Juan Martinez Sykora

• Sand, Mats Ola; Noraz, Quentin; Aulanier, Guillaume; Martinez-Sykora, Juan; Carlsson, Mats & Voort, Luc Rouppe van der (2026). Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202557602. Full text in Research Archive Show summary

  Context. The formation mechanism for the dynamic type II spicules has remained elusive for many years. Their dynamical behaviour has long been linked to magnetic reconnection, yet no conclusive evidence has been provided. However, one recent observational study found signs of magnetic reconnection, as traced by Ellerman bombs (EBs), at the footpoints of many spicules. The triggering of EBs is generally linked to magnetic reconnection due to flux emergence and convective motions in the photosphere. Aims. We aim to explore whether we can connect EBs to type II spicules, and determine to what extent we can use EBs as an observational proxy to probe magnetic reconnection in this dynamic. We also aim to provide further insight into the mechanisms that trigger EBs. Methods. We used a simulation run with the radiative magnetohydrodynamics code Bifrost to track spicules and study the physical processes underlying their formation. To detect EBs and classify the spicules, we synthesised the chromospheric H α spectral line using the multilevel radiative transfer code RH 1.5D. We also traced shocks and current sheets to decipher the origin of EBs and spicules. We selected one type II spicule with a strong EB near its footpoint and studied their formation in detail. Results. A magnetoacoustic shock advects the magnetic field lines towards an oppositely directed ambient field, creating a current sheet. The current sheet accelerates dense plasma via a whiplash effect generated by magnetic reconnection into the inclined ambient field, launching the spicule. Several EB profiles trace shock- and magnetic-reconnection-induced dynamics during this process at the spicule footpoint. Conclusions. We present a new EB triggering mechanism in which a shock-induced current sheet reconnects, triggering an EB in the lower solar atmosphere. The shock-induced current sheet generates the upwards propagation of a type II spicule via reconnection outflows. These results provide a plausible physical origin for the recently observed connection between EBs and spicules.

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

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

• Sen, Samrat; Sukarmadji, A. Ramada C.; Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martinez-Sykora, Juan & Antolin, Patrick (2026). Extreme-ultraviolet Synthesis of Nanojet-like Ejections due to Coalescing Flux Ropes. Astrophysical Journal Letters. ISSN 2041-8205. 997(2). doi: 10.3847/2041-8213/ae35fc. Full text in Research Archive Show summary

  Detection and characterization of small-scale energetic events such as nanoflares and nanojets remain challenging owing to their short lifetimes, small spatial extent, and relatively low energy release, despite their potential role in coronal heating. Recent observations have identified nanojets as small-scale (length ≲6.6 Mm, width ≲1 Mm), fast (∼ few 100 km s−1), and short-lived (≲30 s) ejections associated with nanoflare-scale energies, providing evidence of magnetic reconnection at small spatial scales. However, the lack of synthetic diagnostics has limited the connection between magnetohydrodynamic (MHD) models and observations. In this Letter, we present synthetic observations of the coalescence of two flux ropes, leading to nanojet-like signatures from a numerical model obtained with the MPI-AMRVAC code. We report synthetic observables in extreme-ultraviolet lines compatible with existing instruments such as Solar Dynamics Observatory/Atmospheric Imaging Assembly and the upcoming Multi-slit Solar Explorer mission and compare the synthetic observables with an existing observation of nanojets. The synthetic diagnostics of the emissivity maps, Doppler velocity, and thermal and nonthermal line broadening produce key observational properties, suggesting a plausible 3D scenario for nanojet generation where tiny flux ropes reconnect within loops. Our results provide predictions for the detectability of nanojets with current and future spectroscopic facilities and establish a bridge between MHD modeling and observations.

• To, Andy S. H.; Burden, Abigail; Baker, Deborah; Eklund, Henrik; Brooks, David H. & Hayes, Laura A. [Show all 15 contributors for this article] (2025). Systematic Nonthermal Velocity Increase Preceding Soft X-Ray Flare Onset: A Large-scale Hinode/EIS Study. The Astrophysical Journal (ApJ). ISSN 0004-637X. 993(1). doi: 10.3847/1538-4357/ae07de. Full text in Research Archive Show summary

  Nonthermal velocities, derived from spectral line broadening, can provide crucial insights into plasma dynamics before and during solar flares. To systematically study the preflare phase, we constructed a Hinode/Extreme-ultraviolet Imaging Spectrometer flare catalog of 1449 flares from 2011 to 2024. This enabled a large-scale analysis of flare loop footpoint nonthermal velocity evolution across different flare magnitudes (C, M, X-classes). Analyzing Fe viii –Fe xxiv emission lines formed at log( T /K) ∼ 5.7–7.3 with piecewise linear fits in the preflare period, we find that nonthermal velocities consistently increase 4–25 minutes before Geostationary Operational Environmental Satellite (GOES) soft X-ray start in C and M-class flares. Onset timing patterns vary with flare magnitude: smaller flares show temperature-dependent progression, while larger flares exhibit more compressed, near-simultaneous onsets across temperatures. While our limited X-class sample ( N = 18) also show onset before GOES, larger statistics are needed to confirm its behavior. M-class flares show a systematic precursor nonthermal velocity onset ∼30–60 minutes before GOES peak. In a subset of M-class flares (2011–2018), coronal-mass-ejection (CME)-associated events show earlier and more uniform precursor onsets (45–74 minutes before peak) than non-CME events, of which only some lines display a precursor, suggesting a strong link between extended preflare nonthermal broadening and successful eruptions. This large scale study establishes preflare nonthermal velocity increase at footpoints as a common precursor observable before any X-ray signature.

