are better constrained than those obtained from the nonsimultaneous inversion of individual lines. Each tuple in the database contains an inverted RP, its corresponding RMA, the solar feature label where the observed RP was clustered, its heliocentric angle, and its date and time of observation. The database may be used as an inversion code for any of the lines present in the RPs, and the RMAs may be used for synthesizing spectral lines in the chromosphere and photosphere or as mean constraints for numerical models of the solar atmosphere.

• Cozzo, G.; Reid, J.; Pagano, P.; Reale, F.; Testa, Paola & Hood, A.W. [Show all 16 contributors for this article] (2024). Coronal energy release by MHD avalanches: II. EUV line emission from a multi-threaded coronal loop. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 689. doi: 10.1051/0004-6361/202450644. Full text in Research Archive Show summary

  Context. Magnetohydrodynamic (MHD) instabilities, such as the kink instability, can trigger the chaotic fragmentation of a twisted magnetic flux tube into small-scale current sheets that dissipate as aperiodic impulsive heating events. In turn, the instability could propagate as an avalanche to nearby flux tubes and lead to a nanoflare storm. Our previous work was devoted to related 3D MHD numerical modeling, which included a stratified atmosphere from the solar chromosphere to the corona, tapering magnetic field, and solar gravity for curved loops with the thermal structure modelled by plasma thermal conduction, along with optically thin radiation and anomalous resistivity for 50 Mm flux tubes. Aims. Using 3D MHD modeling, this work addresses predictions for the extreme-ultraviolet (EUV) imaging spectroscopy of such structure and evolution of a loop, with an average temperature of 2–2.5 MK in the solar corona. We set a particular focus on the forthcoming MUSE mission, as derived from the 3D MHD modeling. Methods. From the output of the numerical simulations, we synthesized the intensities, Doppler shifts, and non-thermal line broadening in 3 EUV spectral lines in the MUSE passbands: Fe IX 171 Å, Fe XV 284 Å, and Fe XIX 108 Å, emitted by ∼1 MK, ∼2 MK, and ∼10 MK plasma, respectively. These data were detectable by MUSE, according to the MUSE expected pixel size, temporal resolution, and temperature response functions. We provide maps showing different view angles (front and top) and realistic spectra. Finally, we discuss the relevant evolutionary processes from the perspective of possible observations. Reults. We find that the MUSE observations might be able to detect the fine structure determined by tube fragmentation. In particular, the Fe IX line is mostly emitted at the loop footpoints, where we might be able to track the motions that drive the magnetic stressing and detect the upward motion of evaporating plasma from the chromosphere. In Fe XV, we might see the bulk of the loop with increasing intensity, with alternating filamentary Doppler and non-thermal components in the front view, along with more defined spots in the topward view. The Fe XIX line is very faint within the chosen simulation parameters; thus, any transient brightening around the loop apex may possibly be emphasized by the folding of sheet-like structures, mainly at the boundary of unstable tubes. Conclusions. In conclusion, we show that coronal loop observations with MUSE can pinpoint some crucial features of MHD-modeled ignition processes, such as the related dynamics, helping to identify the heating processes.

• Lörinčík, Juraj; Dudík, Jaroslav; Dalda, Alberto Sainz; Aulanier, Guillaume; Polito, Vanessa & Pontieu, Bart Walter De (2024). Observation of super-Alfvénic slippage of reconnecting magnetic field lines on the Sun. Nature Astronomy. doi: 10.1038/s41550-024-02396-4. Full text in Research Archive Show summary

  Slipping motions of magnetic field lines are a distinct signature of three-dimensional magnetic reconnection, a fundamental process driving solar and stellar flares. While being a key prediction of numerical experiments, the rapid super-Alfvénic field line slippage driven by the ‘slip-running’ reconnection has remained elusive in previous observations. New frontiers into exploring transient flare phenomena were introduced by recently designed high cadence observing programs of the Interface Region Imaging Spectrograph (IRIS). By exploiting high temporal resolution imagery (~2 s) of IRIS, here we reveal slipping motions of flare kernels at speeds reaching thousands of kilometres per second. The fast kernel motions are direct evidence of slip-running reconnection in quasi-separatrix layers, regions where magnetic field strongly changes its connectivity. Our results provide observational proof of theoretical predictions unaddressed for nearly two decades and extend the range of magnetic field configurations where reconnection-related phenomena can occur.

• Carlsson, Mats & Pontieu, Bart Walter De (2023). An Optically Thin View of the Solar Chromosphere from Observations of the O I 1355Å Spectral Line. The Astrophysical Journal (ApJ). ISSN 0004-637X. 959(2). doi: 10.3847/1538-4357/acf451. Full text in Research Archive Show summary

  Abstract The O i 1355 Å spectral line is one of the only optically thin lines that are both routinely observed and thought to be formed in the chromosphere. We present an analysis of a variety of observations of this line with the Interface Region Imaging Spectrograph (IRIS), and compare it with other IRIS diagnostics as well as diagnostics of the photospheric magnetic field. We utilize special deep exposure modes on IRIS and provide an overview of the statistical properties of this spectral line for several different regions on the Sun. We analyze the spatiotemporal variations of the line intensity and find that it is often significantly enhanced when and where magnetic flux of opposite polarities cancel. Significant emission occurs in association with chromospheric spicules. Because of the optically thin nature of the O i line, the nonthermal broadening can provide insight into unresolved small-scale motions. We find that the nonthermal broadening is modest, with typical values of 5–10 km s −1 , and shows some center-to-limb variation, with a modest increase toward the limb. The dependence with the height of the intensity and line broadening off-limb is compatible with the line broadening being dominated by the superposition of Alfvén waves on different structures. The nonthermal broadening shows a modest but significant enhancement above locations that are in between photospheric magnetic flux concentrations in plage, i.e., where the magnetic field is likely to be more inclined with respect to the line of sight. Our measurements provide strict constraints on future theoretical models of the chromosphere.

• Wargnier, Q.M.; Martinez-Sykora, Juan; Hansteen, Viggo Haraldson & Pontieu, Bart Walter De (2023). Multifluid Simulations of Upper-chromospheric Magnetic Reconnection with Helium-Hydrogen Mixture. The Astrophysical Journal (ApJ). ISSN 0004-637X. 946(2). doi: 10.3847/1538-4357/acbfb1. Full text in Research Archive Show summary

  Abstract Our understanding of magnetic reconnection (MR) under chromospheric conditions remains limited. Recent observations have demonstrated the important role of ion–neutral interactions in the dynamics of the chromosphere. Furthermore, the comparison between the spectral profiles and synthetic observations of reconnection events suggests that current MHD approaches appear to be inconsistent with observations. First, collisions and multithermal aspects of the plasma play a role in these regions. Second, hydrogen and helium ionization effects are relevant to the energy balance of the chromosphere. This work investigates the multifluid multispecies (MFMS) effects on MR in conditions representative of the upper chromosphere using the multifluid Ebysus code. We compare an MFMS approach based on a helium–hydrogen mixture with a two-fluid MHD model based on hydrogen only. The simulations of MR are performed in a Lundquist number regime high enough to develop plasmoids and instabilities. We study the evolution of the MR and compare the two approaches including the structure of the current sheet and plasmoids, the decoupling of the particles, the evolution of the heating mechanisms, and the composition. The presence of helium species leads to more efficient heating mechanisms than the two-fluid case. This scenario, which is out of reach of the two-fluid or single-fluid models, can reach transition region temperatures starting from upper-chromospheric thermodynamic conditions, representative of a quiet Sun scenario. The different dynamics between helium and hydrogen species could lead to chemical fractionation and, under certain conditions, enrichment of helium in the strongest outflows. This could be of significance for recent observations of helium enrichment in the solar wind in switchbacks and coronal mass ejections.

• Testa, Paola; Martinez-Sykora, Juan & Pontieu, Bart Walter De (2023). Coronal Abundances in an Active Region: Evolution and Underlying Chromospheric and Transition Region Properties. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb343. Full text in Research Archive Show summary

  Abstract The element abundances in the solar corona and solar wind are often different from those of the solar photosphere, typically with a relative enrichment of elements with low first ionization potential (FIP effect). Here, we study the spatial distribution and temporal evolution of the coronal chemical composition in an active region (AR) over about 10 days, using Hinode/EIS spectra, and we also analyze coordinated IRIS observations of the chromospheric and transition region emission to investigate any evidence of the footprints of the FIP effect in the lower atmosphere. To derive the coronal abundances, we use a spectral inversion method recently developed for the MUSE investigation. We find that, in the studied active region (AR 12738), the coronal FIP bias, as diagnosed by the Si/S abundance ratio, presents significant spatial variations, with its highest values (∼2.5–3.5) in the outflow regions at the boundary of the AR, but typically modest temporal variability. Some moss regions and some regions around the AR sunspot show enhanced FIP bias (∼2–2.5) with respect to the AR core, which has only a small FIP bias of ∼1.5. The FIP bias appears most variable in these moss regions. The IRIS observations reveal that the chromospheric turbulence, as derived from IRIS 2 inversions of the Mg ii spectra, is enhanced in the outflow regions characterized by the high FIP bias, providing significant new constraints to both models aimed at explaining the formation of AR outflows and models of chemical fractionation.

• Dalda, Alberto Sainz & Pontieu, Bart Walter De (2023). Recovering Thermodynamics from Spectral Profiles Observed by IRIS. (II). Improved Calculation of the Uncertainties Based on Monte Carlo Experiments. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb2c7. Full text in Research Archive Show summary

  Abstract Observations by the Interface Region Imaging Spectrograph (IRIS) of the Mg ii h &amp; k spectral lines have provided a new diagnostic window toward knowledge of the complex physical conditions in the solar chromosphere. Theoretical efforts focused on understanding the behavior of these lines have allowed us to obtain a better and more accurate vision of the chromosphere. These efforts include forward modeling, numerical simulations, and inversions. In this paper, we focus our attention on the uncertainties associated with the thermodynamic model atmosphere obtained after the inversion of the Mg ii h &amp; k lines. We have used ≈50,000 synthetic representative profiles of the IRIS 2 database to characterize the most important sources of uncertainties in the inversion process, viz. : the inherent noise of the observations, the random initialization process, and the selection criteria in a high-dimensional space. We have applied a Monte Carlo approach to this problem. Thus, for a given synthetic representative profile, we have created five randomized noise realizations (representative of the most popular exposure times in the IRIS observations), and inverted these profiles five times with different inversion initializations. The resulting 25 inverted profiles, fit to noisy data, and model atmospheres are then used to determine the uncertainty in the model atmosphere, based on the standard deviation and empirical selection criteria for the goodness of fit. With this approach, the new uncertainties of the models available in the IRIS 2 database are more reliable at the optical depths where the Mg ii h &amp; k lines are sensitive to changes in the thermodynamics.

• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Hansteen, Viggo Haraldson; Testa, Paola; Wargnier, Q.M. & Szydlarski, Mikolaj (2023). The Impact of Multifluid Effects in the Solar Chromosphere on the Ponderomotive Force under SE and NEQ Ionization Conditions. The Astrophysical Journal (ApJ). ISSN 0004-637X. 949(2). doi: 10.3847/1538-4357/acc465. Full text in Research Archive Show summary

  Abstract The ponderomotive force has been suggested to be the main mechanism to produce the so-called first ionization potential (FIP) effect—the enrichment of low-FIP elements observed in the outer solar atmosphere, in the solar wind, and in solar energetic events. It is well known that the ionization of these elements occurs within the chromosphere. Therefore, this phenomenon is intimately tied to the plasma state in the chromosphere and the corona. For this study, we combine IRIS observations, a single-fluid 2.5D radiative magnetohydrodynamics (MHD) model of the solar atmosphere, including ion–neutral interaction effects and nonequilibrium (NEQ) ionization effects, and a novel multifluid multispecies numerical model (based on the Ebysus code). Nonthermal velocities of Si iv measured from IRIS spectra can provide an upper limit for the strength of any high-frequency Alfvén waves. With the single-fluid model, we investigate the possible impact of NEQ ionization within the region where the FIP may occur, as well as the plasma properties in those regions. These models suggest that regions with strongly enhanced network and type II spicules are possible sites of large ponderomotive forces. We use the plasma properties of the single-fluid MHD model and the IRIS observations to initialize our multifluid models to investigate the multifluid effects on the ponderomotive force associated with Alfvén waves. Our multifluid analysis reveals that collisions and NEQ ionization effects dramatically impact the behavior of the ponderomotive force in the chromosphere, and existing theories may need to be revisited.

• Testa, Paola; Bakke, Helle; Voort, Luc Rouppe van der & Pontieu, Bart Walter De (2023). High-resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core. The Astrophysical Journal (ApJ). ISSN 0004-637X. 956(2). doi: 10.3847/1538-4357/acf4f1. Full text in Research Archive Show summary

  Abstract High-resolution spectral observations of the lower solar atmosphere (chromosphere and transition region) during coronal heating events, in combination with predictions from models of impulsively heated loops, provide powerful diagnostics of the properties of the heating in active region cores. Here, we analyze the first coordinated observations of such events with the Interface Region Imaging Spectrograph (IRIS) and the CHROMospheric Imaging Spectrometer (CHROMIS), at the Swedish 1 m Solar Telescope (SST), which provided extremely high spatial resolution and revealed chromospheric brightenings with spatial dimensions down to ∼150 km. We use machine-learning methods ( k -means clustering) and find significant coherence in the spatial and temporal properties of the chromospheric spectra, suggesting, in turn, coherence in the spatial and temporal distribution of the coronal heating. The comparison of IRIS and CHROMIS spectra with simulations suggests that both nonthermal electrons with low energy (low-energy cutoff ∼5 keV) and direct heating in the corona transported by thermal conduction contribute to the heating of the low atmosphere. This is consistent with growing evidence that nonthermal electrons are not uncommon in small heating events (nanoflare to microflares), and that their properties can be constrained by chromospheric and transition region spectral observations.

• Dalda, Alberto Sainz & Pontieu, Bart Walter De (2023). Chromospheric thermodynamic conditions from inversions of complex Mg II h & k profiles observed in flares. Frontiers in Astronomy and Space Sciences. 10. doi: 10.3389/fspas.2023.1133429. Full text in Research Archive Show summary

  The flare activity of the Sun has been studied for decades, using both space- and ground-based telescopes. The former have mainly focused on the corona, while the latter have mostly been used to investigate the conditions in the chromosphere and photosphere. The Interface Region Imaging Spectrograph (IRIS) instrument has served as a gateway between these two cases, given its capability to observe quasi-simultaneously the corona, the transition region, and the chromosphere using different spectral lines in the near- and far-ultraviolet ranges. IRIS thus provides unique diagnostics to investigate the thermodynamics of flares in the solar atmosphere. In particular, the Mg II h&k and the Mg II UV triplet lines provide key information about the thermodynamics of low to upper chromosphere, while the C II 1334 & 1335 Å lines cover the upper-chromosphere and low transition region. The Mg II h&k and the Mg II UV triplet lines show a peculiar, pointy shape before and during the flare activity. The physical interpretation, i.e., the physical conditions in the chromosphere, that can explain these profiles has remained elusive. In this paper, we show the results of a non-LTE inversion of such peculiar profiles. To better constrain the atmospheric conditions, the Mg II h&k and the Mg II UV triplet lines are simultaneously inverted with the C II 1334 & 1335 Å lines. This combined inversion leads to more accurate derived thermodynamic parameters, especially the temperature and the turbulent motions (micro-turbulence velocity). We use an iterative process that looks for the best fit between the observed profile and a synthetic profile obtained by considering non-local thermodynamic equilibrium and partial frequency redistribution of the radiation due to scattered photons. This method is computationally rather expensive (≈6 CPU-hour/profile). Therefore, we use the k-means clustering technique to identify representative profiles and associated representative model atmospheres. By inverting the representative profiles with the most advanced inversion code (STiC), in addition to recover the main physical parameters, we are able to conclude that these unique, pointy profiles are associated with a large gradient in the line-of-sight velocity along the optical depth in the high chromosphere.

• Cho, Kyuhyoun; Testa, Paola; Pontieu, Bart Walter De & Polito, Vanessa (2023). A Statistical Study of IRIS Observational Signatures of Nanoflares and Nonthermal Particles. The Astrophysical Journal (ApJ). ISSN 0004-637X. 945(2). doi: 10.3847/1538-4357/acb7da. Full text in Research Archive Show summary

  Abstract Nanoflares are regarded as one of the major mechanisms of magnetic energy release and coronal heating in the solar outer atmosphere. We conduct a statistical study on the response of the chromosphere and transition region to nanoflares, as observed by the Interface Region Imaging Spectrograph (IRIS), by using an algorithm for the automatic detection of these events. The initial atmospheric response to these small heating events is observed, with IRIS, as transient brightening at the foot points of coronal loops heated to high temperatures (&gt;4 MK). For four active regions, observed over 143 hr, we detected 1082 footpoint brightenings under the IRIS slit, and for those we extracted physical parameters from the IRIS Mg ii and Si iv spectra that are formed in the chromosphere and transition region, respectively. We investigate the distributions of the spectral parameters, and the relationships between the parameters, also comparing them with predictions from RADYN numerical simulations of nanoflare-heated loops. We find that these events, and the presence of nonthermal particles, tend to be more frequent in flare productive active regions, and where the hot 94 Å emission measured by the Atmospheric Imaging Assembly is higher. We find evidence for highly dynamic motions characterized by strong Si iv nonthermal velocities (not dependent on the heliocentric x -coordinate, i.e., on the angle between the magnetic field and the line of sight) and asymmetric Mg ii spectra. These findings provide tight new constraints on the properties of nanoflares and nonthermal particles in active regions, and their effects on the lower atmosphere.

• Yardley, Stephanie L.; Owen, Christopher J.; Long, David M.; Baker, Deborah; Brooks, David H. & Polito, Vanessa [Show all 79 contributors for this article] (2023). Slow Solar Wind Connection Science during Solar Orbiter’s First Close Perihelion Passage. Astrophysical Journal Supplement Series. ISSN 0067-0049. 267(1). doi: 10.3847/1538-4365/acd24b. Full text in Research Archive Show summary

  Abstract The Slow Solar Wind Connection Solar Orbiter Observing Plan (Slow Wind SOOP) was developed to utilize the extensive suite of remote-sensing and in situ instruments on board the ESA/NASA Solar Orbiter mission to answer significant outstanding questions regarding the origin and formation of the slow solar wind. The Slow Wind SOOP was designed to link remote-sensing and in situ measurements of slow wind originating at open–closed magnetic field boundaries. The SOOP ran just prior to Solar Orbiter’s first close perihelion passage during two remote-sensing windows (RSW1 and RSW2) between 2022 March 3–6 and 2022 March 17–22, while Solar Orbiter was at respective heliocentric distances of 0.55–0.51 and 0.38–0.34 au from the Sun. Coordinated observation campaigns were also conducted by Hinode and IRIS. The magnetic connectivity tool was used, along with low-latency in situ data and full-disk remote-sensing observations, to guide the target pointing of Solar Orbiter. Solar Orbiter targeted an active region complex during RSW1, the boundary of a coronal hole, and the periphery of a decayed active region during RSW2. Postobservation analysis using the magnetic connectivity tool, along with in situ measurements from MAG and SWA/PAS, showed that slow solar wind originating from two out of three of the target regions arrived at the spacecraft with velocities between ∼210 and 600 km s −1 . The Slow Wind SOOP, despite presenting many challenges, was very successful, providing a blueprint for planning future observation campaigns that rely on the magnetic connectivity of Solar Orbiter.

• Bose, Souvik; Siverio, Daniel Elias Nóbrega; Pontieu, Bart Walter De & Voort, Luc Rouppe van der (2023). The Chromosphere Underneath a Coronal Bright Point. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb544. Full text in Research Archive Show summary

  Abstract Coronal bright points (CBPs) are sets of small-scale coronal loops, connecting opposite magnetic polarities, primarily characterized by their enhanced extreme-ultraviolet (EUV) and X-ray emission. Being ubiquitous, they are thought to play an important role in heating the solar corona. We aim at characterizing the barely explored chromosphere underneath CBPs, focusing on the related spicular activity and on the effects of small-scale magnetic flux emergence on CBPs. We used high-resolution observations of a CBP in H β and Fe i 617.3 nm from the Swedish 1 m Solar Telescope in coordination with the Solar Dynamics Observatory. This work presents the first high-resolution observation of spicules imaged in H β . The spicules were automatically detected using advanced image processing techniques, which were applied to the Dopplergrams derived from H β . Here we report their abundant occurrence close to the CBP “footpoints” and find that the orientation of such spicules is aligned along the EUV loops, indicating that they constitute a fundamental part of the whole CBP magnetic structure. Spatiotemporal analysis across multiple channels indicates that there are coronal propagating disturbances associated with the studied spicules, producing transient EUV intensity variations of the individual CBP loops. Two small-scale flux emergence episodes appearing below the CBP were analyzed, one of them leading to quiet-Sun Ellerman bombs and enhancing the nearby spicular activity. This paper presents unique evidence of the tight coupling between the lower and upper atmosphere of a CBP, thus helping to unravel the dynamic phenomena underneath CBPs and their impact on the latter.

