Viggo Haraldson Hansteen

• Lee, Jeongwoo; Lim, Eun Kyung & Hansteen, Viggo Haraldson (2026). Spatial and Dynamical Relations between Spicules and Network Bright Points. The Astrophysical Journal (ApJ). ISSN 0004-637X. 997(2). doi: 10.3847/1538-4357/ae2747. Fulltekst i vitenarkiv Vis sammendrag

  Spicules are among the most ubiquitous small-scale, jet-like features in the solar chromosphere and are widely believed to play a significant role in transporting mass and energy into the solar corona, with their mechanisms not fully understood. We utilize high-resolution Hα images acquired from the 1.6 m Goode Solar Telescope at Big Bear Solar Observatory to investigate spatial and dynamical properties of both spicules and network bright points (NBPs), and for the first time, incorporated NBP motions in the analyses of spicules. Our main results are as follows: (1) The speed distributions of blueshifted spicules and NBPs both exhibit distinct peaks, whereas that of redshifted spicules is monotonically decreasing. (2) Torsional motions of spicules inferred from alternating signs of Doppler shifts are faster than the NBPs’ transversal motions by a factor of 10–10^2, which may imply the mass density ratio in two different heights as 10^2–10^4. (3) Blueshifted spicules are found to be more abundant than redshifted spicules in general, but their relative population difference reduces to ∼10% at Doppler speeds above ∼35 km s^−1. (4) Redshifted spicules lying at higher heights share morphological and dynamical similarity with the blueshifted spicules, which implies the same driving mechanism operating in both directions. (5) These two populations appear above NBPs concentrated under the AIA 193 Å bright region. We interpret these results in favor of a scenario that Alfvén waves generated by NBPs' motions impart their energies to spicules in both torsional and field-aligned motions, and also contribute to the coronal heating and possibly the acceleration of the solar wind.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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. [Vis alle 16 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Monga, Aabha; Inoue, Satoshi; Lee, Jeongwoo; Wang, Haimin & Hansteen, Viggo Haraldson (2025). Driving Mechanisms of Solar Active Region Geysers: The Role of Interacting Magnetic Flux. Astrophysical Journal Letters. ISSN 2041-8205. 996(1). doi: 10.3847/2041-8213/ae29e8. Fulltekst i vitenarkiv Vis sammendrag

  Active region (AR) recurrent jets are manifestations of episodic magnetic energy release processes driven by complex interactions in the lower solar atmosphere. While magnetic flux emergence and cancellation are widely recognized as key contributors to jet formation, the mechanisms behind repeated magnetic reconnection remain poorly understood. In this Letter, we report a sequence of nine recurrent jets originating from AR 12715 during its decay phase, where the jet activity was associated with a complex distribution of fragmented magnetic flux. Nonlinear force-free field extrapolations reveal the presence of low-lying, current-carrying loops beneath overarching open magnetic fields near the jet footpoints. These magnetic structures were perturbed by: (1) emerging flux elements; and (2) interactions between oppositely polarized moving magnetic features (MMFs). To interpret these observations, we compare them with a 3D radiative MHD simulation from the Bifrost model, which reproduces jet formation driven by interacting bipolar MMFs, leading to subsequent flux cancellation in the photosphere. Our results emphasize the critical role of MMF-driven flux interactions in initiating and sustaining recurrent jet activity in ARs.

• Moreno-Insertis, Fernando; Hansteen, Viggo Haraldson & Siverio, Daniel Elias Nóbrega (2025). Magnetic Flux Cancellation in the Solar Atmosphere through 3D Realistic Numerical Modeling. The Astrophysical Journal (ApJ). ISSN 0004-637X. 995(2). doi: 10.3847/1538-4357/ae14fb. Fulltekst i vitenarkiv Vis sammendrag

  We present a radiation-magnetohydrodynamics (RMHD) simulation of a magnetic cancellation event. The model is calculated with the Bifrost code and spans from the uppermost convection zone to the corona. The cancellation occurs between the positive polarity of an emerged magnetic bipole and a preexisting negative polarity. We try both to understand the RMHD aspects as well as to carry out comparison to observations, in part via spectral synthesis of optically thick photospheric and chromospheric lines using the RH1.5D code, and optically thin coronal ones. The reconnection between the opposite flux systems takes place at chromospheric heights without null points. Sharp V-shaped upward-moving field lines and highly warped downward-moving post-reconnection loops are created. The chromospheric reconnection is in full swing when the colliding magnetic patches are still separated by a granular cell at the photosphere. In a later phase, photospheric cancellation takes place with submergence of the closed magnetic loops linking the opposite polarities. We carry out comparisons with the observations of the photospheric magnetic flux loss rates, as well as of the horizontal magnetic field and vertical velocity at the polarity inversion line. The reconnection outflows cause intensity brightenings, jets, and different spectral features in the synthesized chromospheric spectral lines, strongly reminiscent of those found in recent observations. Coherent, twisted magnetic flux ropes are created by the flows associated with the process. Including coronal levels is crucial for proper modeling, even if no major ejection or brightening is produced in the corona in this event.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Cozzo, G.; Pagano, P.; Reale, F.; Testa, Paola; Petralia, A. & Martinez-Sykora, Juan [Vis alle 8 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Moe, Thore Espedal; Pereira, Tiago; Voort, Luc Rouppe van der; Carlsson, Mats; Hansteen, Viggo Haraldson & Calvo, Flavio [Vis alle 7 forfattere av denne artikkelen] (2024). Comparative clustering analysis of Ca II 854.2 nm spectral profiles from simulations and observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 682. doi: 10.1051/0004-6361/202347328. Fulltekst i vitenarkiv Vis sammendrag

  Context. Synthetic spectra from 3D models of the solar atmosphere have become increasingly successful at reproducing observations, but there are still some outstanding discrepancies for chromospheric spectral lines, such as Ca II and Mg II , particularly regarding the width of the line cores. It has been demonstrated that using sufficiently high spatial resolution in the simulations significantly diminishes the differences in width between the mean spectra in observations and simulations, but a detailed investigation into how this impacts subgroups of individual profiles is currently lacking. Aims. We compare and contrast the typical shapes of synthetic Ca II 854.2 nm spectra found in Bifrost simulations having different magnetic activity with the spectral shapes found in a quiet-Sun observation from the Swedish 1-m Solar Telescope (SST). Methods. We used clustering techniques to extract the typical Ca II 854.2 nm profile shapes synthesized from Bifrost simulations with varying amounts of magnetic activity. We degraded the synthetic profiles to observational conditions and repeated the clustering, and we compared our synthetic results with actual observations. Subsequently, we examined the atmospheric structures in our models for some select sets of clusters, with the intention of uncovering why they do or do not resemble actual observations. Results. While the mean spectra for our high resolution simulations compare reasonably well with the observations, we find that there are considerable differences between the clusters of observed and synthetic intensity profiles, even after the synthetic profiles have been degraded to match observational conditions. The typical absorption profiles from the simulations are both narrower and display a steeper transition from the inner wings to the line core. Furthermore, even in our most quiescent simulation, we find a far larger fraction of profiles with local emission around the core, or other exotic profile shapes, than in the quiet-Sun observations. Looking into the atmospheric structure for a selected set of synthetic clusters, we find distinct differences in the temperature stratification for the clusters most and least similar to the observations. The narrow and steep profiles are associated with either weak gradients in temperature or temperatures rising to a local maximum in the line wing forming region before sinking to a minimum in the line core forming region. The profiles that display less steep transitions show extended temperature gradients that are steeper in the range−3 ≲ log τ 5000 ≲ −1.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Panesar, Navdeep K.; Hansteen, Viggo Haraldson; Tiwari, Sanjiv K.; Cheung, Mark C. M.; Berghmans, David & Müller, Daniel (2023). Solar Orbiter and SDO Observations, and a Bifrost Magnetohydrodynamic Simulation of Small-scale Coronal Jets. The Astrophysical Journal (ApJ). ISSN 0004-637X. 943(1). doi: 10.3847/1538-4357/aca1c1. Fulltekst i vitenarkiv Vis sammendrag