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

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

• Evans, Samuel; Oppenheim, Meers; Martinez-Sykora, Juan & Dimant, Yakov (2025). Multifluid and Kinetic 2D and 3D Simulations of Thermal Farley-Buneman Instability Turbulence in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 986(1). doi: 10.3847/1538-4357/adcd70. Full text in Research Archive Show summary

  Models currently fail to reproduce observations of the coldest regions in the Sun’s atmosphere, though recent work suggests the thermal Farley–Buneman instability (TFBI) may play a critical role. This meter-scale, electrostatic, multifluid plasma instability causes turbulence and heating in the coldest regions of the solar chromosphere. This paper describes how TFBI turbulence and heating varies across multifluid 2D, kinetic 2D, and kinetic 3D simulations. It also presents the first 3D simulations of the TFBI. We find that multifluid and kinetic 2D simulations produce similar results overall, despite using vastly different approaches. Additionally, our kinetic 3D simulations produce a similar or somewhat larger amount of heating compared to 2D, as contributions from the parallel electric field account for only (13 ± 2.5)% of the total turbulent heating in 3D.

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

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

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

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

• Færder, Øystein Håvard; Siverio, Daniel Elias Nóbrega; Carlsson, Mats & Martinez-Sykora, Juan (2024). Extreme-ultraviolet (EUV) observables of simulated plasmoid-mediated reconnection in the solar corona. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202449357. Full text in Research Archive Show summary

  Context. Understanding the role of magnetic reconnection in the heating and dynamics of the solar atmosphere requires detailed observational data of any observable aspect of the reconnection process, including small-scale features such as plasmoids. Aims. Here, we examine the capability of active and upcoming instruments to detect plasmoids generated by reconnection in the corona including low-density regimes. Methods. We used the Bifrost code to perform simulations of plasmoid-mediated reconnection in the corona with a 2D idealized setup: a fan-spine topology with uniform density including thermal conduction. Through a forward-modeling of extreme-ultraviolet (EUV) observables, we checked whether our simulated plasmoids could be detected with the instruments of Solar Dynamics Observatory (SDO) and Solar Orbiter (SO), as well as the upcoming Multi-Slit Solar Explorer (MUSE) and Solar-C missions. Results. Short-lived (∼10 − 20 s) small-scale (∼0.2 − 0.5 Mm) coronal plasmoids are not resolvable with the Atmospheric Imaging Assembly (AIA) on board SDO. In contrast, they could be captured with the EUV High-Resolution Imager at the Extreme Ultraviolet Imager (EUI-HRIEUV) of SO. The spatial and temporal high-resolution planned for the MUSE spectrograph (SG) is adequate to obtain full spectral information of these plasmoids. To achieve a sufficient signal-to-noise ratio (S/N) for ∼0.8 MK plasmoids in the MUSE/SG 171 Å channel, full-raster images are attainable for regions with electron densities above 109 cm−3, while sit-and-stare observations are recommended for lower-density regions. The future Solar-C mission could also capture these coronal plasmoids using the EUV High-Throughput Spectroscopic Telescope (EUVST), considering rapid changes in Doppler shift and line widths in different EUV lines caused by plasmoid motions along the current sheet. Conclusions. With the combined spectra of MUSE/SG and Solar-C/EUVST in multiple emission lines, along with high-resolution images from SO/EUI-HRIEUV and MUSE/CI, it should be possible to gain new insights about plasmoid formation in the corona.

• Furuseth, Sondre Vik; Cherry, George & Martinez-Sykora, Juan (2024). On thermal conduction in the solar atmosphere: An analytical solution for nonlinear diffusivity without compact support. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 691. doi: 10.1051/0004-6361/202451707. Full text in Research Archive Show summary