• Hansteen, Viggo Haraldson; Martinez-Sykora, Juan; Carlsson, Mats; Pontieu, Bart Walter De; Gošić, Milan & Bose, Souvik (2023). Numerical Simulations and Observations of Mg II in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb33c. Full text in Research Archive Show summary

  The Mg ii h and k lines are among the best diagnostic tools of the upper solar chromosphere. This region of the atmosphere is of particular interest, as it is the lowest region of the Sun's atmosphere where the magnetic field is dominant in the energetics and dynamics, defining its structure. While highly successful in the photosphere and lower to mid-chromosphere, numerical models have produced synthetic Mg ii lines that do not match the observations well. We present a number of large-scale models with magnetic field topologies representative of the quiet Sun, ephemeral flux regions and plage, and also models where the numerical resolution is high and where we go beyond the MHD paradigm. The results of this study show models with a much improved correspondence with IRIS observations in terms of both intensities and widths, especially underscoring the importance of chromospheric mass loading and of capturing the magnetic field topology and evolution in simulations. This comes in addition to the importance of capturing the generation of small-scale velocity fields and including nonequilibrium ionization and ion−neutral interaction effects. However, it should be noted that difficulties in achieving a good correspondence remain, especially when considering the width of Mg ii h and k lines in plage. Understanding and modeling all these effects and their relative importance is necessary in order to reproduce observed spectral features and in isolating the missing pieces necessary to fully comprehend Mg ii formation.

• Martinez-Sykora, Juan; Rodriguez, Jaime de la Cruz; Gošić, Milan; Dalda, Alberto Sainz; Hansteen, Viggo Haraldson & Pontieu, Bart Walter De (2023). Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion under Nonequilibrium Conditions. Astrophysical Journal Letters. ISSN 2041-8205. 943(2). doi: 10.3847/2041-8213/acafe9. Full text in Research Archive Show summary

  Abstract The heating of the chromosphere in internetwork regions remains one of the foremost open questions in solar physics. In the present study, we tackle this old problem by using a very-high-spatial-resolution simulation of quiet-Sun conditions performed with radiative MHD numerical models and interface region imaging spectrograph (IRIS) observations. We have expanded a previously existing 3D radiative MHD numerical model of the solar atmosphere, which included self-consistently locally driven magnetic amplification in the chromosphere, by adding ambipolar diffusion and time-dependent nonequilibrium hydrogen ionization to the model. The energy of the magnetic field is dissipated in the upper chromosphere, providing a large temperature increase due to ambipolar diffusion and nonequilibrium ionization (NEQI). At the same time, we find that adding the ambipolar diffusion and NEQI in the simulation has a minor impact on the local growth of the magnetic field in the lower chromosphere and its dynamics. Our comparison between synthesized Mg ii profiles from these high-spatial-resolution models, with and without ambipolar diffusion and NEQI, and quiet-Sun and coronal hole observations from IRIS now reveal a slightly better correspondence. The intensity of profiles is increased, and the line cores are slightly broader when ambipolar diffusion and NEQI effects are included. Therefore, the Mg ii profiles are closer to those observed than in previous models, though some differences still remain.

• Bakke, Helle; Carlsson, Mats; Voort, Luc Rouppe van der; Gudiksen, Boris Vilhelm; Polito, V. & Testa, Paola [Show all 7 contributors for this article] (2022). Chromospheric emission from nanoflare heating in RADYN simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 659. doi: 10.1051/0004-6361/202142842. Full text in Research Archive Show summary

  Context. Heating signatures from small-scale magnetic reconnection events in the solar atmosphere have proven to be difficult to detect through observations. Numerical models that reproduce flaring conditions are essential in understanding how nanoflares may act as a heating mechanism of the corona. Aims. We study the effects of non-thermal electrons in synthetic spectra from 1D hydrodynamic RADYN simulations of nanoflare heated loops to investigate the diagnostic potential of chromospheric emission from small-scale events. Methods. The Mg  II h and k, Ca  II H and K, Ca  II 854.2 nm, and H α and H β chromospheric lines were synthesised from various RADYN models of coronal loops subject to electron beams of nanoflare energies. The contribution function to the line intensity was computed to better understand how the atmospheric response to the non-thermal electrons affects the formation of spectral lines and the detailed shape of their spectral profiles. Results. The spectral line signatures arising from the electron beams highly depend on the density of the loop and the lower cutoff energy of the electrons. Low-energy (5 keV) electrons deposit their energy in the corona and transition region, producing strong plasma flows that cause both redshifts and blueshifts of the chromospheric spectra. Higher-energy (10 and 15 keV) electrons deposit their energy in the lower transition region and chromosphere, resulting in increased emission from local heating. Our results indicate that effects from small-scale events can be observed with ground-based telescopes, expanding the list of possible diagnostics for the presence and properties of nanoflares.

• Wargnier, Q.M.; Martinez-Sykora, Juan; Hansteen, Viggo Haraldson & Pontieu, Bart Walter De (2022). Detailed Description of the Collision Frequency in the Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 933(2). doi: 10.3847/1538-4357/ac6e62. Full text in Research Archive Show summary

  This work aims to provide an accurate description and calculations of collision frequencies in conditions relevant to the solar atmosphere. To do so, we focus on the detailed description of the collision frequency in the solar atmosphere based on a classical formalism with Chapman–Cowling collision integrals, as described by Zhdanov. These collision integrals allow linking the macroscopic transport fluxes of multifluid models to the kinetic scales involved in the Boltzmann equations. In this context, the collision frequencies are computed accurately while being consistent at the kinetic level. We calculate the collision frequencies based on this formalism and compare them with approaches commonly used in the literature for conditions typical of the solar atmosphere. To calculate the collision frequencies, we focus on the collision integral data provided by Bruno et al., which is based on a multicomponent hydrogen–helium mixture used for conditions typical for the atmosphere of Jupiter. We perform a comparison with the classical formalism of Vranjes & Krstic and Leake & Linton. We highlight the differences obtained in the distribution of the cross sections as functions of the temperature. Then, we quantify the disparities obtained in numerical simulations of a 2.5D solar atmosphere by calculating collision frequencies and ambipolar diffusion. This strategy allows us to validate and assess the accuracy of these collision frequencies for conditions typical of the solar atmosphere.

• Song, Donguk; Ishikawa, Ryohko; Kano, Ryouhei; McKenzie, David E.; Bueno, Javier Trujillo & Auchère, Frédéric [Show all 28 contributors for this article] (2022). Polarization Accuracy Verification of the Chromospheric LAyer SpectroPolarimeter. Solar Physics. ISSN 0038-0938. 297(10). doi: 10.1007/s11207-022-02064-8. Full text in Research Archive
• Rachmeler, Laurel A.; Bueno, Javier Trujillo; McKenzie, D.E.; Ishikawa, R.; Auchère, F. & Kobayashi, K. [Show all 28 contributors for this article] (2022). Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg ii h and k Lines. The Astrophysical Journal (ApJ). ISSN 0004-637X. 936(1). doi: 10.3847/1538-4357/ac83b8. Full text in Research Archive Show summary

  Abstract The CLASP2 (Chromospheric LAyer Spectro-Polarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg ii h and k spectral region around 2800 Å along a 200 slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near-ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg ii h and k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semiempirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.

• Peter, Hardi; Chitta, Lakshmi Pradeep; Chen, Feng; Pontin, David I.; Winebarger, Amy R. & Golub, Leon [Show all 18 contributors for this article] (2022). Parallel Plasma Loops and the Energization of the Solar Corona. The Astrophysical Journal (ApJ). ISSN 0004-637X. 933(2). doi: 10.3847/1538-4357/ac7219. Full text in Research Archive Show summary

  The outer atmosphere of the Sun is composed of plasma heated to temperatures well in excess of the visible surface. We investigate short cool and warm (<1 MK) loops seen in the core of an active region to address the role of field-line braiding in energizing these structures. We report observations from the High-resolution Coronal imager (Hi-C) that have been acquired in a coordinated campaign with the Interface Region Imaging Spectrograph (IRIS). In the core of the active region, the 172 Å band of Hi-C and the 1400 Å channel of IRIS show plasma loops at different temperatures that run in parallel. There is a small but detectable spatial offset of less than 1'' between the loops seen in the two bands. Most importantly, we do not see observational signatures that these loops might be twisted around each other. Considering the scenario of magnetic braiding, our observations of parallel loops imply that the stresses put into the magnetic field have to relax while the braiding is applied: the magnetic field never reaches a highly braided state on these length scales comparable to the separation of the loops. This supports recent numerical 3D models of loop braiding in which the effective dissipation is sufficiently large that it keeps the magnetic field from getting highly twisted within a loop.

• Panesar, Navdeep K.; Tiwari, Sanjiv K.; Moore, Ronald L.; Sterling, Alphonse C. & Pontieu, Bart Walter De (2022). Genesis and Coronal-jet-generating Eruption of a Solar Minifilament Captured by IRIS Slit-raster Spectra. The Astrophysical Journal (ApJ). ISSN 0004-637X. 939(1), p. 1–12. doi: 10.3847/1538-4357/ac8d65. Full text in Research Archive Show summary

  Abstract We present the first Mg ii slit-raster spectra from the Interface Region Imaging Spectrograph (IRIS) that fully capture the genesis and coronal-jet-generating eruption of a central-disk solar minifilament. The minifilament arose in a negative-magnetic-polarity coronal hole. The Mg ii spectroheliograms verify that the minifilament plasma temperature is chromospheric. The Mg ii spectra show that the erupting minifilament’s plasma has blueshifted upflow in the onset of the jet spire and simultaneous redshifted downflow at the location of the compact jet bright point (JBP). From the Mg ii spectra together with Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV images and SDO/Helioseismic and Magnetic Imager (HMI) magnetograms, we find: (i) the minifilament forms above a flux-cancelation neutral line at an edge of a negative-polarity network flux clump; (ii) during the onset of the minifilament’s fast eruption and jet spire, the JBP begins brightening over the flux-cancelation neutral line. From IRIS 2 inversion of the Mg ii spectra, the JBP’s Mg ii bright plasma has electron density, temperature, and downward (redshift) Doppler speed of 10 12 cm −3 , 6000 K, and 10 km s −1 , respectively, and the growing spire shows clockwise spin. We speculate: (i) during the slow rise of the erupting minifilament-carrying a twisted flux rope, the top of the erupting flux-rope loop, by writhing, makes its field direction opposite to that of the encountered ambient far-reaching field; (ii) the erupting kink then can reconnect with the far-reaching field to create the spire and reconnect internally to create the JBP. We conclude that this coronal jet is normal in that magnetic flux cancelation builds a minifilament-carrying twisted flux rope and triggers the JBP-generating and jet-spire-generating eruption of the flux rope.