  Abstract We report high-resolution, high-cadence observations of five small-scale coronal jets in an on-disk quiet Sun region observed with Solar Orbiter’s EUI/HRI EUV in 174 Å. We combine the HRI EUV images with the EUV images of SDO/AIA and investigate the magnetic setting of the jets using coaligned line-of-sight magnetograms from SDO/HMI. The HRI EUV jets are miniature versions of typical coronal jets as they show narrow collimated spires with a base brightening. Three out of five jets result from a detectable minifilament eruption following flux cancelation at the neutral line under the minifilament, analogous to coronal jets. To better understand the physics of jets, we also analyze five small-scale jets from a high-resolution Bifrost MHD simulation in synthetic Fe ix /Fe x emissions. The jets in the simulation reside above neutral lines and four out of five jets are triggered by magnetic flux cancelation. The temperature maps show evidence of cool gas in the same four jets. Our simulation also shows the signatures of opposite Doppler shifts (of the order of ±10 s of km s −1 ) in the jet spire, which is evidence of untwisting motion of the magnetic field in the jet spire. The average jet duration, spire length, base width, and speed in our observations (and in synthetic Fe ix /Fe x images) are 6.5 ± 4.0 min (9.0 ± 4.0 minutes), 6050 ± 2900 km (6500 ± 6500 km), 2200 ± 850 km, (3900 ± 2100 km), and 60 ± 8 km s −1 (42 ± 20 km s −1 ), respectively. Our observation and simulation results provide a unified picture of small-scale solar coronal jets driven by magnetic reconnection accompanying flux cancelation. This picture also aligns well with the most recent reports of the formation and eruption mechanisms of larger coronal jets.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Cheung, Mark C. M.; Martinez-Sykora, Juan; Testa, Paola; Pontieu, Bart Walter De; Chintzoglou, Georgios & Rempel, Matthias [Vis alle 27 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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 [Vis alle 26 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv
• Finley, Adam J.; Brun, Allan Sacha; Carlsson, Mats; Hansteen, Viggo Haraldson & Shoda, M. (2022). Stirring the base of the solar wind: On heat transfer and vortex formation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 665. doi: 10.1051/0004-6361/202243947. Fulltekst i vitenarkiv Vis sammendrag

  Context. Current models of the solar wind must approximate (or ignore) the small-scale dynamics within the solar atmosphere; however, these are likely important in shaping the emerging wave-turbulence spectrum that ultimately heats and accelerates the coronal plasma. Aims. This study strives to make connections between small-scale vortex motions at the base of the solar wind and the resulting heating and acceleration of the coronal plasma. Methods. The Bifrost code produces realistic simulations of the solar atmosphere which facilitate the analysis of spatial and temporal scales which are currently at, or beyond, the limit of modern solar telescopes. For this study, the Bifrost simulation is configured to represent the solar atmosphere in a coronal hole region, from which the fast solar wind emerges. The simulation extends from the upper-convection zone (2.5 Mm below the photosphere) to the low corona (14.5 Mm above the photosphere), with a horizontal extent of 24 Mm × 24 Mm. The network of magnetic funnels in the computational domain influence the movement of plasma, as well as the propagation of magnetohydrodynamic waves into the low corona. Results. The twisting of the coronal magnetic field by photospheric flows efficiently injects energy into the low corona. Poynting fluxes of up to 2 − 4 kWm −2 are commonly observed inside twisted magnetic structures with diameters in the low corona of 1–5 Mm. Torsional Alfvén waves are favourably transmitted along these structures, and subsequently escape into the solar wind. However, reflections of these waves from the upper boundary condition make it difficult to unambiguously quantify the emerging Alfvén wave-energy flux. Conclusions. This study represents a first step in quantifying the conditions at the base of the solar wind using Bifrost simulations. It is shown that the coronal magnetic field is readily braided and twisted by photospheric flows. Temperature and density contrasts form between regions with active stirring motions and those without. Stronger whirlpool-like flows in the convection, concurrent with magnetic concentrations, launch torsional Alfvén waves up through the magnetic funnel network, which are expected to enhance the turbulent generation of magnetic switchbacks in the solar wind.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Noda, Carlos Quintero; Schlichenmaier, R.; Rubio, L.R. Bellot; Löfdahl, M.G.; Khomenko, E. & Jurčák, J. [Vis alle 281 forfattere av denne artikkelen] (2022). The European Solar Telescope. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 666. doi: 10.1051/0004-6361/202243867. Fulltekst i vitenarkiv Vis sammendrag

  The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l’Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Pontieu, Bart Walter De; Polito, Vanessa; Hansteen, Viggo Haraldson; Testa, Paola; Reeves, Katharine K. & Antolin, Patrick [Vis alle 14 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Libbrecht, Tine; Bjørgen, Johan Pires; Leenaarts, Jorrit; Rodríguez, Jaime de la Cruz; Hansteen, Viggo Haraldson & Joshi, Jayant (2021). Line formation of He I D3 and He I 10 830 Å in a small-scale reconnection event. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 652. doi: 10.1051/0004-6361/202039788. Fulltekst i vitenarkiv Vis sammendrag

  Context. Ellerman bombs (EBs) and UV bursts are small-scale reconnection events that occur in the region of the upper photosphere to the chromosphere. It has recently been discovered that these events can have emission signatures in the He  I D 3 and He  I 10 830 Å lines, suggesting that their temperatures are higher than previously expected. Aims. We aim to explain the line formation of He  I D 3 and He  I 10 830 Å in small-scale reconnection events. Methods. We used a simulated EB in a Bifrost-generated radiative magnetohydrodynamics snapshot. The resulting He  I D 3 and He  I 10 830 Å line intensities were synthesized in 3D using the non-local thermal equilibrium (non-LTE) Multi3D code. The presence of coronal extreme UV (EUV) radiation was included self-consistently. We compared the synthetic helium spectra with observed raster scans of EBs in He  I 10 830 Å and He  I D 3 obtained at the Swedish Solar Telescope with the TRI-Port Polarimetric Echelle-Littrow Spectrograph. Results. Emission in He  I D 3 and He  I 10 830 Å is formed in a thin shell around the EB at a height of ∼0.8 Mm, while the He  I D 3 absorption is formed above the EB at ∼4 Mm. The height at which the emission is formed corresponds to the lower boundary of the EB, where the temperature increases rapidly from 6 × 10 3 K to 10 6 K. The synthetic line profiles at a heliocentric angle of μ = 0.27 are qualitatively similar to the observed profiles at the same μ-angle in dynamics, broadening, and line shape: emission in the wing and absorption in the line core. The opacity in He  I D 3 and He  I 10 830 Å is generated through photoionization-recombination driven by EUV radiation that is locally generated in the EB at temperatures in the range of 2 × 10 4  − 2 × 10 6 K and electron densities between 10 11 and 10 13 cm −3 . The synthetic emission signals are a result of coupling to local conditions in a thin shell around the EB, with temperatures between 7 × 10 3 and 10 4 K and electron densities ranging from ∼10 12 to 10 13 cm −3 . This shows that both strong non-LTE and thermal processes play a role in the formation of He  I D 3 and He  I 10 830 Å in the synthetic EB/UV burst that we studied. Conclusions. In conclusion, the synthetic He  I D 3 and He  I 10 830 Å emission signatures are an indicator of temperatures of at least 2 × 10 4 K; in this case, as high as ∼10 6 K.

• Parenti, Susanna; Chifu, Iulia; Zanna, Giulio Del; Edmondson, Justin; Giunta, Alessandra & Hansteen, Viggo H. [Vis alle 16 forfattere av denne artikkelen] (2021). Linking the Sun to the Heliosphere Using Composition Data and Modelling: A Test Case with a Coronal Jet. Space Science Reviews. ISSN 0038-6308. 217(8). doi: 10.1007/s11214-021-00856-1. Fulltekst i vitenarkiv Vis sammendrag

  Our understanding of the formation and evolution of the corona and the heliosphere is linked to our capability of properly interpret the data from remote sensing and in-situ observations. In this respect, being able to correctly connect in-situ observations with their source regions on the Sun is the key for solving this problem. In this work we aim at testing a diagnostics method for this connectivity. This paper makes use of a coronal jet observed on 2010 August 2nd in active region 11092 as a test for our connectivity method. This combines solar EUV and in-situ data together with magnetic field extrapolation, large scale MHD modeling and FIP (First Ionization Potential) bias modeling to provide a global picture from the source region of the jet to its possible signatures at 1 AU. Our data analysis reveals the presence of outflow areas near the jet which are within open magnetic flux regions and which present FIP bias consistent with the FIP model results. In our picture, one of these open areas is the candidate jet source. Using a back-mapping technique we identified the arrival time of this solar plasma at the ACE spacecraft. The in-situ data show signatures of changes in the plasma and magnetic field parameters, with FIP bias consistent with the possible passage of the jet material. Our results highlight the importance of remote sensing and in-situ coordinated observations as a key to solve the connectivity problem. We discuss our results in view of the recent Solar Orbiter launch which is currently providing such unique data.