  Context. The scientific community employs complicated multiphysics simulations to understand the physics in solar, stellar, and interstellar media. These must be tested against known solutions to ensure their validity. Several well-known tests exist, such as the Sod shock tube test. However, a test for nonlinear diffusivity is missing. This problem is highly relevant in the solar atmosphere, where various events release energy that subsequently diffuses by Spitzer thermal conductivity. Aims. The aim is to derive an analytical solution for nonlinear diffusivity in 1D, 2D, and 3D, which allows for a nonzero background value. The solution is used to design a test for numerical solvers and study Spitzer conductivity in the solar atmosphere. Methods. There exists an ideal solution assuming zero background value. We performed an analytical first-order perturbation of this solution. The first-order solution was first tested against a dedicated nonlinear diffusion solver, whereupon it was used to benchmark the single- and multifluid radiative magnetohydrodynamics code Ebysus, used to study the Sun. The theory and numerical modeling were used to investigate the role of Spitzer conductivity in the transport of energy released in a nanoflare. Results. The derived analytical solution models nonlinear diffusivity accurately within its region of validity and approximately beyond. Various numerical schemes implemented in the Ebysus code is found to model Spitzer conductivity correctly. The energy from a representative nanoflare is found to diffuse 9 Mm within the first second of its lifetime due to Spitzer conductivity alone, strongly dependent on the electron density. Conclusions. The analytical first-order solution is a step forward in ensuring the physical validity of intricate simulations of the Sun. Additionally, since the derivation and argumentation are general, they can easily be followed to treat other nonlinear diffusion problems.

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

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

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

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

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

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

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

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

• Evans, Samuel; Oppenheim, Meers; Martinez-Sykora, Juan; Dimant, Yakov & Xiao, Richard (2023). Multifluid Simulation of Solar Chromospheric Turbulence and Heating Due to Thermal Farley-Buneman Instability. The Astrophysical Journal (ApJ). ISSN 0004-637X. 949(2). doi: 10.3847/1538-4357/acc5e5. Full text in Research Archive Show summary

  Abstract Models fail to reproduce observations of the coldest parts of the Sun’s atmosphere, where interactions between multiple ionized and neutral species prevent an accurate MHD representation. This paper argues that a meter-scale electrostatic plasma instability develops in these regions and causes heating. We refer to this instability as the Thermal Farley–Buneman Instability (TFBI). Using parameters from a 2.5D radiative MHD Bifrost simulation, we show that the TFBI develops in many of the colder regions in the chromosphere. This paper also presents the first multifluid simulation of the TFBI and validates this new result by demonstrating close agreement with theory during the linear regime. The simulation eventually develops turbulence, and we characterize the resulting wave-driven heating, plasma transport, and turbulent motions. These results all contend that the effects of the TFBI contribute to the discrepancies between solar observations and radiative MHD models.

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

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

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

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

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

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

• Pontieu, Bart Walter De; Testa, Paola; Martinez-Sykora, Juan; Antolin, Patrick; Karampelas, Konstantinos & Hansteen, Viggo Haraldson [Show all 26 contributors for this article] (2022). Probing the Physics of the Solar Atmosphere with the Multi-slit Solar Explorer (MUSE). I. Coronal Heating. The Astrophysical Journal (ApJ). ISSN 0004-637X. 926(1). doi: 10.3847/1538-4357/ac4222. Full text in Research Archive
• Antolin, Patrick; Martinez-Sykora, Juan & Şahin, Seray (2022). Thermal Instability-Induced Fundamental Magnetic Field Strands in the Solar Corona. Astrophysical Journal Letters. ISSN 2041-8205. 926(2). doi: 10.3847/2041-8213/ac51dd. Full text in Research Archive Show summary

  Thermal instability is a fundamental process of astrophysical plasmas. It is expected to occur whenever the cooling is dominated by radiation and cannot be compensated for by heating. In this work, we conduct 2.5D radiation MHD simulations with the Bifrost code of an enhanced activity network in the solar atmosphere. Coronal loops are produced self-consistently, mainly through Joule heating, which is sufficiently stratified and symmetric to produce thermal nonequilibrium. During the cooling and driven by thermal instability, coronal rain is produced along the loops. Due to flux freezing, the catastrophic cooling process leading to a rain clump produces a local enhancement of the magnetic field, thereby generating a distinct magnetic strand within the loop up to a few Gauss stronger than the surrounding coronal field. These strands, which can be considered fundamental, are a few hundred kilometers in width, span most of the loop leg, and emit strongly in the UV and extreme UV (EUV), thereby establishing a link between the commonly seen rain strands in the visible spectrum with the observed EUV coronal strands at high resolution. The compression downstream leads to an increase in temperature that generates a plume-like structure, a strongly emitting spicule-like feature, and short-lived brightening in the UV during the rain impact, providing an explanation for similar phenomena seen with IRIS. Thermal instability and nonequilibrium can therefore be associated with localized and intermittent UV brightening in the transition region and chromosphere, as well as contribute to the characteristic filamentary morphology of the solar corona in the EUV.