• Cheung, Mark C. M.; Martinez-Sykora, Juan; Testa, Paola; Pontieu, Bart Walter De; Chintzoglou, Georgios & Rempel, Matthias [Show all 27 contributors for this article] (2022). Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). II. Flares and Eruptions. The Astrophysical Journal (ApJ). ISSN 0004-637X. 926(1). doi: 10.3847/1538-4357/ac4223. Full text in Research Archive Show summary

  Current state-of-the-art spectrographs cannot resolve the fundamental spatial (subarcseconds) and temporal (less than a few tens of seconds) scales of the coronal dynamics of solar flares and eruptive phenomena. The highest-resolution coronal data to date are based on imaging, which is blind to many of the processes that drive coronal energetics and dynamics. As shown by the Interface Region Imaging Spectrograph for the low solar atmosphere, we need high-resolution spectroscopic measurements with simultaneous imaging to understand the dominant processes. In this paper: (1) we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne observatory to fill this observational gap by providing high-cadence (<20 s), subarcsecond-resolution spectroscopic rasters over an active region size of the solar transition region and corona; (2) using advanced numerical models, we demonstrate the unique diagnostic capabilities of MUSE for exploring solar coronal dynamics and for constraining and discriminating models of solar flares and eruptions; (3) we discuss the key contributions MUSE would make in addressing the science objectives of the Next Generation Solar Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope (and other ground-based observatories) can operate as a distributed implementation of the NGSPM. This is a companion paper to De Pontieu et al., which focuses on investigating coronal heating with MUSE.

• Pontieu, Bart Walter De; Testa, Paola; Martinez-Sykora, Juan; Antolin, Patrick; Karampelas, Konstantinos & Hansteen, Viggo Haraldson [Show all 26 contributors for this article] (2022). Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). I. Coronal Heating. The Astrophysical Journal (ApJ). ISSN 0004-637X. 926(1). doi: 10.3847/1538-4357/ac4222. Full text in Research Archive
• Lörinčík, Juraj; Polito, Vanessa; Pontieu, Bart Walter De; Yu, Sijie & Freij, Nabil (2022). Rapid variations of Si IV spectra in a flare observed by interface region imaging spectrograph at a sub-second cadence. Frontiers in Astronomy and Space Sciences. 9. doi: 10.3389/fspas.2022.1040945. Full text in Research Archive Show summary

  We report on observations of highly-varying Si IV 1402.77 Å line profiles observed with the Interface Region Imaging Spectrograph (IRIS) during the M-class flare from 18 January 2022 at an unprecedented 0.8 s cadence. Moment analysis of this line observed in flare ribbon kernels showed that the intensity, Doppler velocity, and non-thermal broadening exhibited variations with periods below 10 s. These variations were found to be correlated with properties of the Gaussian fit to a well-resolved secondary component of the line redshifted by up to 70 km s−1, while the primary component was consistently observed near the rest wavelength of the line. A particularly high correlation was found between the non-thermal broadening of the line resulting from the moment analysis and the redshift of the secondary component. This means that the oscillatory enhancements in the line broadening were due to plasma flows (away from the observer) with varying properties. A simple de-projection of the Doppler velocities of the secondary component based on a three-dimensional reconstruction of flare loops rooted in the kernel suggests that the observed flows were caused by downflows and compatible with strong condensation flows recently predicted by numerical simulations. Furthermore, peaks of the intensity and the trends of Doppler velocity of the Gaussian fit to the secondary component (averaged in the ribbon) were found to correspond to one of the quasi-periodic pulsations (QPPs) detected during the event in the soft X-ray flux (as measured by the Geostationary Operational Environmental Satellite, GOES) and the microwave radio flux (as measured by the Expanded Owens Valley Solar Array, EOVSA). This result supports a scenario in which the QPPs were driven by repeated magnetic reconnection.

• Martinez-Sykora, Juan; Hansteen, Viggo Haraldson; Pontieu, Bart Walter De & Landi, Enrico (2022). A Novel Inversion Method to Determine the Coronal Magnetic Field Including the Impact of Bound-Free Absorption. The Astrophysical Journal (ApJ). ISSN 0004-637X. 938(1). doi: 10.3847/1538-4357/ac8d5b. Full text in Research Archive Show summary

  The magnetic field governs the corona; hence, it is a crucial parameter to measure. Unfortunately, existing techniques for estimating its strength are limited by strong assumptions and limitations. These techniques include photospheric or chromospheric field extrapolation using potential or nonlinear force-free methods, estimates based on coronal seismology, or direct observations via, e.g., the Cryo-NIRSP instrument on DKIST, which will measure the coronal magnetic field but only off the limb. Alternately, in this work, we investigate a recently developed approach based on the magnetic-field-induced transition (MIT) of Fe x 257.261Å In order to examine this approach, we have synthesized several Fe x lines from two 3D magnetohydrodynamic simulations, one modeling an emerging flux region and the second an established mature active region. In addition, we take bound–free absorption from neutral hydrogen and helium and singly ionized helium into account. The absorption from cool plasma that occurs at coronal heights has a significant impact on determining the magnetic field. We investigate in detail the challenges of using these Fe x lines to measure the field, considering their density and temperature dependence. We present a novel approach to deriving the magnetic field from the MIT using inversions of the differential emission measure as a function of the temperature, density, and magnetic field. This approach successfully estimates the magnetic field strength (up to 18% relative error) in regions that do not suffer from significant absorption and that have relatively strong coronal magnetic fields (>250 G). This method allows regions where absorption is significant to be masked.

• Tiwari, Sanjiv K.; Hansteen, Viggo Haraldson; Pontieu, Bart Walter De; Panesar, Navdeep K. & Berghmans, David (2022). SolO/EUI Observations of Ubiquitous Fine-scale Bright Dots in an Emerging Flux Region: Comparison with a Bifrost MHD Simulation. The Astrophysical Journal (ApJ). ISSN 0004-637X. 929(1). doi: 10.3847/1538-4357/ac5d46. Full text in Research Archive Show summary

  We report on the presence of numerous tiny bright dots in and around an emerging flux region (an X-ray/coronal bright point) observed with SolO's EUI/HRIEUV in 174 Å. These dots are roundish and have a diameter of 675 ± 300 km, a lifetime of 50 ± 35 s, and an intensity enhancement of 30% ± 10% above their immediate surroundings. About half of the dots remain isolated during their evolution and move randomly and slowly (<10 km s−1). The other half show extensions, appearing as a small loop or surge/jet, with intensity propagations below 30 km s−1. Many of the bigger and brighter HRIEUV dots are discernible in the SDO/AIA 171 Å channel, have significant emissivity in the temperature range of 1–2 MK, and are often located at polarity inversion lines observed in SDO/HMI LOS magnetograms. Although not as pervasive as in observations, a Bifrost MHD simulation of an emerging flux region does show dots in synthetic Fe ix/x images. These dots in the simulation show distinct Doppler signatures—blueshifts and redshifts coexist, or a redshift of the order of 10 km s−1 is followed by a blueshift of similar or higher magnitude. The synthetic images of O v/vi and Si iv lines, which represent transition region radiation, also show the dots that are observed in Fe ix/x images, often expanded in size, or extended as a loop, and always with stronger Doppler velocities (up to 100 km s−1) than that in Fe ix/x lines. Our observation and simulation results, together with the field geometry of dots in the simulation, suggest that most dots in emerging flux regions form in the lower solar atmosphere (at ≈ 1 Mm) by magnetic reconnection between emerging and preexisting/emerged magnetic field. Some dots might be manifestations of magnetoacoustic shocks through the line formation region of Fe ix/x emission.

• Gošić, Milan; Pontieu, Bart Walter De; Rubio, Luis R. Bellot; Dalda, Alberto Sainz & Pozuelo, Sara Esteban (2021). Emergence of Internetwork Magnetic Fields through the Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 911(1). doi: 10.3847/1538-4357/abe7e0. Full text in Research Archive
• Pontieu, Bart Walter De; Polito, Vanessa; Hansteen, Viggo Haraldson; Testa, Paola; Reeves, Katharine K. & Antolin, Patrick [Show all 14 contributors for this article] (2021). A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS). Solar Physics. ISSN 0038-0938. 296. doi: 10.1007/s11207-021-01826-0. Full text in Research Archive Show summary

  Abstract The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. IRIS is the highest resolution observatory to provide seamless coverage of spectra and images from the photosphere into the low corona. The unique combination of near- and far-ultraviolet spectra and images at sub-arcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the surface and the corona or solar wind. IRIS has enabled research into the fundamental physical processes thought to play a role in the low solar atmosphere such as ion–neutral interactions, magnetic reconnection, the generation, propagation, and dissipation of waves, the acceleration of non-thermal particles, and various small-scale instabilities. IRIS has provided insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nano-flares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvénic waves, energy release and jet-like dynamics associated with braiding of magnetic-field lines, the role of turbulence and the tearing-mode instability in reconnection, the contribution of waves, turbulence, and non-thermal particles in the energy deposition during flares and smaller-scale events such as UV bursts, and the role of flux ropes and various other mechanisms in triggering and driving CMEs. IRIS observations have also been used to elucidate the physical mechanisms driving the solar irradiance that impacts Earth’s upper atmosphere, and the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine-learning techniques have played a key role. With the advent of exciting new instrumentation both on the ground, e.g. the Daniel K. Inouye Solar Telescope (DKIST) and the Atacama Large Millimeter/submillimeter Array (ALMA), and space-based, e.g. the Parker Solar Probe and the Solar Orbiter , we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges.

• Ayres, Thomas; Pontieu, Bart Walter De & Testa, Paola (2021). In the Trenches of the Solar-stellar Connection. IV. Solar Full-disk Scans of C ii, Si iv, and Mg ii by the Interface Region Imaging Spectrograph. The Astrophysical Journal (ApJ). ISSN 0004-637X. 916(1). doi: 10.3847/1538-4357/abfa92. Full text in Research Archive
• Rast, Mark P.; González, Nazaret Bello; Rubio, Luis R. Bellot; Cao, Wenda; Cauzzi, Gianna & DeLuca, Edward [Show all 86 contributors for this article] (2021). Critical Science Plan for the Daniel K. Inouye Solar Telescope (DKIST). Solar Physics. ISSN 0038-0938. 296(4). doi: 10.1007/s11207-021-01789-2. Full text in Research Archive Show summary

  The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) will revolutionize our ability to measure, understand, and model the basic physical processes that control the structure and dynamics of the Sun and its atmosphere. The first-light DKIST images, released publicly on 29 January 2020, only hint at the extraordinary capabilities that will accompany full commissioning of the five facility instruments. With this Critical Science Plan (CSP) we attempt to anticipate some of what those capabilities will enable, providing a snapshot of some of the scientific pursuits that the DKIST hopes to engage as start-of-operations nears. The work builds on the combined contributions of the DKIST Science Working Group (SWG) and CSP Community members, who generously shared their experiences, plans, knowledge, and dreams. Discussion is primarily focused on those issues to which DKIST will uniquely contribute.