• Chintzoglou, Georgios; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Hansteen, Viggo; Rodriguez, Jaime de la Cruz & Szydlarski, Mikolaj Marcin [Vis alle 10 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv
• Chintzoglou, Georgios; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Hansteen, Viggo; Rodriguez, Jaime de la Cruz & Szydlarski, Mikolaj Marcin [Vis alle 10 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv
• Ortiz, Ada; Hansteen, Viggo; Siverio, Daniel Elias Nóbrega & Voort, Luc Rouppe van der (2020). Ellerman bombs and UV bursts: reconnection at different atmospheric layers. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 633(A58). doi: 10.1051/0004-6361/201936574. Fulltekst i vitenarkiv Vis sammendrag

  The emergence of magnetic flux through the photosphere and into the outer solar atmosphere produces, amongst other dynamical phenomena, Ellerman bombs (EBs), which are observed in the wings of Hα and are due to magnetic reconnection in the photosphere below the chromospheric canopy. Signs of magnetic reconnection are also observed in other spectral lines, typical of the chromosphere or the transition region. An example are the ultraviolet (UV) bursts observed in the transition region lines of Si IV and the upper chromospheric lines of Mg II. In this work we analyze high-cadence, high-resolution coordinated observations between the Swedish 1m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft. Hα images from the SST provide us with the positions, timings, and trajectories of EBs in an emerging flux region. Simultaneous, co-aligned IRIS slit-jaw images at 133 (C II, transition region), 140 (Si IV, transition region), and 279.6 (Mg II k, core, upper chromosphere) nm as well as spectroscopy in the far- and near-ultraviolet from the fast spectrograph raster allow us to study the possible chromospheric and transition region counterparts of those EBs. Our main goal is to study the possible temporal and spatial relationship between several reconnection events at different layers in the atmosphere (namely EBs and UV bursts), the timing history between them, and the connection of these dynamical phenomena to the ejection of surges in the chromosphere. We also investigate the properties of an extended UV burst and their variations across the burst domain. Our results suggest a scenario where simultaneous and co-spatial EBs and UV bursts are part of the same reconnection system occurring sequentially along a vertical or nearly vertical current sheet. Heating and bidirectional jets trace the location where reconnection takes place. These results support and expand those obtained from recent numerical simulations of magnetic flux emergence.

• Martinez-Sykora, Juan; Leenarts, Jorrit; Pontieu, Bart Walter De; Siverio, Daniel Elias Nóbrega; Hansteen, Viggo & Carlsson, Mats [Vis alle 7 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv
• Santos, João Manuel da Silva; Rodriguez, Jaime de la Cruz; White, S. M.; Leenaarts, Jorrit; Vissers, Gregal Joan Maria & Hansteen, Viggo (2020). ALMA observations of transient heating in a solar active region. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 643. doi: 10.1051/0004-6361/202038755. Fulltekst i vitenarkiv Vis sammendrag

  Aims. We aim to investigate the temperature enhancements and formation heights of solar active-region brightenings such as Ellerman bombs (EBs), ultraviolet bursts (UVBs), and flaring active-region fibrils (FAFs) using interferometric observations in the millimeter (mm) continuum provided by the Atacama Large Millimeter/submillimeter Array (ALMA). Methods. We examined 3 mm signatures of heating events identified in Solar Dynamics Observatory observations of an active region and compared the results with synthetic spectra from a 3D radiative magnetohydrodynamic simulation. We estimated the contribution from the corona to the mm brightness using differential emission measure analysis. Results. We report the null detection of EBs in the 3 mm continuum at ∼1.2 spatial resolution, which is evidence that they are sub-canopy events that do not significantly contribute to heating the upper chromosphere. In contrast, we find the active region to be populated with multiple compact, bright, flickering mm-bursts – reminiscent of UVBs. The high brightness temperatures of up to ∼14 200 K in some events have a contribution (up to ∼7%) from the corona. We also detect FAF-like events in the 3 mm continuum. These events show rapid motions of &gt; 10 kK plasma launched with high plane-of-sky velocities (37 − 340 km s −1 ) from bright kernels. The mm FAFs are the brightest class of warm canopy fibrils that connect magnetic regions of opposite polarities. The simulation confirms that ALMA should be able to detect the mm counterparts of UVBs and small flares and thus provide a complementary diagnostic for localized heating in the solar chromosphere.

• 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. Fulltekst i vitenarkiv
• Kohutova, Petra; Antolin, Patrick; Popovas, Andrius; Szydlarski, Mikolaj Marcin & Hansteen, Viggo (2020). Self-consistent 3D radiative magnetohydrodynamic simulations of coronal rain formation and evolution. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 639. doi: 10.1051/0004-6361/202037899. Fulltekst i vitenarkiv Vis sammendrag

  Context. Coronal rain consists of cool and dense plasma condensations formed in coronal loops as a result of thermal instability. Aims. Previous numerical simulations of thermal instability and coronal rain formation have relied on the practice of artificially adding a coronal heating term to the energy equation. To reproduce large-scale characteristics of the corona, the use of more realistic coronal heating prescription is necessary. Methods. We analysed coronal rain formation and evolution in a three-dimensional radiative magnetohydrodynamic simulation spanning from convection zone to corona which is self-consistently heated by magnetic field braiding as a result of convective motions. Results. We investigate the spatial and temporal evolution of energy dissipation along coronal loops which become thermally unstable. Ohmic dissipation in the model leads to the heating events capable of inducing sufficient chromospheric evaporation into the loop to trigger thermal instability and condensation formation. The cooling of the thermally unstable plasma occurs on timescales that are comparable to the duration of the individual impulsive heating events. The impulsive heating has sufficient duration to trigger thermal instability in the loop but does not last long enough to lead to coronal rain limit cycles. We show that condensations can either survive and fall into the chromosphere or be destroyed by strong bursts of Joule heating associated with a magnetic reconnection events. In addition, we find that condensations can also form along open magnetic field lines. Conclusions. We modelled, for the first time, coronal rain formation in a self-consistent 3D radiative magnetohydrodynamic simulation, in which the heating occurs mainly through the braiding and subsequent Ohmic dissipation of the magnetic field. The heating is stratified enough and lasts for long enough along specific field lines to produce the necessary chromospheric evaporation that triggers thermal instability in the corona.

• 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. Fulltekst i vitenarkiv
• Anderson, Mark; Appourchaux, T.; Auchère, Frédéric; Cuadrado, R. Aznar; Barbay, J. & Baudin, F. [Vis alle 89 forfattere av denne artikkelen] (2020). The Solar Orbiter SPICE instrument: An extreme UV imaging spectrometer. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 642. doi: 10.1051/0004-6361/201935574. Fulltekst i vitenarkiv Vis sammendrag

  Aims. The Spectral Imaging of the Coronal Environment (SPICE) instrument is a high-resolution imaging spectrometer operating at extreme ultraviolet wavelengths. In this paper, we present the concept, design, and pre-launch performance of this facility instrument on the ESA/NASA Solar Orbiter mission. Methods. The goal of this paper is to give prospective users a better understanding of the possible types of observations, the data acquisition, and the sources that contribute to the instrument’s signal. Results. The paper discusses the science objectives, with a focus on the SPICE-specific aspects, before presenting the instrument’s design, including optical, mechanical, thermal, and electronics aspects. This is followed by a characterisation and calibration of the instrument’s performance. The paper concludes with descriptions of the operations concept and data processing. Conclusions. The performance measurements of the various instrument parameters meet the requirements derived from the mission’s science objectives. The SPICE instrument is ready to perform measurements that will provide vital contributions to the scientific success of the Solar Orbiter mission.

• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Carlsson, Mats; Rodriguez, Jaime de la Cruz; Bose, Souvik & Chintzoglou, Georgios [Vis alle 33 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Hilde, Janne Mykland; Hisdal, Jonny; Skjørten, Ingunn; Hansteen, Viggo; Melsom, Morten Nissen & Grøtta, Ole Jørgen [Vis alle 11 forfattere av denne artikkelen] (2020). Left ventricular dysfunction in COPD without pulmonary hypertension. PLOS ONE. 15(7), s. 1–15. doi: 10.1371/journal.pone.0235075. Fulltekst i vitenarkiv Vis sammendrag

  Objectives: We aimed to assess prevalence of left ventricular (LV) systolic and diastolic function in stable cohort of COPD patients, where LV disease had been thoroughly excluded in advance. Methods: 100 COPD outpatients in GOLD II-IV and 34 controls were included. Patients were divided by invasive mean pulmonary artery pressure (mPAP) in COPD-PH (≥25 mmHg) and COPD-non-PH (<25 mmHg), which was subdivided in mPAP ≤20 mmHg and 21–24 mmHg. LV myocardial performance index (LV MPI) and strain by tissue Doppler imaging (TDI) were used for evaluation of LV global and systolic function, respectively. LV MPI ≥0.51 and strain ≤-15.8% were considered abnormal. LV diastolic function was assessed by the ratio between peak early (E) and late (A) velocity, early TDI E´, E/E´, isovolumic relaxation time, and left atrium volume. Results: LV MPI ≥0.51 was found in 64.9% and 88.5% and LV strain ≤-15.8% in 62.2.% and 76.9% in the COPD-non-PH and COPD-PH patients, respectively. Similarly, LV MPI and LV strain were impaired even in patients with mPAP <20 mmHg. In multiple regression analyses, residual volume and stroke volume were best associated to LV MPI and LV strain, respectively. Except for isovolumic relaxation time, standard diastolic echo indices as E/A, E´, E/E´ and left atrium volume did not change from normal individuals to COPD-non-PH. Conclusions: Subclinical LV systolic dysfunction was a frequent finding in this cohort of COPD patients, even in those with normal pulmonary artery pressure. Evidence of LV diastolic dysfunction was hardly present as measured by conventional echo indices.