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

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

• Pontieu, Bart Walter De; Polito, Vanessa; Hansteen, Viggo Haraldson; Testa, Paola; Reeves, Katharine K. & Antolin, Patrick [Show all 14 contributors for this article] (2021). A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS). Solar Physics. ISSN 0038-0938. 296. doi: 10.1007/s11207-021-01826-0. Full text in Research Archive Show summary

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

• Schmit, Don; Martinez-Sykora, Juan; Pereira, Tiago & Ramos, Andrés Asensio (2021). Probing Uncertainties in Diagnostics of a Synthetic Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 913(1). doi: 10.3847/1538-4357/abf4d2. Full text in Research Archive
• Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge; Siverio, Daniel Elias Nóbrega & Martinez-Sykora, Juan [Show all 7 contributors for this article] (2021). Evidence of the multi-thermal nature of spicular downflows. Impact on solar atmospheric heating. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 654(51). doi: 10.1051/0004-6361/202141404. Full text in Research Archive Show summary

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

• Chintzoglou, Georgios; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Hansteen, Viggo; Rodriguez, Jaime de la Cruz & Szydlarski, Mikolaj Marcin [Show all 10 contributors for this article] (2021). ALMA and IRIS Observations of the Solar Chromosphere. I. An On-Disk Type II Spicule. The Astrophysical Journal (ApJ). ISSN 0004-637X. 906(2). doi: 10.3847/1538-4357/abc9b1. Full text in Research Archive
• Chintzoglou, Georgios; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Hansteen, Viggo; Rodriguez, Jaime de la Cruz & Szydlarski, Mikolaj Marcin [Show all 10 contributors for this article] (2021). ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages. The Astrophysical Journal (ApJ). ISSN 0004-637X. 906(2). doi: 10.3847/1538-4357/abc9b0. Full text in Research Archive
• Martinez-Sykora, Juan; Leenarts, Jorrit; Pontieu, Bart Walter De; Siverio, Daniel Elias Nóbrega; Hansteen, Viggo & Carlsson, Mats [Show all 7 contributors for this article] (2020). Ion–neutral Interactions and Nonequilibrium Ionization in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 889(2). doi: 10.3847/1538-4357/ab643f. Full text in Research Archive
• Siverio, Daniel Elias Nóbrega; Moreno-Insertis, Fernando; Martinez-Sykora, Juan; Carlsson, Mats & Szydlarski, Mikolaj Marcin (2020). Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 633. doi: 10.1051/0004-6361/201936944. Full text in Research Archive Show summary

  Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized. Aims. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of the flux emergence process. Methods. Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the ambipolar diffusion term of the generalized Ohm’s law. Results. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the ambipolar diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the ambipolar diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of ambipolar diffusion in the atmosphere.

• Martinez-Sykora, Juan; Szydlarski, Mikolaj Marcin; Hansteen, Viggo & Pontieu, Bart Walter De (2020). On the Velocity Drift between Ions in the Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 900(2). doi: 10.3847/1538-4357/ababa3. Full text in Research Archive
• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Rodriguez, Jaime de la Cruz & Chintzoglou, Georgios (2020). The Formation Height of Millimeter-wavelength Emission in the Solar Chromosphere. Astrophysical Journal Letters. ISSN 2041-8205. 891(1). doi: 10.3847/2041-8213/ab75ac. Full text in Research Archive
• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Carlsson, Mats; Rodriguez, Jaime de la Cruz; Bose, Souvik & Chintzoglou, Georgios [Show all 33 contributors for this article] (2020). High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038732. Full text in Research Archive Show summary

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

• Siverio, Daniel Elias Nóbrega; Martinez-Sykora, Juan; Moreno-Insertis, Fernando & Carlsson, Mats (2020). Ambipolar diffusion in the Bifrost code. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 638. doi: 10.1051/0004-6361/202037809. Full text in Research Archive Show summary

  Context. Ambipolar diffusion is a physical mechanism related to the drift between charged and neutral particles in a partially ionized plasma that is key to many different astrophysical systems. However, understanding its effects is challenging due to basic uncertainties concerning relevant microphysical aspects and the strong constraints it imposes on the numerical modeling. Aims. Our aim is to introduce a numerical tool that allows us to address complex problems involving ambipolar diffusion in which, additionally, departures from ionization equilibrium are important or high resolution is needed. The primary application of this tool is for solar atmosphere calculations, but the methods and results presented here may also have a potential impact on other astrophysical systems. Methods. We have developed a new module for the stellar atmosphere Bifrost code that improves its computational capabilities of the ambipolar diffusion term in the generalized Ohm’s law. This module includes, among other things, collision terms adequate to processes in the coolest regions in the solar chromosphere. As the main feature of the module, we have implemented the super time stepping (STS) technique, which allows an important acceleration of the calculations. We have also introduced hyperdiffusion terms to guarantee the stability of the code. Results. We show that to have an accurate value for the ambipolar diffusion coefficient in the solar atmosphere it is necessary to include as atomic elements in the equation of state not only hydrogen and helium, but also the main electron donors like sodium, silicon, and potassium. In addition, we establish a range of criteria to set up an automatic selection of the free parameters of the STS method that guarantees the best performance, optimizing the stability and speed for the ambipolar diffusion calculations. We validate the STS implementation by comparison with a self-similar analytical solution.

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

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

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

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

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

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

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

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

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

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

• 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. Full text in Research Archive Show summary

  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.