• Ishikawa, Ryohko; Bueno, Javier Trujillo; Alemán, Tanausú del Pino; Okamoto, Takenori J.; McKenzie, David E. & Auchère, Frédéric [Show all 28 contributors for this article] (2021). Mapping solar magnetic fields from the photosphere to the base of the corona. Science Advances. 7(8). doi: 10.1126/sciadv.abe8406. Full text in Research Archive Show summary

  Routine ultraviolet imaging of the Sun’s upper atmosphere shows the spectacular manifestation of solar activity; yet, we remain blind to its main driver, the magnetic field. Here, we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg ii h & k and Mn i) and visible (Fe i) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere, the field strengths reach more than 300 G, strongly correlated with the Mg ii k line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere.

• Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge; Siverio, Daniel Elias Nóbrega & Martinez-Sykora, Juan [Show all 7 contributors for this article] (2021). Evidence of the multi-thermal nature of spicular downflows. Impact on solar atmospheric heating. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 654(51). doi: 10.1051/0004-6361/202141404. Full text in Research Archive Show summary

  Context. Spectroscopic observations of the emission lines formed in the solar transition region commonly show persistent downflows on the order of 10−15 km s −1 . The cause of such downflows, however, is still not fully clear and has remained a matter of debate. Aims. We aim to understand the cause of such downflows by studying the coronal and transition region responses to the recently reported chromospheric downflowing rapid redshifted excursions (RREs) and their impact on the heating of the solar atmosphere. Methods. We have used two sets of coordinated data from the Swedish 1 m Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory for analyzing the response of the downflowing RREs in the transition region and corona. To provide theoretical support, we use an already existing 2.5D magnetohydrodynamic simulation of spicules performed with the Bifrost code. Results. We find ample occurrences of downflowing RREs and show several examples of their spatio-temporal evolution, sampling multiple wavelength channels ranging from the cooler chromospheric to the hotter coronal channels. These downflowing features are thought to be likely associated with the returning components of the previously heated spicular plasma. Furthermore, the transition region Doppler shifts associated with them are close to the average redshifts observed in this region, which further implies that these flows could (partly) be responsible for the persistent downflows observed in the transition region. We also propose two mechanisms – (i) a typical upflow followed by a downflow and (ii) downflows along a loop –from the perspective of a numerical simulation that could explain the ubiquitous occurrence of such downflows. A detailed comparison between the synthetic and observed spectral characteristics reveals a distinctive match and further suggests an impact on the heating of the solar atmosphere. Conclusions. We present evidence that suggests that at least some of the downflowing RREs are the chromospheric counterparts of the transition region and lower coronal downflows.

• Chintzoglou, Georgios; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Hansteen, Viggo; Rodriguez, Jaime de la Cruz & Szydlarski, Mikolaj Marcin [Show all 10 contributors for this article] (2021). ALMA and IRIS Observations of the Solar Chromosphere. I. An On-Disk Type II Spicule. The Astrophysical Journal (ApJ). ISSN 0004-637X. 906(2). doi: 10.3847/1538-4357/abc9b1. Full text in Research Archive
• Chintzoglou, Georgios; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Hansteen, Viggo; Rodriguez, Jaime de la Cruz & Szydlarski, Mikolaj Marcin [Show all 10 contributors for this article] (2021). ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages. The Astrophysical Journal (ApJ). ISSN 0004-637X. 906(2). doi: 10.3847/1538-4357/abc9b0. Full text in Research Archive
• Woods, Magnus M.; Dalda, Alberto Sainz & Pontieu, Bart De (2021). Unsupervised Machine Learning for the Identification of Preflare Spectroscopic Signatures. The Astrophysical Journal (ApJ). ISSN 0004-637X. 922(2). doi: 10.3847/1538-4357/ac2667. Full text in Research Archive
• Santos, João Manuel da Silva; Rodriguez, Jaime de la Cruz; Leenaarts, Jorrit; Chintzoglou, Georgios; Pontieu, Bart Walter De & Wedemeyer, Sven [Show all 7 contributors for this article] (2020). The multi-thermal chromosphere: Inversions of ALMA and IRIS data. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 634. doi: 10.1051/0004-6361/201937117. Full text in Research Archive Show summary

  Context. Numerical simulations of the solar chromosphere predict a diverse thermal structure with both hot and cool regions. Observations of plage regions in particular typically feature broader and brighter chromospheric lines, which suggests that they are formed in hotter and denser conditions than in the quiet Sun, but also implies a nonthermal component whose source is unclear. Aims. We revisit the problem of the stratification of temperature and microturbulence in plage and the quiet Sun, now adding millimeter (mm) continuum observations provided by the Atacama Large Millimiter Array (ALMA) to inversions of near-ultraviolet Interface Region Imaging Spectrograph (IRIS) spectra as a powerful new diagnostic to disentangle the two parameters. We fit cool chromospheric holes and track the fast evolution of compact mm brightenings in the plage region. Methods. We use the STiC nonlocal thermodynamic equilibrium (NLTE) inversion code to simultaneously fit real ultraviolet and mm spectra in order to infer the thermodynamic parameters of the plasma. Results. We confirm the anticipated constraining potential of ALMA in NLTE inversions of the solar chromosphere. We find significant differences between the inversion results of IRIS data alone compared to the results of a combination with the mm data: the IRIS+ALMA inversions have increased contrast and temperature range, and tend to favor lower values of microturbulence (∼3−6 km s−1 in plage compared to ∼4−7 km s−1 from IRIS alone) in the chromosphere. The average brightness temperature of the plage region at 1.25 mm is 8500 K, but the ALMA maps also show much cooler (∼3000 K) and hotter (∼11 000 K) evolving features partially seen in other diagnostics. To explain the former, the inversions require the existence of localized low-temperature regions in the chromosphere where molecules such as CO could form. The hot features could sustain such high temperatures due to non-equilibrium hydrogen ionization effects in a shocked chromosphere – a scenario that is supported by low-frequency shock wave patterns found in the Mg II lines probed by IRIS.

• Guo, L.-J.; Pontieu, Bart Walter De; Huang, Y.-M.; Peter, Hardi & Bhattacharjee, A. (2020). Observations and Modeling of the Onset of Fast Reconnection in the Solar Transition Region. The Astrophysical Journal (ApJ). ISSN 0004-637X. 901. doi: 10.3847/1538-4357/abb2a7. Full text in Research Archive
• Warren, Harry P.; Reep, Jeffrey W.; Crump, Nicholas A.; Ugarte-Urra, Ignacio; Brooks, David H. & Winebarger, Amy R. [Show all 18 contributors for this article] (2020). Observation and Modeling of High-temperature Solar Active Region Emission during the High-resolution Coronal Imager Flight of 2018 May 29. The Astrophysical Journal (ApJ). ISSN 0004-637X. 896(1). doi: 10.3847/1538-4357/ab917c. Full text in Research Archive
• Bryans, Paul; McIntosh, Scott W.; Brooks, David H. & Pontieu, Bart Walter De (2020). Investigating the Chromospheric Footpoints of the Solar Wind. Astrophysical Journal Letters. ISSN 2041-8205. 905. doi: 10.3847/2041-8213/abce69. Full text in Research Archive
• Martinez-Sykora, Juan; Leenarts, Jorrit; Pontieu, Bart Walter De; Siverio, Daniel Elias Nóbrega; Hansteen, Viggo & Carlsson, Mats [Show all 7 contributors for this article] (2020). Ion–neutral Interactions and Nonequilibrium Ionization in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 889(2). doi: 10.3847/1538-4357/ab643f. Full text in Research Archive
• Martinez-Sykora, Juan; Szydlarski, Mikolaj Marcin; Hansteen, Viggo & Pontieu, Bart Walter De (2020). On the Velocity Drift between Ions in the Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 900(2). doi: 10.3847/1538-4357/ababa3. Full text in Research Archive
• Brooks, David H.; Winebarger, Amy R.; Savage, Sabrina L.; Warren, Harry P.; Pontieu, Bart Walter De & Peter, Hardi [Show all 16 contributors for this article] (2020). The Drivers of Active Region Outflows into the Slow Solar Wind. The Astrophysical Journal (ApJ). ISSN 0004-637X. 894(2). doi: 10.3847/1538-4357/ab8a4c. Full text in Research Archive
• Testa, Paola; Polito, Vanessa & Pontieu, Bart Walter De (2020). IRIS Observations of Short-term Variability in Moss Associated with Transient Hot Coronal Loops. The Astrophysical Journal (ApJ). ISSN 0004-637X. 889(2). doi: 10.3847/1538-4357/ab63cf. Full text in Research Archive
• Polito, Vanessa; Pontieu, Bart Walter De; Testa, Paola; Brooks, David H. & Hansteen, Viggo (2020). IRIS Observations of the Low-atmosphere Counterparts of Active Region Outflows. The Astrophysical Journal (ApJ). ISSN 0004-637X. 903(1). doi: 10.3847/1538-4357/abba1d. Full text in Research Archive
• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Rodriguez, Jaime de la Cruz & Chintzoglou, Georgios (2020). The Formation Height of Millimeter-wavelength Emission in the Solar Chromosphere. Astrophysical Journal Letters. ISSN 2041-8205. 891(1). doi: 10.3847/2041-8213/ab75ac. Full text in Research Archive
• Schmit, Donald; Pontieu, Bart Walter De; Winebarger, Amy R.; Rachmeler, Laurel A. & Daw, A. (2020). Understanding the Structure of Rapid Intensity Fluctuations in the Chromosphere with IRIS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 889(2). doi: 10.3847/1538-4357/ab5f6b. Full text in Research Archive
• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Carlsson, Mats; Rodriguez, Jaime de la Cruz; Bose, Souvik & Chintzoglou, Georgios [Show all 33 contributors for this article] (2020). High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038732. Full text in Research Archive Show summary

  NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe  I lines and spectrally resolved imaging in the chromospheric Ca  II 8542 Å, H α , and Ca  II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes.