• Cheung, Mark C. M.; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Testa, Paola; Winebarger, Amy R. & Daw, Adrian N. [Vis alle 11 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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).

• Pontieu, Bart Walter De; Martinez-Sykora, Juan; Testa, Paola; Winebarger, Amy R.; Daw, Adrian N. & Hansteen, Viggo [Vis alle 8 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.; Testa, Paola & Martinez-Sykora, Juan [Vis alle 14 forfattere av denne artikkelen] (2019). A comprehensive three-dimensional radiative magnetohydrodynamic simulation of a solar flare. Nature Astronomy. 3, s. 160–166. doi: 10.1038/s41550-018-0629-3. Fulltekst i vitenarkiv
• 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. Fulltekst i vitenarkiv Vis sammendrag

  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, s. 189–226. doi: 10.1146/annurev-astro-081817-052044. Fulltekst i vitenarkiv
• Hansteen, Viggo; Ortiz, Ada; Archontis, Vasilis; Carlsson, Mats; Pereira, Tiago Mendes Domingos & Bjørgen, Johan Pires (2019). Ellerman bombs and UV bursts: transient events in chromospheric current sheets. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 626. doi: 10.1051/0004-6361/201935376. Fulltekst i vitenarkiv Vis sammendrag

  Context: Ellerman bombs (EBs), observed in the photospheric wings of the Hα line, and UV bursts, observed in the transition region Si IV line, are both brightenings related to flux emergence regions and specifically to magnetic flux of opposite polarity that meet in the photosphere. These two reconnection-related phenomena, nominally formed far apart, occasionally occur in the same location and at the same time, thus challenging our understanding of reconnection and heating of the lower solar atmosphere. Aims: We consider the formation of an active region, including long fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5 Mm below the photosphere, is studied as it pierces the photosphere and interacts with the preexisting ambient field. Specifically, we aim to study whether EBs and UV bursts are generated as a result of such flux emergence and examine their physical relationship. Methods: The Bifrost radiative magnetohydrodynamics code was used to model flux emerging into a model atmosphere that contained a fairly strong ambient field, constraining the emerging field to a limited volume wherein multiple reconnection events occur as the field breaks through the photosphere and expands into the outer atmosphere. Synthetic spectra of the different reconnection events were computed using the 1.5D RH code and the fully 3D MULTI3D code. Results: The formation of UV bursts and EBs at intensities and with line profiles that are highly reminiscent of observed spectra are understood to be a result of the reconnection of emerging flux with itself in a long-lasting current sheet that extends over several scale heights through the chromosphere. Synthetic spectra in the Hα and Si IV 139.376 nm lines both show characteristics that are typical of the observations. These synthetic diagnostics suggest that there are no compelling reasons to assume that UV bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally formed at opposite ends of a long current sheet that resides in an extended bubble of cool gas.

• Bjørgen, Johan Pires; Sukhorukov, Andrii V.; Leenaarts, Jorrit; Carlsson, Mats; Rodriguez, Jaime de la Cruz & Scharmer, Göran B. [Vis alle 7 forfattere av denne artikkelen] (2018). Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 611:A62. doi: 10.1051/0004-6361/201731926. Fulltekst i vitenarkiv Vis sammendrag

  Context: CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar Telescope (SST), can observe the chromosphere in the H and K lines of Ca II at high spatial and spectral resolution. Accurate modeling as well as an understanding of the formation of these lines are needed to interpret the SST/CHROMIS observations. Such modeling is computationally challenging because these lines are influenced by strong departures from local thermodynamic equilibrium, three-dimensional radiative transfer, and partially coherent resonance scattering of photons. Aim: We aim to model the Ca II H and K lines in 3D model atmospheres to understand their formation and to investigate their diagnostic potential for probing the chromosphere. Methods: We model the synthetic spectrum of Ca II using the radiative transfer code Multi3D in three different radiation-magnetohydrodynamic model atmospheres computed with the Bifrost code. We classify synthetic intensity profiles according to their shapes and study how their features are related to the physical properties in the model atmospheres. We investigate whether the synthetic data reproduce the observed spatially-averaged line shapes, center-to-limb variation and compare this data with SST/CHROMIS images. Results: The spatially-averaged synthetic line profiles show too low central emission peaks, and too small separation between the peaks. The trends of the observed center-to-limb variation of the profiles properties are reproduced by the models. The Ca II H and K line profiles provide a temperature diagnostic of the temperature minimum and the temperature at the formation height of the emission peaks. The Doppler shift of the central depression is an excellent probe of the velocity in the upper chromosphere.

• Skjørten, Ingunn; Hilde, Janne Mykland; Melsom, Morten Nissen; Hisdal, Jonny; Hansteen, Viggo & Steine, Kjetil [Vis alle 7 forfattere av denne artikkelen] (2018). Exercise capacity in COPD patients with exercise-induced pulmonary hypertension. The International Journal of Chronic Obstructive Pulmonary Disease. ISSN 1176-9106. 13, s. 3599–3610. doi: 10.2147/COPD.S161175. Fulltekst i vitenarkiv Vis sammendrag

  Background: Pulmonary hypertension (PH) in patients with COPD is associated with reduced exercise capacity. A subgroup of COPD patients has normal mean pulmonary artery pressure (mPAP) at rest, but develops high mPAP relative to cardiac output (CO) during exercise, a condition we refer to as exercise-induced pulmonary hypertension (EIPH). We hypothesized that COPD patients with EIPH could be identified by cardiopulmonary exercise test (CPET) and that these patients have lower exercise capacity and more abnormal CPET parameters compared to COPD patients with normal hemodynamic exercise response. Methods: Ninety-three stable outpatients with COPD underwent right heart catheterization with the measurement of mPAP, CO, and capillary wedge pressure at rest and during supine exercise. Resting mPAP <25 mmHg with ΔmPAP/ΔCO slope above or below 3 mmHg/L/min were defined as COPD-EIPH and COPD-normal, respectively. Pulmonary function tests and CPET with arterial blood gases were performed. Linear mixed models were fitted to estimate differences between the groups with adjustment for gender, age, and airflow obstruction. Results: EIPH was observed in 45% of the study population. Maximal workload was lower in COPD-EIPH compared to COPD-normal, whereas other CPET measurements at peak exercise in % predicted values were similar between the two groups. After adjustment for gender, age, and airflow obstruction, patients with COPD-EIPH showed significantly greater increase in oxygen uptake, ventilation, respiratory frequency, heart rate, and lactate with increasing work load, as well as more reduction in pH compared to those with normal hemodynamic responses. Conclusion: COPD-EIPH could not be discriminated from COPD-normal by CPET. However, COPD-EIPH experienced a higher cost of exercise in terms of higher oxygen uptake, ventilation, respiratory frequency, heart rate, and lactate for a given increase in workload compared to COPD-normal.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Pereira, Tiago Mendes Domingos; Voort, Luc Rouppe van der; Hansteen, Viggo & Pontieu, Bart Walter De (2018). Chromospheric counterparts of solar transition region unresolved fine structure loops. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 611. doi: 10.1051/0004-6361/201832762. Fulltekst i vitenarkiv Vis sammendrag

  Low-lying loops have been discovered at the solar limb in transition region temperatures by the Interface Region Imaging Spectrograph (IRIS). They do not appear to reach coronal temperatures, and it has been suggested that they are the long-predicted unresolved fine structures (UFS). These loops are dynamic and believed to be visible during both heating and cooling phases. Making use of coordinated observations between IRIS and the Swedish 1-m Solar Telescope, we study how these loops impact the solar chromosphere. We show for the first time that there is indeed a chromospheric signal of these loops, seen mostly in the form of strong Doppler shifts and a conspicuous lack of chromospheric heating. In addition, we find that several instances have a inverse Y-shaped jet just above the loop, suggesting that magnetic reconnection is driving these events. Our observations add several puzzling details to the current knowledge of these newly discovered structures; this new information must be considered in theoretical models. © 2018 ESO

• Moreno-Insertis, F.; Hansteen, Viggo; Martinez-Sykora, Juan & Munoz, D. (2018). Small-scale Magnetic Flux Emergence in the Quiet Sun. Astrophysical Journal Letters. ISSN 2041-8205. 859(2). doi: 10.3847/2041-8213/aac648. Fulltekst i vitenarkiv Vis sammendrag

  Small bipolar magnetic features are observed to appear in the interior of individual granules in the quiet Sun, signaling the emergence of tiny magnetic loops from the solar interior. We study the origin of those features as part of the magnetoconvection process in the top layers of the convection zone. Two quiet-Sun magnetoconvection models, calculated with the radiation-magnetohydrodynamic (MHD) Bifrost code and with domain stretching from the top layers of the convection zone to the corona, are analyzed. Using 3D visualization as well as a posteriori spectral synthesis of Stokes parameters, we detect the repeated emergence of small magnetic elements in the interior of granules, as in the observations. Additionally, we identify the formation of organized horizontal magnetic sheets covering whole granules. Our approach is twofold, calculating statistical properties of the system, like joint probability density functions (JPDFs), and pursuing individual events via visualization tools. We conclude that the small magnetic loops surfacing within individual granules in the observations may originate from sites at or near the downflows in the granular and mesogranular levels, probably in the first 1 or 1.5 Mm below the surface. We also document the creation of granule-covering magnetic sheet-like structures through the sideways expansion of a small subphotospheric magnetic concentration picked up and pulled out of the interior by a nascent granule. The sheet-like structures that we found in the models may match the recent observations of Centeno et al.