• Pontieu, Bart Walter De; Moortel, I. De; Martinez-Sykora, Juan & McIntosh, S. W. (2017). Observations and Numerical Models of Solar Coronal Heating Associated with Spicules. Astrophysical Journal Letters. ISSN 2041-8205. 845(2). doi: 10.3847/2041-8213/aa7fb4. Full text in Research Archive Show summary

  Spicules have been proposed as significant contributors to the mass and energy balance of the corona. While previous observations have provided a glimpse of short-lived transient brightenings in the corona that are associated with spicules, these observations have been contested and are the subject of a vigorous debate both on the modeling and the observational side. Therefore, it remains unclear whether plasma is heated to coronal temperatures in association with spicules. We use high-resolution observations of the chromosphere and transition region (TR) with the Interface Region Imaging Spectrograph and of the corona with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory to show evidence of the formation of coronal structures associated with spicular mass ejections and heating of plasma to TR and coronal temperatures. Our observations suggest that a significant fraction of the highly dynamic loop fan environment associated with plage regions may be the result of the formation of such new coronal strands, a process that previously had been interpreted as the propagation of transient propagating coronal disturbances. Our observations are supported by 2.5D radiative MHD simulations that show heating to coronal temperatures in association with spicules. Our results suggest that heating and strong flows play an important role in maintaining the substructure of loop fans, in addition to the waves that permeate this low coronal environment. © 2017. The American Astronomical Society. All rights reserved.

• Pontieu, Bart Walter De; Martinez-Sykora, Juan & Chintzoglou, Georgios (2017). What Causes the High Apparent Speeds in Chromospheric and Transition Region Spicules on the Sun? Astrophysical Journal Letters. ISSN 2041-8205. 849(1). doi: 10.3847/2041-8213/aa9272. Full text in Research Archive Show summary

  Spicules are the most ubuiquitous type of jets in the solar atmosphere. The advent of high-resolution imaging and spectroscopy from the Interface Region Imaging Spectrograph (IRIS) and ground-based observatories has revealed the presence of very high apparent motions of order 100–300 km s−1 in spicules, as measured in the plane of the sky. However, line of sight measurements of such high speeds have been difficult to obtain, with values deduced from Doppler shifts in spectral lines typically of order 30–70 km s−1. In this work, we resolve this long-standing discrepancy using recent 2.5D radiative MHD simulations. This simulation has revealed a novel driving mechanism for spicules in which ambipolar diffusion resulting from ion-neutral interactions plays a key role. In our simulation, we often see that the upward propagation of magnetic waves and electrical currents from the low chromosphere into already existing spicules can lead to rapid heating when the currents are rapidly dissipated by ambipolar diffusion. The combination of rapid heating and the propagation of these currents at Alfvénic speeds in excess of 100 km s−1 leads to the very rapid apparent motions, and often wholesale appearance, of spicules at chromospheric and transition region temperatures. In our simulation, the observed fast apparent motions in such jets are actually a signature of a heating front, and much higher than the mass flows, which are of order 30–70 km s−1. Our results can explain the behavior of transition region "network jets" and the very high apparent speeds reported for some chromospheric spicules. © 2017. The American Astronomical Society. All rights reserved.

• Nobrega-Siverio, Daniel; Martinez-Sykora, Juan; Moreno-Insertis, F. & Voort, Luc Rouppe van der (2017). Surges and Si IV Bursts in the Solar Atmosphere: Understanding IRIS and SST Observations through RMHD Experiments. The Astrophysical Journal (ApJ). ISSN 0004-637X. 850(2). doi: 10.3847/1538-4357/aa956c. Full text in Research Archive Show summary

  © 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. Full text in Research Archive Show summary

  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), p. 1269–1272. doi: 10.1126/science.aah5412. Full text in Research Archive Show summary

  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

• 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. Full text in Research Archive
• 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. Full text in Research Archive Show summary

  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

• 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. Full text in Research Archive
• Cheung, Mark C. M.; Pontieu, Bart Walter De; Tarbell, Theodore D.; Fu, Y.; Tian, H. & Testa, Paola [Show all 21 contributors for this article] (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. Full text in Research Archive Show summary

  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

• Pontieu, Bart Walter De; McIntosh, Scott; Martinez-Sykora, Juan; Peter, Hardi & Pereira, Tiago Mendes Domingos (2015). Why is non-thermal line broadening of spectral lines in the lower transition region of the sun independent of spatial resolution? Astrophysical Journal Letters. ISSN 2041-8205. 799(1). doi: 10.1088/2041-8205/799/1/L12. Full text in Research Archive Show summary