• Jafarzadeh, Shahin; Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Pontieu, Bart Walter De; Rezaei, Rezza & Carlsson, Mats (2019). The solar chromosphere at millimetre and ultraviolet wavelengths: I. Radiation temperatures and a detailed comparison. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 622. doi: 10.1051/0004-6361/201834205. Full text in Research Archive Show summary

  Solar observations with the Atacama Large Millimeter/submillimeter Array (ALMA) provide us with direct measurements of the brightness temperature in the solar chromosphere. We study the temperature distributions obtained with ALMA Band 6 (in four sub-bands at 1.21, 1.22, 1.29, and 1.3 mm) for various areas at, and in the vicinity of, a sunspot, comprising quasi-quiet and active regions with different amounts of underlying magnetic fields. We compare these temperatures with those obtained at near- and far-ultraviolet (UV) wavelengths (and with the line-core intensities of the optically-thin far-UV spectra), co-observed with the Interface Region Imaging Spectrograph (IRIS) explorer. These include the emission peaks and cores of the Mg II k 279.6 nm and Mg II h 280.4 nm lines as well as the line cores of C II 133.4 nm, O I 135.6 nm, and Si IV 139.4 nm, sampling the mid-to-high chromosphere and the low transition region. Splitting the ALMA sub-bands resulted in an slight increase of spatial resolution in individual temperature maps, thus, resolving smaller-scale structures compared to those produced with the standard averaging routines. We find that the radiation temperatures have different, though somewhat overlapping, distributions in different wavelengths and in the various magnetic regions. Comparison of the ALMA temperatures with those of the UV diagnostics should, however, be interpreted with great caution, the former is formed under the local thermodynamic equilibrium (LTE) conditions, the latter under non-LTE. The mean radiation temperature of the ALMA Band 6 is similar to that extracted from the IRIS C II line in all areas with exception of the sunspot and pores where the C II poses higher radiation temperatures. In all magnetic regions, the Mg II lines associate with the lowest mean radiation temperatures in our sample. These will provide constraints for future numerical models.

• Kowalski, Adam F.; Butler, Elizabeth; Daw, Adrian N.; Fletcher, Lyndsay; Allred, Joel C. & Pontieu, Bart Walter De [Show all 8 contributors for this article] (2019). Spectral Evidence for Heating at Large Column Mass in Umbral Solar Flare Kernels. I. IRIS Near-UV Spectra of the X1 Solar Flare of 2014 October 25. The Astrophysical Journal (ApJ). ISSN 0004-637X. 878(2). doi: 10.3847/1538-4357/ab1f8b. Full text in Research Archive Show summary

  The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flare observed with the Interface Region Imaging Spectrograph (IRIS) in the near-UV (NUV) and far-UV. One of the flare ribbons crossed a sunspot umbra, producing a dramatic, ~1000% increase in the NUV continuum radiation. We comprehensively analyze the UV spectral data of the umbral flare brightenings, which provide new challenges for radiative−hydrodynamic modeling of the chromospheric velocity field and the white-light continuum radiation. The emission line profiles in the umbral flare brightenings exhibit redshifts and profile asymmetries, but these are significantly smaller than in another, well-studied X-class solar flare. We present a ratio of the NUV continuum intensity to the Fe ii λ2814.45 intensity. This continuum-to-line ratio is a new spectral diagnostic of significant heating at high column mass (log m/[g cm−2] > −2) during solar flares because the continuum and emission line radiation originate from relatively similar temperatures but moderately different optical depths. The full spectral readout of these IRIS data also allow for a comprehensive survey of the flaring NUV landscape: in addition to many lines of Fe ii and Cr ii, we identify a new solar flare emission line, He i λ2829.91 (as previously identified in laboratory and early-type stellar spectra). The Fermi/GBM hard X-ray data provide inputs to radiative−hydrodynamic models (which will be presented in Paper II) in order to better understand the large continuum-to-line ratios, the origin of the white-light continuum radiation, and the role of electron beam heating in the low atmosphere.

• Graham, David R.; Pontieu, Bart Walter De & Testa, Paola (2019). Automated Detection of Rapid Variability of Moss Using SDO/AIA and Its Connection to the Solar Corona. Astrophysical Journal Letters. ISSN 2041-8205. 880(1), p. 1–8. doi: 10.3847/2041-8213/ab2f91. Full text in Research Archive Show summary

  Active region moss—the upper transition region of hot loops—was observed exhibiting rapid intensity variability on timescales of order 15 s by Testa et al. in a short time series (~150 s) data set from Hi-C (High-resolution Coronal Imager). The intensity fluctuations in the subarcsecond 193A images (~1.5 MK plasma) were uncharacteristic of steadily heated moss and were considered an indication of heating events connected to the corona. Intriguingly, these brightenings displayed a connection to the ends of transient hot loops seen in the corona. Following the same active region, AR11520, for 6 days, we demonstrate an algorithm designed to detect the same temporal variability in lower resolution Atmospheric Imaging Assembly (AIA) data, significantly expanding the number of events detected. Multiple analogous regions to the Hi-C data are successfully detected, showing moss that appears to "sparkle" prior to clear brightening of connected high-temperature loops; this is confirmed by the hot AIA channels and the isolated Fe xviii emission. The result is illuminating, as the same behavior has recently been shown by Polito et al. while simulating nanoflares with a beam of electrons depositing their energy in the lower atmosphere. Furthermore, the variability is localized mostly to the hot core of the region, hence we reinforce the diagnostic potential of moss variability as the driver of energy release in the corona. The ubiquitous nature of this phenomenon, and the ability to detect it in data with extended time series, and large fields of view, opens a new window into investigating the coronal heating mechanism.

• Cheung, Mark C. M.; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Testa, Paola; Winebarger, Amy R. & Daw, Adrian N. [Show all 11 contributors for this article] (2019). Multi-component Decomposition of Astronomical Spectra by Compressed Sensing. The Astrophysical Journal (ApJ). ISSN 0004-637X. 882(1). doi: 10.3847/1538-4357/ab263d. Full text in Research Archive Show summary

  The signal measured by an astronomical spectrometer may be due to radiation from a multi-component mixture of plasmas with a range of physical properties (e.g., temperature, Doppler velocity). Confusion between multiple components may be exacerbated if the spectrometer sensor is illuminated by overlapping spectra dispersed from different slits, with each slit being exposed to radiation from a different portion of an extended astrophysical object. We use a compressed sensing method to robustly retrieve the different components. This method can be adopted for a variety of spectrometer configurations, including single-slit, multi-slit (e.g., the proposed MUlti-slit Solar Explorer mission), and slot spectrometers (which produce overlappograms).

• Polito, Vanessa; Testa, Paola & Pontieu, Bart Walter De (2019). Can the Superposition of Evaporative Flows Explain Broad Fe XXI Profiles during Solar Flares? Astrophysical Journal Letters. ISSN 2041-8205. 879(2). doi: 10.3847/2041-8213/ab290b. Full text in Research Archive
• Tiwari, Sanjiv K.; Panesar, Navdeep K.; Moore, Ronald L.; Pontieu, Bart Walter De; Winebarger, Amy R. & Golub, Leon [Show all 17 contributors for this article] (2019). Fine-scale Explosive Energy Release at Sites of Prospective Magnetic Flux Cancellation in the Core of the Solar Active Region Observed by Hi-C 2.1, IRIS, and SDO. The Astrophysical Journal (ApJ). ISSN 0004-637X. 887(1). doi: 10.3847/1538-4357/ab54c1. Full text in Research Archive Show summary

  The second Hi-C flight (Hi-C 2.1) provided unprecedentedly high spatial and temporal resolution (~250 km, 4.4 s) coronal EUV images of Fe ix/x emission at 172 Å of AR 12712 on 2018 May 29, during 18:56:21–19:01:56 UT. Three morphologically different types (I: dot-like; II: loop-like; III: surge/jet-like) of fine-scale sudden-brightening events (tiny microflares) are seen within and at the ends of an arch filament system in the core of the AR. Although type Is (not reported before) resemble IRIS bombs (in size, and brightness with respect to surroundings), our dot-like events are apparently much hotter and shorter in span (70 s). We complement the 5 minute duration Hi-C 2.1 data with SDO/HMI magnetograms, SDO/AIA EUV images, and IRIS UV spectra and slit-jaw images to examine, at the sites of these events, brightenings and flows in the transition region and corona and evolution of magnetic flux in the photosphere. Most, if not all, of the events are seated at sites of opposite-polarity magnetic flux convergence (sometimes driven by adjacent flux emergence), implying likely flux cancellation at the microflare's polarity inversion line. In the IRIS spectra and images, we find confirming evidence of field-aligned outflow from brightenings at the ends of loops of the arch filament system. In types I and II the explosion is confined, while in type III the explosion is ejective and drives jet-like outflow. The light curves from Hi-C, AIA, and IRIS peak nearly simultaneously for many of these events, and none of the events display a systematic cooling sequence as seen in typical coronal flares, suggesting that these tiny brightening events have chromospheric/transition region origin.

• Pontieu, Bart Walter De; Martinez-Sykora, Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian N. & Hansteen, Viggo [Show all 8 contributors for this article] (2019). The Multi-slit Approach to Coronal Spectroscopy with the Multi-slit Solar Explorer (MUSE). The Astrophysical Journal (ApJ). ISSN 0004-637X. 888(1). doi: 10.3847/1538-4357/ab5b03. Full text in Research Archive Show summary

  The Multi-slit Solar Explorer (MUSE) is a proposed mission aimed at understanding the physical mechanisms driving the heating of the solar corona and the eruptions that are at the foundation of space weather. MUSE contains two instruments, a multi-slit extreme ultraviolet (EUV) spectrograph and a context imager. It will simultaneously obtain EUV spectra (along 37 slits) and context images with the highest resolution in space (0farcs33–0farcs4) and time (1–4 s) ever achieved for the transition region (TR) and corona. The MUSE science investigation will exploit major advances in numerical modeling, and observe at the spatial and temporal scales on which competing models make testable and distinguishable predictions, thereby leading to a breakthrough in our understanding of coronal heating and the drivers of space weather. By obtaining spectra in four bright EUV lines (Fe ix 171 Å, Fe xv 284 Å, Fe xix 108Å, Fe xxi 108 Å) covering a wide range of TR and coronal temperatures along 37 slits simultaneously, MUSE will be able to "freeze" the evolution of the dynamic coronal plasma. We describe MUSE's multi-slit approach and show that the optimization of the design minimizes the impact of spectral lines from neighboring slits, generally allowing line parameters to be accurately determined. We also describe a Spectral Disambiguation Code to resolve multi-slit ambiguity in locations where secondary lines are bright. We use simulations of the corona and eruptions to perform validation tests and show that the multi-slit disambiguation approach allows accurate determination of MUSE observables in locations where significant multi-slit contamination occurs.