• Zacharias, Pia; Hansteen, Viggo; Leenaarts, Jorrit; Carlsson, Mats & Gudiksen, Boris Vilhelm (2018). Disentangling flows in the solar transition region. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 614. doi: 10.1051/0004-6361/201732055. Fulltekst i vitenarkiv Vis sammendrag

  Context: The measured average velocities in solar and stellar spectral lines formed at transition region temperatures have been difficult to interpret. The dominant redshifts observed in the lower transition region naturally leads to the question of how the upper layers of the solar (and stellar) atmosphere can be maintained. Likewise, no ready explanation has been made for the average blueshifts often found in upper transition region lines. However, realistic three-dimensional radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere are able to reproduce the observed dominant line shifts and may thus hold the key to resolve these issues. Aims. These new 3D rMHD simulations aim to shed light on how mass flows between the chromosphere and corona and on how the coronal mass is maintained. These simulations give new insights into the coupling of various atmospheric layers and the origin of Doppler shifts in the solar transition region and corona. Methods: The passive tracer particles, so-called corks, allow the tracking of parcels of plasma over time and thus the study of changes in plasma temperature and velocity not only locally, but also in a co-moving frame. By following the trajectories of the corks, we can investigate mass and energy flows and understand the composition of the observed velocities. Results: Our findings show that most of the transition region mass is cooling. The preponderance of transition region redshifts in the model can be explained by the higher percentage of downflowing mass in the lower and middle transition region. The average upflows in the upper transition region can be explained by a combination of both stronger upflows than downflows and a higher percentage of upflowing mass. The most common combination at lower and middle transition region temperatures are corks that are cooling and traveling downward. For these corks, a strong correlation between the pressure gradient along the magnetic field line and the velocity along the magnetic field line has been observed, indicating a formation mechanism that is related to downward propagating pressure disturbances. Corks at upper transition region temperatures are subject to a rather slow and highly variable but continuous heating process. Conclusions: Corks are shown to be an essential tool in 3D rMHD models in order to study mass and energy flows. We have shown that most transition region plasma is cooling after having been heated slowly to upper transition region temperatures several minutes before. Downward propagating pressure disturbances are identified as one of the main mechanisms responsible for the observed redshifts at transition region temperatures. © 2018 ESO

• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Scharmer, G. B.; Rodriguez, Jaime de la Cruz; Martinez-Sykora, Juan & Nobrega-Siverio, Daniel [Vis alle 12 forfattere av denne artikkelen] (2017). Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere. Astrophysical Journal Letters. ISSN 2041-8205. 851(1). doi: 10.3847/2041-8213/aa99dd. Fulltekst i vitenarkiv Vis sammendrag

  Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry–Pérot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in so-called UV bursts in the low solar atmosphere. The Si iv 1403 Å UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca ii K 3934 Å line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0farcs2 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere. © 2017. The American Astronomical Society. All rights reserved.

• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Carlsson, Mats; Hansteen, Viggo; Nobrega-Siverio, Daniel & Gudiksen, Boris Vilhelm (2017). Two-dimensional Radiative Magnetohydrodynamic Simulations of Partial Ionization in the Chromosphere. II. Dynamics and Energetics of the Low Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 847(1). doi: 10.3847/1538-4357/aa8866. Fulltekst i vitenarkiv Vis sammendrag

  We investigate the effects of interactions between ions and neutrals on the chromosphere and overlying corona using 2.5D radiative MHD simulations with the Bifrost code. We have extended the code capabilities implementing ion–neutral interaction effects using the generalized Ohm's law, i.e., we include the Hall term and the ambipolar diffusion (Pedersen dissipation) in the induction equation. Our models span from the upper convection zone to the corona, with the photosphere, chromosphere, and transition region partially ionized. Our simulations reveal that the interactions between ionized particles and neutral particles have important consequences for the magnetothermodynamics of these modeled layers: (1) ambipolar diffusion increases the temperature in the chromosphere; (2) sporadically the horizontal magnetic field in the photosphere is diffused into the chromosphere, due to the large ambipolar diffusion; (3) ambipolar diffusion concentrates electrical currents, leading to more violent jets and reconnection processes, resulting in (3a) the formation of longer and faster spicules, (3b) heating of plasma during the spicule evolution, and (3c) decoupling of the plasma and magnetic field in spicules. Our results indicate that ambipolar diffusion is a critical ingredient for understanding the magnetothermodynamic properties in the chromosphere and transition region. The numerical simulations have been made publicly available, similar to previous Bifrost simulations. This will allow the community to study realistic numerical simulations with a wider range of magnetic field configurations and physics modules than previously possible. © 2017. The American Astronomical Society. All rights reserved.

• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Hansteen, Viggo; Voort, Luc Rouppe van der; Carlsson, Mats & Pereira, Tiago Mendes Domingos (2017). On the generation of solar spicules and Alfvenic waves. Science. ISSN 0036-8075. 356(6344), s. 1269–1272. doi: 10.1126/science.aah5412. Fulltekst i vitenarkiv Vis sammendrag

  In the lower solar atmosphere, the chromosphere is permeated by jets known as spicules, in which plasma is propelled at speeds of 50 to 150 kilometers per second into the corona. The origin of the spicules is poorly understood, although they are expected to play a role in heating the million-degree corona and are associated with Alfvénic waves that help drive the solar wind. We compare magnetohydrodynamic simulations of spicules with observations from the Interface Region Imaging Spectrograph and the Swedish 1-m Solar Telescope. Spicules are shown to occur when magnetic tension is amplified and transported upward through interactions between ions and neutrals or ambipolar diffusion. The tension is impulsively released to drive flows, heat plasma (through ambipolar diffusion), and generate Alfvénic waves. This is the author’s version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science on 23 Jun 2017, DOI: 10.1126/science.aah5412

• Guerreiro, Nuno Miguel Rodrigues; Haberreiter, M.; Hansteen, Viggo & Schmutz, W. (2017). Small-scale heating events in the solar atmosphere: II. Lifetime, total energy, and magnetic properties. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 603. doi: 10.1051/0004-6361/201629795. Fulltekst i vitenarkiv Vis sammendrag

  Context. Small-scale heating events (SSHEs) are believed to play a fundamental role in understanding the process responsible for heating of the solar corona, the pervading redshifts in the transition region, and the acceleration of spicules. Aims. We determine the properties of the SSHEs and the atmospheric response to them in 3D magnetohydrodynamics (3D-MHD) simulations of the solar atmosphere. Methods. We developed a method for identifying and following SSHEs over their lifetime, and applied it to two simulation models. We identified the locations where the energy dissipation is greatest inside the SSHEs volume, and we traced the SSHEs by following the spatial and temporal evolution of the maximum energy dissipation inside the SSHEs volume. Results. The method is effective in following the SSHEs. We can determine their lifetime, total energy, and properties of the plasma, as well as the magnetic field orientation in the vicinity of the SSHEs. Conclusions. We determine that the SSHEs that have the potential to heat the corona live less than 4 min, and typically the energy they release ranges from 1020 to 1024 erg. In addition, the directional change of the field lines on the two sides of the current sheet constituting the SSHEs ranges from 5° to 15° at the moment of the absolute maximum energy dissipation. ESO, 2017

• Hansteen, Viggo; Archontis, Vasilis; Pereira, Tiago Mendes Domingos; Carlsson, Mats; Voort, Luc Rouppe van der & Leenaarts, Jorrit (2017). Bombs and Flares at the Surface and Lower Atmosphere of the Sun. The Astrophysical Journal (ApJ). ISSN 0004-637X. 839(1). doi: 10.3847/1538-4357/aa6844. Fulltekst i vitenarkiv Vis sammendrag

  A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Hα line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma "bombs" (UV bursts) with high temperatures of perhaps up to 8 × 104 K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (106 K) in the upper chromosphere. These results provide new insights into the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere. © 2017. The American Astronomical Society. All rights reserved.