  Spectral observations of the solar transition region (TR) and corona show broadening of spectral lines beyond what is expected from thermal and instrumental broadening. The remaining non-thermal broadening is significant (5–30 km s−1) and correlated with intensity. Here we study spectra of the TR Si iv 1403 Å line obtained at high resolution with the Interface Region Imaging Spectrograph (IRIS). We find that the large improvement in spatial resolution (0farcs33) of IRIS compared to previous spectrographs (2'') does not resolve the non-thermal line broadening which, in most regions, remains at pre-IRIS levels of about 20 km s−1. This invariance to spatial resolution indicates that the processes behind the broadening occur along the line-of-sight (LOS) and/or on spatial scales (perpendicular to the LOS) smaller than 250 km. Both effects appear to play a role. Comparison with IRIS chromospheric observations shows that, in regions where the LOS is more parallel to the field, magneto-acoustic shocks driven from below impact the TR and can lead to significant non-thermal line broadening. This scenario is supported by MHD simulations. While these do not show enough non-thermal line broadening, they do reproduce the long-known puzzling correlation between non-thermal line broadening and intensity. This correlation is caused by the shocks, but only if non-equilibrium ionization is taken into account. In regions where the LOS is more perpendicular to the field, the prevalence of small-scale twist is likely to play a significant role in explaining the invariance and correlation with intensity. © American Astronomical Society. All rights reserved.

• Martinez-Sykora, Juan; Rouppe, Van Der Voort Luc; Carlsson, Mats; Pontieu, Bart Walter De; Pereira, Tiago Mendes Domingos & Boerner, P. [Show all 21 contributors for this article] (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. Full text in Research Archive Show summary

  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

• Kleint, L.; Antolin, Patrick; Tian, H; Judge, Philip G.; Testa, Paola & Pontieu, Bart Walter De [Show all 21 contributors for this article] (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. Full text in Research Archive Show summary

  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.

• Peter, H.; Tian, H.; Curdt, W.; Schmit, D.; Innes, D. & Pontieu, Bart Walter De [Show all 23 contributors for this article] (2014). Hot explosions in the cool atmosphere of the Sun. Science. ISSN 0036-8075. 346(6207). doi: 10.1126/science.1255726. Full text in Research Archive
• Testa, Paola; Pontieu, Bart Walter De; Allred, J.; Carlsson, Mats; Reale, F. & Daw, A. [Show all 24 contributors for this article] (2014). Evidence of nonthermal particles in coronal loops heated impulsively by nanoflares. Science. ISSN 0036-8075. 346(6207). doi: 10.1126/science.1255724. Full text in Research Archive
• Tian, H.; DeLuca, E.E.; Cranmer, S.R.; Pontieu, Bart Walter De; Peter, H. & Martinez-Sykora, Juan [Show all 27 contributors for this article] (2014). Prevalence of small-scale jets from the networks of the solar transition region and chromosphere. Science. ISSN 0036-8075. 346(6207). doi: 10.1126/science.1255711. Full text in Research Archive
• Pereira, Tiago Mendes Domingos; Pontieu, Bart Walter De; Carlsson, Mats; Hansteen, Viggo; Tarbell, T.D. & Lemen, J. [Show all 20 contributors for this article] (2014). An interface region imaging spectrograph first view on solar spicules. Astrophysical Journal Letters. ISSN 2041-8205. 792(1). doi: 10.1088/2041-8205/792/1/L15. Full text in Research Archive Show summary

  Solar spicules have eluded modelers and observers for decades. Since the discovery of the more energetic type II, spicules have become a heated topic but their contribution to the energy balance of the low solar atmosphere remains unknown. Here we give a first glimpse of what quiet-Sun spicules look like when observed with NASA's recently launched Interface Region Imaging Spectrograph (IRIS). Using IRIS spectra and filtergrams that sample the chromosphere and transition region, we compare the properties and evolution of spicules as observed in a coordinated campaign with Hinode and the Atmospheric Imaging Assembly. Our IRIS observations allow us to follow the thermal evolution of type II spicules and finally confirm that the fading of Ca II H spicules appears to be caused by rapid heating to higher temperatures. The IRIS spicules do not fade but continue evolving, reaching higher and falling back down after 500-800 s. Ca II H type II spicules are thus the initial stages of violent and hotter events that mostly remain invisible in Ca II H filtergrams. These events have very different properties from type I spicules, which show lower velocities and no fading from chromospheric passbands. The IRIS spectra of spicules show the same signature as their proposed disk counterparts, reinforcing earlier work. Spectroheliograms from spectral rasters also confirm that quiet-Sun spicules originate in bushes from the magnetic network. Our results suggest that type II spicules are indeed the site of vigorous heating (to at least transition region temperatures) along extensive parts of the upward moving spicular plasma. © American Astronomical Society. All rights reserved.