• Panesar, Navdeep K.; Sterling, Alphonse C.; Moore, Ronald L.; Winebarger, Amy R.; Tiwari, Sanjiv K. & Savage, Sabrina L. [Show all 18 contributors for this article] (2019). Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar Magnetic Network Lanes. Astrophysical Journal Letters. ISSN 2041-8205. 887(1). doi: 10.3847/2041-8213/ab594a. Full text in Research Archive Show summary

  We present high-resolution, high-cadence observations of six, fine-scale, on-disk jet-like events observed by the High-resolution Coronal Imager 2.1 (Hi-C 2.1) during its sounding-rocket flight. We combine the Hi-C 2.1 images with images from the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) and the Interface Region Imaging Spectrograph (IRIS), and investigate each event's magnetic setting with co-aligned line-of-sight magnetograms from the SDO/Helioseismic and Magnetic Imager (HMI). We find that (i) all six events are jetlet-like (having apparent properties of jetlets), (ii) all six are rooted at edges of magnetic network lanes, (iii) four of the jetlet-like events stem from sites of flux cancelation between majority-polarity network flux and merging minority-polarity flux, and (iv) four of the jetlet-like events show brightenings at their bases reminiscent of the base brightenings in coronal jets. The average spire length of the six jetlet-like events (9000 ± 3000 km) is three times shorter than that for IRIS jetlets (27,000 ± 8000 km). While not ruling out other generation mechanisms, the observations suggest that at least four of these events may be miniature versions of both larger-scale coronal jets that are driven by minifilament eruptions and still-larger-scale solar eruptions that are driven by filament eruptions. Therefore, we propose that our Hi-C events are driven by the eruption of a tiny sheared-field flux rope, and that the flux rope field is built and triggered to erupt by flux cancelation.

• Rutten, Robert; Voort, Luc Rouppe van der & Pontieu, Bart Walter De (2019). Solar Hα features with hot onsets: IV. Network fibrils. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 632. doi: 10.1051/0004-6361/201936113. Full text in Research Archive Show summary

  Even in quiet areas underneath coronal holes the solar chromosphere contains ubiquitous heating events. They tend to be small scale and short lived, hence difficult to identify. Here we do not address their much-debated contribution to outer-atmosphere heating, but their aftermaths. We performed a statistical analysis of high-resolution observations in the Balmer Hα line to suggest that many slender dark Hα fibrils spreading out from network represent cooling gas that outlines tracks of preceding rapid type II spicule events or smaller similar but as yet unresolved heating agents in which the main gas constituent, hydrogen, ionizes at least partially. Subsequent recombination then causes dark Hα fibrils enhanced by nonequilibrium overopacity. We suggest that the extraordinary fibrilar appearance of the Hα chromosphere around network results from intermittent, frequent small-scale prior heating.

• Dalda, Alberto Sainz; Rodriguez, Jaime de la Cruz; Pontieu, Bart Walter De & Gošić, Milan (2019). Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach. Astrophysical Journal Letters. ISSN 2041-8205. 875(2). doi: 10.3847/2041-8213/ab15d9. Full text in Research Archive Show summary

  Inversion codes allow the reconstruction of a model atmosphere from observations. With the inclusion of optically thick lines that form in the solar chromosphere, such modeling is computationally very expensive because a non-LTE evaluation of the radiation field is required. In this study, we combine the results provided by these traditional methods with machine and deep learning techniques to obtain similar-quality results in an easy-to-use, much faster way. We have applied these new methods to Mg ii h and k lines observed by the Interface Region Imaging Spectrograph (IRIS). As a result, we are able to reconstruct the thermodynamic state (temperature, line-of-sight velocity, nonthermal velocities, electron density, etc.) in the chromosphere and upper photosphere of an area equivalent to an active region in a few CPU minutes, speeding up the process by a factor of 105 − 106. The open-source code accompanying this Letter will allow the community to use IRIS observations to open a new window to a host of solar phenomena.

• Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.; Testa, Paola & Martinez-Sykora, Juan [Show all 14 contributors for this article] (2019). A comprehensive three-dimensional radiative magnetohydrodynamic simulation of a solar flare. Nature Astronomy. 3, p. 160–166. doi: 10.1038/s41550-018-0629-3. Full text in Research Archive
• Martinez-Sykora, Juan; Hansteen, Viggo; Gudiksen, Boris Vilhelm; Carlsson, Mats; Pontieu, Bart Walter De & Gošić, Milan (2019). On the Origin of the Magnetic Energy in the Quiet Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 878(1). doi: 10.3847/1538-4357/ab1f0b. Full text in Research Archive Show summary

  The presence of magnetic field is crucial in the transport of energy through the solar atmosphere. Recent ground-based and space-borne observations of the quiet Sun have revealed that magnetic field accumulates at photospheric heights, via a local dynamo or from small-scale flux emergence events. However, most of this small-scale magnetic field may not expand into the chromosphere due to the entropy drop with height at the photosphere. Here we present a study that uses a high-resolution 3D radiative MHD simulation of the solar atmosphere with non-gray and non-LTE radiative transfer and thermal conduction along the magnetic field to reveal that (1) the net magnetic flux from the simulated quiet photosphere is not sufficient to maintain a chromospheric magnetic field (on average), (2) processes in the lower chromosphere, in the region dominated by magnetoacoustic shocks, are able to convert kinetic energy into magnetic energy, (3) the magnetic energy in the chromosphere increases linearly in time until the rms of the magnetic field strength saturates at roughly 4–30 G (horizontal average) due to conversion from kinetic energy, (4) and that the magnetic features formed in the chromosphere are localized to this region.

• Carlsson, Mats; Pontieu, Bart Walter De & Hansteen, Viggo (2019). New View of the Solar Chromosphere. Annual Review of Astronomy and Astrophysics. ISSN 0066-4146. 57, p. 189–226. doi: 10.1146/annurev-astro-081817-052044. Full text in Research Archive
• Rachmeler, Laurel A.; Winebarger, Amy R.; Savage, Sabrina L.; Golub, Leon; Kobayashi, Ken & Vigil, Genevieve D. [Show all 52 contributors for this article] (2019). The High-Resolution Coronal Imager, Flight 2.1. Solar Physics. ISSN 0038-0938. 294:174(12), p. 1–22. doi: 10.1007/s11207-019-1551-2. Full text in Research Archive
• Brajša, R.; Sudar, D.; Benz, A. O.; Skokić, I.; Bárta, M. & Pontieu, Bart Walter De [Show all 14 contributors for this article] (2018). First analysis of solar structures in 1.21 mm full-disc ALMA image of the Sun. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 613. doi: 10.1051/0004-6361/201730656. Full text in Research Archive Show summary

  Context: Various solar features can be seen in emission or absorption on maps of the Sun in the millimetre and submillimetre wavelength range. The recently installed Atacama Large Millimetre/submillimetre Array (ALMA) is capable of observing the Sun in that wavelength range with an unprecedented spatial, temporal and spectral resolution. To interpret solar observations with ALMA, the first important step is to compare solar ALMA maps with simultaneous images of the Sun recorded in other spectral ranges. Aims: The first aim of the present work is to identify different structures in the solar atmosphere seen in the optical, infrared, and EUV parts of the spectrum (quiet Sun, active regions, prominences on the disc, magnetic inversion lines, coronal holes and coronal bright points) in a full-disc solar ALMA image. The second aim is to measure the intensities (brightness temperatures) of those structures and to compare them with the corresponding quiet Sun level. Methods: A full-disc solar image at 1.21 mm obtained on December 18, 2015, during a CSV-EOC campaign with ALMA is calibrated and compared with full-disc solar images from the same day in Hα line, in He I 1083 nm line core, and with various SDO images (AIA at 170 nm, 30.4 nm, 21.1 nm, 19.3 nm, and 17.1 nm and HMI magnetogram). The brightness temperatures of various structures are determined by averaging over corresponding regions of interest in the calibrated ALMA image. Results: Positions of the quiet Sun, active regions, prominences on the disc, magnetic inversion lines, coronal holes and coronal bright points are identified in the ALMA image. At the wavelength of 1.21 mm, active regions appear as bright areas (but sunspots are dark), while prominences on the disc and coronal holes are not discernible from the quiet Sun background, despite having slightly less intensity than surrounding quiet Sun regions. Magnetic inversion lines appear as large, elongated dark structures and coronal bright points correspond to ALMA bright points. Conclusions: These observational results are in general agreement with sparse earlier measurements at similar wavelengths. The identification of coronal bright points represents the most important new result. By comparing ALMA and other maps, it was found that the ALMA image was oriented properly and that the procedure of overlaying the ALMA image with other images is accurate at the 5 arcsec level. The potential of ALMA for physics of the solar chromosphere is emphasised.

• Tiwari, Sanjiv K.; Moore, Ronald L.; Pontieu, Bart Walter De; Tarbell, Theodore D.; Panesar, Navdeep K. & Winebarger, Amy R. [Show all 7 contributors for this article] (2018). Evidence of Twisting and Mixed-polarity Solar Photospheric Magnetic Field in Large Penumbral Jets: IRIS and Hinode Observations. The Astrophysical Journal (ApJ). ISSN 0004-637X. 869(2). doi: 10.3847/1538-4357/aaf1b8. Full text in Research Archive Show summary

  A recent study using Hinode (Solar Optical Telescope/Filtergraph [SOT/FG]) data of a sunspot revealed some unusually large penumbral jets that often repeatedly occurred at the same locations in the penumbra, namely, at the tail of a penumbral filament or where the tails of multiple penumbral filaments converged. These locations had obvious photospheric mixed-polarity magnetic flux in Na i 5896 Stokes-V images obtained with SOT/FG. Several other recent investigations have found that extreme-ultraviolet (EUV)/X-ray coronal jets in quiet-Sun regions (QRs), in coronal holes (CHs), and near active regions (ARs) have obvious mixed-polarity fluxes at their base, and that magnetic flux cancellation prepares and triggers a minifilament flux-rope eruption that drives the jet. Typical QR, CH, and AR coronal jets are up to 100 times bigger than large penumbral jets, and in EUV/X-ray images they show a clear twisting motion in their spires. Here, using Interface Region Imaging Spectrograph (IRIS) Mg ii k λ2796 SJ images and spectra in the penumbrae of two sunspots, we characterize large penumbral jets. We find redshift and blueshift next to each other across several large penumbral jets, and we interpret these as untwisting of the magnetic field in the jet spire. Using Hinode/SOT (FG and SP) data, we also find mixed-polarity magnetic flux at the base of these jets. Because large penumbral jets have a mixed-polarity field at their base and have a twisting motion in their spires, they might be driven the same way as QR, CH, and AR coronal jets.