• Chatterjee, Piyali; Hansteen, Viggo & Carlsson, Mats (2016). Modeling Repeatedly Flaring δ Sunspots. Physical Review Letters. ISSN 0031-9007. 116(10). doi: 10.1103/PhysRevLett.116.101101. Fulltekst i vitenarkiv Vis sammendrag

  Active regions (ARs) appearing on the surface of the Sun are classified into α, β, γ, and δ by the rules of the Mount Wilson Observatory, California on the basis of their topological complexity. Amongst these, the δ sunspots are known to be superactive and produce the most x-ray flares. Here, we present results from a simulation of the Sun by mimicking the upper layers and the corona, but starting at a more primitive stage than any earlier treatment. We find that this initial state consisting of only a thin subphotospheric magnetic sheet breaks into multiple flux tubes which evolve into a colliding-merging system of spots of opposite polarity upon surface emergence, similar to those often seen on the Sun. The simulation goes on to produce many exotic δ sunspot associated phenomena: repeated flaring in the range of typical solar flare energy release and ejective helical flux ropes with embedded cool-dense plasma filaments resembling solar coronal mass ejections. © American Physical Society

• Ortiz, Ada; Hansteen, Viggo; Rubio, L. R. Bellot; Rodriguez, Jaime de la Cruz; Pontieu, Bart Walter De & Carlsson, Mats [Vis alle 7 forfattere av denne artikkelen] (2016). Emergence of Granular-Sized Magnetic Bubbles through the Solar Atmosphere. III. The Path to the Transition Region. The Astrophysical Journal (ApJ). ISSN 0004-637X. 825(2). doi: 10.3847/0004-637X/825/2/93. Fulltekst i vitenarkiv Vis sammendrag

  We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Hα 656.28 nm and Ca ii 854.2 nm lines, plus the photospheric Fe i 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0farcs14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C ii, transition region), 140 (Si iv, transition region), 279.6 (Mg ii k, core, upper chromosphere), and 283.2 nm (Mg ii k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far- and near-ultraviolet, of which we have used the Si iv 140.3 and the Mg ii k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Carlsson, Mats; Hansteen, Viggo; Gudiksen, Boris Vilhelm; Leenaarts, Jorrit & Pontieu, Bart Walter De (2016). A publicly available simulation of an enhanced network region of the Sun. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 585:A4. doi: 10.1051/0004-6361/201527226. Fulltekst i vitenarkiv Vis sammendrag

  Context. The solar chromosphere is the interface between the solar surface and the solar corona. Modelling of this region is difficult because it represents the transition from optically thick to thin radiation escape, from gas-pressure domination to magnetic-pressure domination, from a neutral to an ionised state, from MHD to plasma physics, and from near-equilibrium (LTE) to non-equilibrium conditions. Aims. Our aim is to provide the community with realistic simulations of the magnetic solar outer atmosphere. This will enable detailed comparison of existing and upcoming observations with synthetic observables from the simulations, thereby elucidating the complex interactions of magnetic fields and plasma that are crucial for our understanding of the dynamic outer atmosphere. Methods. We used the radiation magnetohydrodynamics code Bifrost to perform simulations of a computational volume with a magnetic field topology similar to an enhanced network area on the Sun. Results. The full simulation cubes are made available from the Hinode Science Data Centre Europe. The general properties of the simulation are discussed, and limitations are discussed. Reproduced with permission from Astronomy & Astrophysics, © ESO

• Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Gudiksen, Boris Vilhelm; Wedemeyer, Sven & Carlsson, Mats (2016). Chromospheric and Coronal Wave Generation in a Magnetic Flux Sheath. The Astrophysical Journal (ApJ). ISSN 0004-637X. 827(1). doi: 10.3847/0004-637X/827/1/7. Fulltekst i vitenarkiv
• Testa, Paola; Pontieu, Bart Walter De & Hansteen, Viggo (2016). High Spatial Resolution Fe XII Observations of Solar Active Regions. The Astrophysical Journal (ApJ). ISSN 0004-637X. 827(2). doi: 10.3847/0004-637X/827/2/99. Fulltekst i vitenarkiv
• Abbo, L.; Ofman, L.; Antiochos, S. K.; Hansteen, Viggo; Harra, L. & Ko, Y.-K. [Vis alle 12 forfattere av denne artikkelen] (2016). Slow Solar Wind: Observations and Modeling. Space Science Reviews. ISSN 0038-6308. 201(1-4), s. 55–108. doi: 10.1007/s11214-016-0264-1. Fulltekst i vitenarkiv
• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Carlsson, Mats & Hansteen, Viggo (2016). On the Misalignment between Chromospheric Features and the Magnetic Field on the Sun. Astrophysical Journal Letters. ISSN 2041-8205. 831(1). doi: 10.3847/2041-8205/831/1/L1. Fulltekst i vitenarkiv
• Hilde, Janne Mykland; Skjørten, Ingunn; Hansteen, Viggo; Melsom, Morten Nissen; Atar, Dan & Hisdal, Jonny [Vis alle 8 forfattere av denne artikkelen] (2016). Assessment of right ventricular afterload in COPD. COPD: Journal of Chronic Obstructive Pulmonary Disease. ISSN 1541-2555. 13(2), s. 176–185. doi: 10.3109/15412555.2015.1057275. Fulltekst i vitenarkiv
• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Hansteen, Viggo & Gudiksen, Boris Vilhelm (2016). TIME DEPENDENT NONEQUILIBRIUM IONIZATION of TRANSITION REGION LINES OBSERVED with IRIS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 817(1). doi: 10.3847/0004-637X/817/1/46. Fulltekst i vitenarkiv Vis sammendrag

  The properties of nonstatistical equilibrium ionization of silicon and oxygen ions are analyzed in this work. We focus on five solar targets (quiet Sun; coronal hole; plage; quiescent active region, AR; and flaring AR) as observed with the Interface Region Imaging Spectrograph (IRIS). IRIS is best suited for this work owing to the high cadence (up to 0.5 s), high spatial resolution (up to 0farcs32), and high signal-to-noise ratios for O iv λ1401 and Si iv λ1402. We find that the observed intensity ratio between lines of three times ionized silicon and oxygen ions depends on their total intensity and that this correlation varies depending on the region observed (quiet Sun, coronal holes, plage, or active regions) and on the specific observational objects present (spicules, dynamic loops, jets, microflares, or umbra). In order to interpret the observations, we compare them with synthetic profiles taken from 2D self-consistent radiative MHD simulations of the solar atmosphere, where the statistical equilibrium or nonequilibrium treatment of silicon and oxygen is applied. These synthetic observations show vaguely similar correlations to those in the observations, i.e., between the intensity ratios and their intensities, but only in the nonequilibrium case do we find that (some of) the observations can be reproduced. We conclude that these lines are formed out of statistical equilibrium. We use our time-dependent nonequilibrium ionization simulations to describe the physical mechanisms behind these observed properties. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Hansteen, Viggo; Guerreiro, Nuno Miguel Rodrigues; Pontieu, Bart Walter De & Carlsson, Mats (2015). NUMERICAL SIMULATIONS of CORONAL HEATING THROUGH FOOTPOINT BRAIDING. The Astrophysical Journal (ApJ). ISSN 0004-637X. 811(2). doi: 10.1088/0004-637X/811/2/106. Fulltekst i vitenarkiv Vis sammendrag

  Advanced three-dimensional (3D) radiative MHD simulations now reproduce many properties of the outer solar atmosphere. When including a domain from the convection zone into the corona, a hot chromosphere and corona are self-consistently maintained. Here we study two realistic models, with different simulated areas, magnetic field strength and topology, and numerical resolution. These are compared in order to characterize the heating in the 3D-MHD simulations which self-consistently maintains the structure of the atmosphere. We analyze the heating at both large and small scales and find that heating is episodic and highly structured in space, but occurs along loop-shaped structures, and moves along with the magnetic field. On large scales we find that the heating per particle is maximal near the transition region and that widely distributed opposite-polarity field in the photosphere leads to a greater heating scale height in the corona. On smaller scales, heating is concentrated in current sheets, the thicknesses of which are set by the numerical resolution. Some current sheets fragment in time, this process occurring more readily in the higher-resolution model leading to spatially highly intermittent heating. The large-scale heating structures are found to fade in less than about five minutes, while the smaller, local, heating shows timescales of the order of two minutes in one model and one minutes in the other, higher-resolution, model. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Pereira, Tiago Mendes Domingos; Carlsson, Mats; Pontieu, Bart Walter De & Hansteen, Viggo (2015). THE FORMATION of IRIS DIAGNOSTICS. IV. the Mg ii TRIPLET LINES AS A NEW DIAGNOSTIC FOR LOWER CHROMOSPHERIC HEATING. The Astrophysical Journal (ApJ). ISSN 0004-637X. 806(1). doi: 10.1088/0004-637X/806/1/14. Fulltekst i vitenarkiv Vis sammendrag

  A triplet of subordinate lines of Mg ii exists in the region around the h&k lines. In solar spectra these lines are seen mostly in absorption, but in some cases can become emission lines. The aim of this work is to study the formation of this triplet, and investigate any diagnostic value they can bring. Using 3D radiative magnetohydrodynamic simulations of quiet Sun and flaring flux emergence, we synthesize spectra and investigate how spectral features respond to the underlying atmosphere. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Hansteen, Viggo & Carlsson, Mats (2015). The role of partial ionization effects in the chromosphere. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. ISSN 1364-503X. 373(2042). doi: 10.1098/rsta.2014.0268. Fulltekst i vitenarkiv
• Cheung, Mark C. M.; Pontieu, Bart Walter De; Tarbell, Theodore D.; Fu, Y.; Tian, H. & Testa, Paola [Vis alle 21 forfattere av denne artikkelen] (2015). Homologous helical jets: Observations by IRIS, SDO, and Hinode and magnetic modeling with data-driven simulations. The Astrophysical Journal (ApJ). ISSN 0004-637X. 801(2). doi: 10.1088/0004-637X/801/2/83. Fulltekst i vitenarkiv Vis sammendrag