• Tian, H; DeLuca, E.; Reeves, K. K.; McKillop, S.; Pontieu, Bart Walter De & Martinez-Sykora, Juan [Show all 23 contributors for this article] (2014). HIGH-RESOLUTION OBSERVATIONS OF THE SHOCK WAVE BEHAVIOR FOR SUNSPOT OSCILLATIONS WITH THE INTERFACE REGION IMAGING SPECTROGRAPH. The Astrophysical Journal (ApJ). ISSN 0004-637X. 786(2). doi: 10.1088/0004-637X/786/2/137. Full text in Research Archive Show summary

  We present the first results of sunspot oscillations from observations by the Interface Region Imaging Spectrograph. The strongly nonlinear oscillation is identified in both the slit-jaw images and the spectra of several emission lines formed in the transition region and chromosphere. We first apply a single Gaussian fit to the profiles of the Mg II 2796.35 Å, C II 1335.71 Å, and Si IV 1393.76 Å lines in the sunspot. The intensity change is ~30%. The Doppler shift oscillation reveals a sawtooth pattern with an amplitude of ~10 km s–1 in Si IV. The Si IV oscillation lags those of C II and Mg II by ~6 and ~25 s, respectively. The line width suddenly increases as the Doppler shift changes from redshift to blueshift. However, we demonstrate that this increase is caused by the superposition of two emission components. We then perform detailed analysis of the line profiles at a few selected locations on the slit. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blueshift in Si IV, whereas the intensity enhancement slightly precedes the maximum blueshift in Mg II. We find a positive correlation between the maximum velocity and deceleration, a result that is consistent with numerical simulations of upward propagating magnetoacoustic shock waves. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Hansteen, Viggo; Pontieu, Bart Walter De; Carlsson, Mats; Lemen, James R.; Title, Alan M. & Boerner, Paul F.X. [Show all 21 contributors for this article] (2014). The unresolved fine structure resolved: IRIS observations of the solar transition region. Science. ISSN 0036-8075. 346(6207). doi: 10.1126/science.1255757. Full text in Research Archive
• Pontieu, Bart Walter De; Rouppe, Van Der Voort Luc; McIntosh, S.W.; Pereira, Tiago Mendes Domingos; Carlsson, Mats & Hansteen, Viggo [Show all 24 contributors for this article] (2014). On the prevalence of small-scale twist in the solar chromosphere and transition region. Science. ISSN 0036-8075. 346(6207). doi: 10.1126/science.1255732. Full text in Research Archive
• Testa, Paola; Pontieu, Bart De; Martinez-Sykora, Juan; DeLuca, E.; Hansteen, Viggo & Cirtain, J. [Show all 15 contributors for this article] (2013). OBSERVING CORONAL NANOFLARES IN ACTIVE REGION MOSS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 770(1). doi: 10.1088/2041-8205/770/1/L1. Full text in Research Archive
• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Leenaarts, Jorrit; Pereira, Tiago Mendes Domingos; Carlsson, Mats & Hansteen, Viggo [Show all 10 contributors for this article] (2013). A DETAILED COMPARISON BETWEEN THE OBSERVED AND SYNTHESIZED PROPERTIES OF A SIMULATED TYPE II SPICULE. The Astrophysical Journal (ApJ). ISSN 0004-637X. 771(1). doi: 10.1088/0004-637X/771/1/66. Full text in Research Archive Show summary