• Panesar, Navdeep K.; Sterling, Alphonse C.; Moore, Ronald L.; Tiwari, Sanjiv K.; Pontieu, Bart Walter De & Norton, Aimee A. (2018). IRIS and SDO Observations of Solar Jetlets Resulting from Network-edge Flux Cancelation. Astrophysical Journal Letters. ISSN 2041-8205. 868(2). doi: 10.3847/2041-8213/aaef37. Full text in Research Archive Show summary

  Recent observations show that the buildup and triggering of minifilament eruptions that drive coronal jets result from magnetic flux cancelation at the neutral line between merging majority- and minority-polarity magnetic flux patches. We investigate the magnetic setting of 10 on-disk small-scale UV/EUV jets (jetlets, smaller than coronal X-ray jets but larger than chromospheric spicules) in a coronal hole by using IRIS UV images and SDO/AIA EUV images and line-of-sight magnetograms from SDO/HMI. We observe recurring jetlets at the edges of magnetic network flux lanes in the coronal hole. From magnetograms coaligned with the IRIS and AIA images, we find, clearly visible in nine cases, that the jetlets stem from sites of flux cancelation proceeding at an average rate of ~1.5 × 1018 Mx hr−1, and show brightenings at their bases reminiscent of the base brightenings in larger-scale coronal jets. We find that jetlets happen at many locations along the edges of network lanes (not limited to the base of plumes) with average lifetimes of 3 minutes and speeds of 70 km s−1. The average jetlet-base width (4000 km) is three to four times smaller than for coronal jets (~18,000 km). Based on these observations of 10 obvious jetlets, and our previous observations of larger-scale coronal jets in quiet regions and coronal holes, we infer that flux cancelation is an essential process in the buildup and triggering of jetlets. Our observations suggest that network jetlet eruptions might be small-scale analogs of both larger-scale coronal jets and the still-larger-scale eruptions producing CMEs.

• Chintzoglou, G.; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Pereira, Tiago Mendes Domingos; Vourlidas, A. & Beltran, Samuel Tun (2018). Bridging the gap: Capturing the Lyα Counterpart of a Type-II Spicule and its Heating Evolution with VAULT2.0 and IRIS Observations. The Astrophysical Journal (ApJ). ISSN 0004-637X. 857(1). doi: 10.3847/1538-4357/aab607. Full text in Research Archive Show summary

  We present results from an observing campaign in support of the VAULT2.0 sounding rocket launch on 2014 September 30. VAULT2.0 is a Lyα (1216 Å) spectroheliograph capable of providing spectroheliograms at high cadence. Lyα observations are highly complementary to the IRIS observations of the upper chromosphere and the low transition region (TR) but have previously been unavailable. The VAULT2.0 data provide new constraints on upper-chromospheric conditions for numerical models. The observing campaign was closely coordinated with the IRIS mission. Taking advantage of this simultaneous multi-wavelength coverage of target AR 12172 and by using state-of-the-art radiative-MHD simulations of spicules, we investigate in detail a type-II spicule associated with a fast (300 km s−1) network jet recorded in the campaign observations. Our analysis suggests that spicular material exists suspended high in the atmosphere but at lower temperatures (seen in Lyα) until it is heated and becomes visible in TR temperatures as a network jet. The heating begins lower in the spicule and propagates upwards as a rapidly propagating thermal front. The front is then observed as fast, plane-of-the-sky motion typical of a network jet, but contained inside the pre-existing spicule. This work supports the idea that the high speeds reported in network jets should not be taken as real mass upflows but only as apparent speeds of a rapidly propagating heating front along the pre-existing spicule.

• Nindos, A.; Alissandrakis, C. E.; Bastian, T. S.; Patsourakos, S.; Pontieu, Bart Walter De & Warren, H. [Show all 12 contributors for this article] (2018). First high-resolution look at the quiet Sun with ALMA at 3mm. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 619. doi: 10.1051/0004-6361/201834113. Full text in Research Archive Show summary

  We present an overview of high-resolution quiet Sun observations, from disk center to the limb, obtained with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm. Seven quiet-Sun regions were observed at a resolution of up to 2.5 by 4.5. We produced both average and snapshot images by self-calibrating the ALMA visibilities and combining the interferometric images with full-disk solar images. The images show well the chromospheric network, which, based on the unique segregation method we used, is brighter than the average over the fields of view of the observed regions by ∼305 K while the intranetwork is less bright by ∼280 K, with a slight decrease of the network/intranetwork contrast toward the limb. At 3 mm the network is very similar to the 1600 Å images, with somewhat larger size. We detect, for the first time, spicular structures, rising up to 15 above the limb with a width down to the image resolution and brightness temperature of ∼1800 K above the local background. No trace of spicules, either in emission or absorption, is found on the disk. Our results highlight the potential of ALMA for the study of the quiet chromosphere.

• Martinez-Sykora, Juan; Moortel, Ineke De; Pontieu, Bart Walter De; Hansteen, Viggo & Carlsson, Mats (2018). Impact of Type II Spicules in the Corona: Simulations and Synthetic Observables. The Astrophysical Journal (ApJ). ISSN 0004-637X. 860(2). doi: 10.3847/1538-4357/aac2ca. Full text in Research Archive Show summary

  The role of type II spicules in the corona has been a much debated topic in recent years. This paper aims to shed light on the impact of type II spicules in the corona using novel 2.5D radiative MHD simulations, including ion–neutral interaction effects with the Bifrost code. We find that the formation of simulated type II spicules, driven by the release of magnetic tension, impacts the corona in various manners. Associated with the formation of spicules, the corona exhibits (1) magneto-acoustic shocks and flows, which supply mass to coronal loops, and (2) transversal magnetic waves and electric currents that propagate at Alfvén speeds. The transversal waves and electric currents, generated by the spicule's driver and lasting for many minutes, are dissipated and heat the associated loop. These complex interactions in the corona can be connected with blueshifted secondary components in coronal spectral lines (red–blue asymmetries) observed with Hinode/EIS and SOHO/SUMER, as well as the EUV counterpart of type II spicules and propagating coronal disturbances observed with the 171 Å and 193 Å SDO/AIA channels.

• Bueno, Javier Trujillo; Stepan, Jiri; Belluzzi, Luca; Ramos, Andrés Asensio; Sainz, Manso Manso & Alemán, T. del Pino [Show all 28 contributors for this article] (2018). CLASP Constraints on the Magnetization and Geometrical Complexity of the Chromosphere-Corona Transition Region. Astrophysical Journal Letters. ISSN 2041-8205. 866:L15. doi: 10.3847/2041-8213/aae25a. Full text in Research Archive Show summary

  The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a suborbital rocket experiment that on 2015 September 3 measured the linear polarization produced by scattering processes in the hydrogen Lyα line of the solar disk radiation. The line-center photons of this spectral line radiation mostly stem from the chromosphere-corona transition region (TR). These unprecedented spectropolarimetric observations revealed an interesting surprise, namely that there is practically no center-to-limb variation (CLV) in the Q/I line-center signals. Using an analytical model, we first show that the geometric complexity of the corrugated surface that delineates the TR has a crucial impact on the CLV of the Q/I and U/I line-center signals. Second, we introduce a statistical description of the solar atmosphere based on a 3D model derived from a state-of-the-art radiation magnetohydrodynamic simulation. Each realization of the statistical ensemble is a 3D model characterized by a given degree of magnetization and corrugation of the TR, and for each such realization we solve the full 3D radiative transfer problem taking into account the impact of the CLASP instrument degradation on the calculated polarization signals. Finally, we apply the statistical inference method presented in a previous paper to show that the TR of the 3D model that produces the best agreement with the CLASP observations has a relatively weak magnetic field and a relatively high degree of corrugation. We emphasize that a suitable way to validate or refute numerical models of the upper solar chromosphere is by confronting calculations and observations of the scattering polarization in ultraviolet lines sensitive to the Hanle effect.

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• Pontieu, Bart Walter De (2025). Status of IRIS and MUSE. Full text in Research Archive
• Pontieu, Bart Walter De (2025). Solar chromospheric dynamics. Full text in Research Archive Show summary

  I will review recent progress in observations and numerical simulations of the dynamics of the solar chromosphere. During the last decade, novel high-resolution space-based and ground-based instrumentation has provided new views of the role of shocks, jets, waves, and magnetic reconnection in the dynamics and energetics of the chromosphere and the layers above. These observations have been accompanied by advances in state-of-the-art numerical modeling of the partially ionized chromosphere, including ion-neutral interactions and multi-fluid effects, providing novel insights into the physics that drive the dynamics of the solar chromosphere. I will review how the synergy between observations and modeling is key to make advances in our understanding of this critical layer in the Sun's atmosphere. I will also discuss the impact of chromospheric dynamics on the transition region and beyond.

• Pontieu, Bart Walter De (2025). Status of the Multi-slit Solar Explorer. Full text in Research Archive Show summary

  In this talk I will provide a status update of the development of the upcoming Multi-slit Solar Explorer (MUSE) mission, which is scheduled for launch no earlier than 2027. MUSE is a NASA MIDEX mission, composed of a multi-slit EUV spectrograph (in three narrow spectral bands centered around 171Å, 284Å, and 108Å) and an EUV context imager (in two narrow passbands around 195Å and 304Å). MUSE will provide spectral and imaging diagnostics of the solar corona at high spatial (~0.5 arcseconds), and temporal resolution (down to ~0.5 seconds) thanks to its innovative multi-slit design. By obtaining spectra in 4 bright EUV lines (Fe IX 171Å, Fe XV 284Å, Fe XIX-XXI 108Å) covering a wide range of transition region and coronal temperatures along 35 slits simultaneously, MUSE will be able to ``freeze" (at a cadence as short as 10 seconds) with a spectroscopic raster the evolution of the dynamic coronal plasma over a wide range of scales: from the spatial scales on which energy is released (<0.5 arcsec) to the large-scale often active-region size (~ 170 arcsec x 170 arcsec) atmospheric response. I will provide an update on the status of MUSE including the critical design review (PDR) in April 2025. I will also describe the multi-slit data approach and the data products that the MUSE team will provide.

• Pontieu, Bart Walter De (2025). Understanding the mysteries of the solar atmosphere. Full text in Research Archive
• Pontieu, Bart Walter De (2025). Understanding the mysteries of the solar atmosphere. Full text in Research Archive
• Pontieu, Bart Walter De (2025). Understanding the mysteries of the solar atmosphere. Full text in Research Archive
• Pontieu, Bart Walter De (2024). MUSE science. Full text in Research Archive
• Pontieu, Bart Walter De (2024). MUSE status at Hinode/IRIS meeting. Full text in Research Archive
• Pontieu, Bart Walter De (2024). MUSE status. Full text in Research Archive
• Pontieu, Bart Walter De (2023). MUSE. Full text in Research Archive
• Pontieu, Bart Walter De (2023). MUSE science. Full text in Research Archive
• Pontieu, Bart Walter De (2023). MUSE science. Full text in Research Archive
• Pontieu, Bart Walter De (2023). The Mulit-Slit Solar Explorer (MUSE). Full text in Research Archive
• Pontieu, Bart Walter De (2023). Future high-resolution observations of the low solar atmosphere from space. Full text in Research Archive

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