  We report on observations of recurrent jets by instruments on board the Interface Region Imaging Spectrograph, Solar Dynamics Observatory (SDO), and Hinode spacecraft. Over a 4 hr period on 2013 July 21, recurrent coronal jets were observed to emanate from NOAA Active Region 11793. Far-ultraviolet spectra probing plasma at transition region temperatures show evidence of oppositely directed flows with components reaching Doppler velocities of ±100 km s−1. Raster Doppler maps using a Si iv transition region line show all four jets to have helical motion of the same sense. Simultaneous observations of the region by SDO and Hinode show that the jets emanate from a source region comprising a pore embedded in the interior of a supergranule. The parasitic pore has opposite polarity flux compared to the surrounding network field. This leads to a spine-fan magnetic topology in the coronal field that is amenable to jet formation. Time-dependent data-driven simulations are used to investigate the underlying drivers for the jets. These numerical experiments show that the emergence of current-carrying magnetic field in the vicinity of the pore supplies the magnetic twist needed for recurrent helical jet formation. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Olluri, Kosovare; Gudiksen, Boris Vilhelm; Hansteen, Viggo & Pontieu, Bart Walter De (2015). Synthesized spectra of optically thin emission lines produced by the Bifrost stellar atmosphere code, including nonequilibrium ionization effects: A study of the intensity, nonthermal line widths, and Doppler shifts. The Astrophysical Journal (ApJ). ISSN 0004-637X. 802(1). doi: 10.1088/0004-637X/802/1/5. Fulltekst i vitenarkiv Vis sammendrag

  In recent years realistic 3D numerical models of the solar atmosphere have become available. The models attempt to recreate the solar atmosphere and mimic observations in the best way, in order to make it possible to couple complicated observations with physical properties such as the temperatures, densities, velocities, and magnetic fields. We here present a study of synthetic spectra created using the Bifrost code in order to assess how well they fit with previously taken solar data. A study of the synthetic intensity, nonthermal line widths, Doppler shifts, and correlations between any two of these three components of the spectra first assuming statistical equilibrium is made, followed by a report on some of the effects nonequilibrium ionization will have on the synthesized spectra. We find that the synthetic intensities compare well with the observations. The synthetic observations depend on the assumed resolution and point-spread function (PSF) of the instrument, and we find a large effect on the results, especially for intensity and nonthermal line width. The Doppler shifts produce the reported persistent redshifts for the transition region (TR) lines and blueshifts for the upper TR and corona lines. The nonthermal line widths reproduce the well-known turnoff point around (2–3) × 105 K, but with much lower values than those observed. The nonthermal line widths tend to increase with decreasing assumed instrumental resolution, also when nonequilibrium ionization is included. Correlations between the nonthermal line width of any two TR line studies as reported by Chae et al. are reproduced, while the correlations of intensity to line width are reproduced only after applying a PSF to the data. Doppler shift correlations reported by Doschek for the TR lines and correlations of Doppler shift to nonthermal line width of the Fe xii 19.5 line reported by Doschek et al. are reproduced. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Rouppe, Van Der Voort Luc; Pontieu, Bart Walter De; Pereira, Tiago Mendes Domingos; Carlsson, Mats & Hansteen, Viggo (2015). Heating signatures in the disk counterparts of solar spicules in interface region imaging spectrograph observations. Astrophysical Journal Letters. ISSN 2041-8205. 799(1). doi: 10.1088/2041-8205/799/1/L3. Fulltekst i vitenarkiv Vis sammendrag

  We use coordinated observations with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1 m Solar Telescope to identify the disk counterpart of type II spicules in upper-chromospheric and transition region (TR) diagnostics. These disk counterparts were earlier identified through short-lived asymmetries in chromospheric spectral lines: rapid blue- or red-shifted excursions (RBEs or RREs). We find clear signatures of RBEs and RREs in Mg II h & k, often with excursions of the central h3 and k3 absorption features in concert with asymmetries in co-temporal and co-spatial Hα spectral profiles. We find spectral signatures for RBEs and RREs in C II 1335 and 1336 Å and Si IV 1394 and 1403 Å spectral lines and interpret this as a sign that type II spicules are heated to at least TR temperatures, supporting other recent work. These C II and Si IV spectral signals are weaker for a smaller network region than for more extended network regions in our data. A number of bright features around extended network regions observed in IRIS slit-jaw imagery SJI 1330 and 1400, recently identified as network jets, can be clearly connected to Hα RBEs and/or RREs in our coordinated data. We speculate that at least part of the diffuse halo around network regions in the IRIS SJI 1330 and 1400 images can be attributed to type II spicules with insufficient opacity in the C II and Si IV lines to stand out as single features in these passbands. © American Astronomical Society. All rights reserved.

• Hansteen, Viggo; Carlsson, Mats & Gudiksen, Boris Vilhelm (2015). IRIS observations and 3D `realistic' MHD models of the solar chromosphere. I Cenarro, A. J.; Figueras, F.; Hernández-Monteagudo, C.; Bueno, J. Trujillo & Valdivielso, L. (Red.), Highlights of Spanish Astrophysics VIII, Proceedings of the XI Scientific Meeting of the Spanish Astronomical Society (SEA) held in Teruel, September 8-12, 2014. Sociedad Española de Astronomía. ISSN 9788460687603. s. 19–28. Fulltekst i vitenarkiv
• Hansteen, Viggo; Carlsson, Mats & Gudiksen, Boris Vilhelm (2015). Numerical Modeling of the Solar Chromosphere and Corona: What Has Been Done? What Should Be Done? Astronomical Society of the Pacific Conference Series. ISSN 1050-3390. 498, s. 141–146. Fulltekst i vitenarkiv
• Guerreiro, Nuno Miguel Rodrigues; Haberreiter, M.; Hansteen, Viggo & Schmutz, W (2015). Small-scale Heating Events in the Solar Atmosphere. I. Identification, Selection, and Implications for Coronal Heating. The Astrophysical Journal (ApJ). ISSN 0004-637X. 813(1). doi: 10.1088/0004-637X/813/1/61. Fulltekst i vitenarkiv Vis sammendrag

  We present a comprehensive method to analyze small-scale heating events in detail in a 3D magnetohydrodynamics simulation for quiet-Sun conditions. The method determines the number, volume, and some general geometric properties of the small-scale heating events at different instants in a simulation with a volume of 16 × 8 × 16 Mm3, spanning from the top of the convection zone to the corona. We found that there are about 104 small-scale heating events at any instant above the simulated area of 128 Mm2. They occur mainly at heights between 1.5 and 3.0 Mm. We determine the average value of their projected vertical extent, which ranges from 375 to 519 km over time, and we show that height, volume, and energy distribution of the events at any instant resemble power laws. Finally, we demonstrate that larger heating structures are a combination of much smaller heating events and that small-scale heating events dissipate enough energy to maintain the coronal energetic balance at any instant. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Martinez-Sykora, Juan; Rouppe, Van Der Voort Luc; Carlsson, Mats; Pontieu, Bart Walter De; Pereira, Tiago Mendes Domingos & Boerner, P. [Vis alle 21 forfattere av denne artikkelen] (2015). Internetwork Chromospheric Bright Grains Observed with IRIS and SST. The Astrophysical Journal (ApJ). ISSN 0004-637X. 803(1). doi: 10.1088/0004-637X/803/1/44. Fulltekst i vitenarkiv Vis sammendrag

  The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid brightenings in the form of bright grains all over coronal holes and the quiet Sun. These bright grains are seen with the IRIS 1330, 1400, and 2796 Å slit-jaw filters. We combine coordinated observations with IRIS and from the ground with the Swedish 1 m Solar Telescope (SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H 3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å, Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps in Fe i 6302 Å at high spatial 0."33, temporal, and spectral resolution. We conclude that the IRIS slit-jaw grains are the counterpart of so-called acoustic grains, i.e., resulting from chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw images (SJIs) with spectra from the IRIS spectrograph. We conclude that the grain intensity in the 2796 Å slit-jaw filter comes from both the Mg ii k core and wings. The signal in the C ii and Si iv lines is too weak to explain the presence of grains in the 1300 and 1400 Å SJIs and we conclude that the grain signal in these passbands comes mostly from the continuum. Although weak, the characteristic shock signatures of acoustic grains can often be detected in IRIS C ii spectra. For some grains, a spectral signature can be found in IRIS Si iv. This suggests that upward propagating acoustic waves sometimes reach all the way up to the transition region. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Rodriguez, Jaime de la Cruz; Hansteen, Viggo; Bellot-Rubio, Luis & Ortiz, Ada (2015). EMERGENCE of GRANULAR-SIZED MAGNETIC BUBBLES THROUGH the SOLAR ATMOSPHERE. II. NON-LTE CHROMOSPHERIC DIAGNOSTICS and INVERSIONS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 810(2). doi: 10.1088/0004-637X/810/2/145. Fulltekst i vitenarkiv Vis sammendrag

  Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photosphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca ii 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain information on the line-of-sight stratifications of temperature, velocity, and the magnetic field. Our analysis explains the peculiar Ca ii 8542 Å profiles observed in the flux emerging region. Additionally, we derive detailed temperature and velocity maps describing the ascent of a magnetic bubble from the photosphere to the chromosphere. The inversions suggest that, in active regions, granular-sized bubbles emerge up to the lower chromosphere where the existing large-scale field hinders their ascent. We report hints of heating when the field reaches the chromosphere. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Kleint, L.; Antolin, Patrick; Tian, H; Judge, Philip G.; Testa, Paola & Pontieu, Bart Walter De [Vis alle 21 forfattere av denne artikkelen] (2014). Detection of supersonic downflows and associated heating events in the transition region above sunspots. Astrophysical Journal Letters. ISSN 2041-8205. 789(2). doi: 10.1088/2041-8205/789/2/L42. Fulltekst i vitenarkiv Vis sammendrag

  Interface Region Imaging Spectrograph data allow us to study the solar transition region (TR) with an unprecedented spatial resolution of 0farcs33. On 2013 August 30, we observed bursts of high Doppler shifts suggesting strong supersonic downflows of up to 200 km s–1 and weaker, slightly slower upflows in the spectral lines Mg II h and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated with brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout the 2 hr observation, with average burst durations of about 20 s. The locations of these short-lived events appear to be the umbral and penumbral footpoints of EUV loops. Fast apparent downflows are observed along these loops in the SJIs and in the Atmospheric Imaging Assembly, suggesting that the loops are thermally unstable. We interpret the observations as cool material falling from coronal heights, and especially coronal rain produced along the thermally unstable loops, which leads to an increase of intensity at the loop footpoints, probably indicating an increase of density and temperature in the TR. The rain speeds are on the higher end of previously reported speeds for this phenomenon, and possibly higher than the free-fall velocity along the loops. On other observing days, similar bright dots are sometimes aligned into ribbons, resembling small flare ribbons. These observations provide a first insight into small-scale heating events in sunspots in the TR. © American Astronomical Society. All rights reserved.

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• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra; Hansteen, Viggo Haraldson & Noraz, Quentin Jean Francois Alain [Vis alle 7 forfattere av denne artikkelen] (2023). Update on global MHD simulations of the solar convective region. Fulltekst i vitenarkiv
• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra; Hansteen, Viggo Haraldson & Noraz, Quentin Jean Francois Alain [Vis alle 7 forfattere av denne artikkelen] (2023). Global MHD simulation of the solar convective region. Fulltekst i vitenarkiv
• Hansteen, Viggo Haraldson & Spilde, Ingrid (2022). Norske forskere skal lage romfartøy for NASA. [Journal]. forskning.no. Fulltekst i vitenarkiv
• Hansteen, Viggo Haraldson (2021). Heating the outer layers of the Sun: braiding, flux emergence, and episodic events. Fulltekst i vitenarkiv
• Hansteen, Viggo Haraldson (2021). Numeriske Modeller av Sola. Astronomi. ISSN 0802-7587. s. 24–24. Fulltekst i vitenarkiv
• Hansteen, Viggo Haraldson (2021). Flux emergence in the Quiet Sun and its implications for chromospheric/coronal heating. Fulltekst i vitenarkiv
• Hansteen, Viggo (2020). IRIS: heating of the solar chromosphere, Ellerman Bombs and UV bursts. Fulltekst i vitenarkiv
• Hansteen, Viggo (2019). The FIP effect in the Chromosphere, Transition Region, Corona, and Solar Wind. Fulltekst i vitenarkiv
• Hansteen, Viggo (2019). EBs and UV-bursts: reconnection at different heights in the solar atmosphere. Fulltekst i vitenarkiv
• Hansteen, Viggo (2019). Ellerman Bombs and UV bursts - observations. Fulltekst i vitenarkiv
• Hansteen, Viggo (2019). Ellerman Bombs and UV-bursts, a result of flux emergence. Fulltekst i vitenarkiv
• Hansteen, Viggo (2019). Ellerman bombs and UV bursts in the solar chromosphere. Fulltekst i vitenarkiv
• Hansteen, Viggo (2019). Ellerman Bombs and UV bursts, numerical simulations and IRIS observations. Fulltekst i vitenarkiv
• Hansteen, Viggo (2019). Ellerman Bombs and UV bursts, reconnection at different heights in the chromosphere? Fulltekst i vitenarkiv
• Kohutova, Petra; Antolin, Patrick; Popovas, Andrius; Szydlarski, Mikolaj Marcin & Hansteen, Viggo (2019). 3D radiative MHD simulations of coronal rain formation and evolution. Fulltekst i vitenarkiv
• Hansteen, Viggo (2018). Ellerman bombs and UV bursts: reconnection at different atmospheric layers? Fulltekst i vitenarkiv
• Hansteen, Viggo (2018). Ion neutral effects in the solar chromosphere and type II spicules. Fulltekst i vitenarkiv
• Hansteen, Viggo (2018). Ellerman bombs and UV bursts: reconnection at different atmospheric layers? Fulltekst i vitenarkiv
• Hansteen, Viggo (2018). Modeling chromospheric heating and dynamics: What do we understand? Fulltekst i vitenarkiv
• Hansteen, Viggo (2018). The Heating of the Solar Chromosphere and Corona, What can "realistic" 3D Numerical Models teach us? Fulltekst i vitenarkiv
• Hansteen, Viggo (2018). Chromospheric Dynamics and Heating Processes Ion-Neutral Effects in the Solar Chromosphere and Type II Spicules. Fulltekst i vitenarkiv
• Hansteen, Viggo (2018). Mass and energy flows between the solar chromosphere, transition region and corona. Fulltekst i vitenarkiv
• Pereira, Tiago Mendes Domingos; Carlsson, Mats; Pontieu, Bart Walter De & Hansteen, Viggo (2015). The Mg II lines as a diagnostic for heating in the lower chromosphere. Fulltekst i vitenarkiv
• Carlsson, Mats & Hansteen, Viggo (2015). Vær og uvær på sola. Fulltekst i vitenarkiv
• Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Carlsson, Mats & Gudiksen, Boris Vilhelm (2014). Chromospheric and Corona Wave Generation in Flux Tubes. Fulltekst i vitenarkiv
• Færder, Øystein Håvard; Carlsson, Mats; Hansteen, Viggo Haraldson & Siverio, Daniel Elias Nóbrega (2024). The role of resistivity in plasmoid-mediated reconnection in the solar corona and its observational imprints. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Magnetic reconnection is a fundamental mechanism in astrophysics and a promising candidate for solving the coronal heating problem. This process has been shown to produce a variety of different observable phenomena on the Sun, including Ellerman bombs, ultraviolet bursts, coronal bright points, surges, coronal jets, and flares. Achieving a better understanding on the role that reconnection plays in the heating and dynamics of the solar atmosphere requires observational data and numerical reproduction of any aspect related to reconnection, including small-scale features such as plasmoids. A common challenge in mimicking reconnection on the Sun numerically is how to model the electrical resistivity, since this quantity is small compared to the diffusive effects caused by the discrete nature of numerical codes. To address this challenge, we ran a series of magnetohydrodynamic (MHD) simulations using the Bifrost code with its inbuilt hyper-resistivity model along with other resistivity models, for which we developed new modules especially for our research. Our experiments proved that the hyper-resistivity model of Bifrost is at least as suitable for mimicking reconnection phenomena on the Sun as other widely used resistivity models and has the advantage of being applicable in any part of the solar atmosphere without the need for re-calibration. Furthermore, we found that when simulating plasmoid-mediated reconnection with a moderate resolution, another takeaway with the hyper-resistivity model is the capability of reproducing plasmoid characteristics in closer resemblance to those attained with uniform resistivity at a significantly higher resolution. Observing small-scale features of reconnection on the Sun, such as plasmoids, can also be demanding, due to the limited resolution provided by currently-active telescopes and spectrographs. Furthermore, plasmoids in the corona are quite dim because of the low densities, hence often difficult to detect due to an underwhelming signal-to-noise ratio. Through forward-modelling of synthetic extreme-ultraviolet (EUV) observables from simulations of plasmoid-mediated reconnection, we found that combined spectra from the upcoming Multi-slit Solar Explorer (MUSE) and Solar-C missions along with high-resolution images from the MUSE context imager and the recently-launched Solar Orbiter will together contribute strongly in providing new insights into plasmoid formation on the Sun. Those insights can be used to constrain our numerical models in terms of further development of state-of-the-art resistivity models, hence lead us one step forward in unravelling the mysteries of our life-giving star.

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