  We have performed a three-dimensional radiative MHD simulation of the solar atmosphere. This simulation shows a jet-like feature that shows similarities to the type II spicules observed for the first time with Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the solar disk are associated with these spicules. Observational results suggest they may contribute significantly in supplying the corona with hot plasma. We perform a detailed comparison of the properties of the simulated jet with those of type II spicules (observed with Hinode) and RBEs (with ground-based instruments). We analyze a wide variety of synthetic emission and absorption lines from the simulations including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition region and coronal temperatures (10,000 K to several million K). We compare their synthetic intensities, line profiles, Doppler shifts, line widths, and asymmetries with observations from Hinode/SOT and EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many properties of the synthetic observables resemble the observations, and we describe in detail the physical processes that lead to these observables. Detailed analysis of the synthetic observables provides insight into how observations should be analyzed to derive information about physical variables in such a dynamic event. For example, we find that line-of-sight superposition in the optically thin atmosphere requires the combination of Doppler shifts and spectral line asymmetry to determine the velocity in the jet. In our simulated type II spicule, the lifetime of the asymmetry of the transition region lines is shorter than that of the coronal lines. Other properties differ from the observations, especially in the chromospheric lines. The mass density of the part of the spicule with a chromospheric temperature is too low to produce significant opacity in chromospheric lines. The synthetic Ca II 8542 Å and Hα profiles therefore do not show signal resembling RBEs. These and other discrepancies are described in detail, and we discuss which mechanisms and physical processes may need to be included in the MHD simulations to mimic the thermodynamic processes of the chromosphere and corona, in particular to reproduce type II spicules. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Testa, Paola; Pontieu, Bart De; Martinez-Sykora, Juan; Hansteen, Viggo & Carlsson, Mats (2012). INVESTIGATING THE RELIABILITY OF CORONAL EMISSION MEASURE DISTRIBUTION DIAGNOSTICS USING THREE-DIMENSIONAL RADIATIVE MAGNETOHYDRODYNAMIC SIMULATIONS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 758(1). doi: 10.1088/0004-637X/758/1/54. Full text in Research Archive
• Martinez-Sykora, Juan (2012). Two Types of Spicules “Observed” in 3D Realistic Models. Astronomical Society of the Pacific Conference Series. ISSN 1050-3390. 454, p. 133–136. Full text in Research Archive
• Martinez-Sykora, Juan; Pontieu, Bart De & Hansteen, Viggo (2012). TWO-DIMENSIONAL RADIATIVE MAGNETOHYDRODYNAMIC SIMULATIONS OF THE IMPORTANCE OF PARTIAL IONIZATION IN THE CHROMOSPHERE. The Astrophysical Journal (ApJ). ISSN 0004-637X. 753(2). doi: 10.1088/0004-637X/753/2/161. Full text in Research Archive
• Dalda, A. S.; Martinez-Sykora, Juan; Rubio, L. R. Bellot & Title, A. (2012). STUDY OF SINGLE-LOBED CIRCULAR POLARIZATION PROFILES IN THE QUIET SUN. The Astrophysical Journal (ApJ). ISSN 0004-637X. 748(1). doi: 10.1088/0004-637X/748/1/38. Full text in Research Archive
• Gudiksen, Boris Vilhelm; Carlsson, Mats; Hayek, Wolfgang; Hansteen, Viggo; Leenaarts, Jorrit & Martinez-Sykora, Juan (2011). The stellar atmosphere simulation code Bifrost. Code description and validation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 531. doi: 10.1051/0004-6361/201116520. Full text in Research Archive
• Martinez-Sykora, Juan; Pontieu, Bart De; Testa, Paola & Hansteen, Viggo (2011). FORWARD MODELING OF EMISSION IN SOLAR DYNAMICS OBSERVATORY/ATMOSPHERIC IMAGING ASSEMBLY PASSBANDS FROM DYNAMIC THREE-DIMENSIONAL SIMULATIONS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 743(1). doi: 10.1088/0004-637X/743/1/23. Full text in Research Archive
• Tian, H; McIntosh, S.W.; Pontieu, Bart De; Martinez-Sykora, Juan; Sechler, M. & Wang, X. (2011). TWO COMPONENTS OF THE SOLAR CORONAL EMISSION REVEALED BY EXTREME-ULTRAVIOLET SPECTROSCOPIC OBSERVATIONS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 738(1). doi: 10.1088/0004-637X/738/1/18. Full text in Research Archive
• Martinez-Sykora, Juan; Pontieu, Bart De; Hansteen, Viggo & McIntosh, Scott (2011). WHAT DO SPECTRAL LINE PROFILE ASYMMETRIES TELL US ABOUT THE SOLAR ATMOSPHERE? The Astrophysical Journal (ApJ). ISSN 0004-637X. 732(2). doi: 10.1088/0004-637X/732/2/84. Full text in Research Archive
• Martinez-Sykora, Juan; Hansteen, Viggo & Moreno-Insertis, F. (2011). ON THE ORIGIN OF THE TYPE II SPICULES: DYNAMIC THREE-DIMENSIONAL MHD SIMULATIONS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 736(1). doi: 10.1088/0004-637X/736/1/9. Full text in Research Archive
• Pontieu, Bart De; McIntosh, S.W.; Carlsson, Mats; Hansteen, Viggo; Tarbell, T.D. & Boerner, P. [Show all 9 contributors for this article] (2011). The Origins of Hot Plasma in the Solar Corona. Science. ISSN 0036-8075. 331(6013), p. 55–58. doi: 10.1126/science.1197738. Full text in Research Archive
• Sykora, Juan Martinez; Hansteen, Viggo & Carlsson, Mats (2009). TWISTED FLUX TUBE EMERGENCE FROM THE CONVECTION ZONE TO THE CORONA. II. LATER STATES. The Astrophysical Journal (ApJ). ISSN 0004-637X. 702(1), p. 129–140. doi: 10.1088/0004-637X/702/1/129. Full text in Research Archive
• Sykora, Juan Martinez; Hansteen, Viggo; Pontieu, Bart De & Carlsson, Mats (2009). SPICULE-LIKE STRUCTURES OBSERVED IN THREE-DIMENSIONAL REALISTIC MAGNETOHYDRODYNAMIC SIMULATIONS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 701(2), p. 1569–1581. doi: 10.1088/0004-637X/701/2/1569. Full text in Research Archive
• Sykora, Juan Martinez; Hansteen, Viggo & Carlsson, Mats (2008). Twisted flux tube emergence from the convection zone to the corona. The Astrophysical Journal (ApJ). ISSN 0004-637X. 679, p. 871–888. Full text in Research Archive

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• Pereira, Tiago & Martinez-Sykora, Juan (2024). MUSE synthesis tools. Full text in Research Archive
• Furuseth, Sondre Vik; Cherry, George & Martinez-Sykora, Juan (2024). Benchmarking Solar Simulations: An Analytical Solution for Non-Linear Diffusivity. Full text in Research Archive
• Furuseth, Sondre Vik; Cherry, George & Martinez-Sykora, Juan (2024). Benchmarking Astrophysical Simulations: An Analytical Solution for Non-Linear Diffusivity. Full text in Research Archive
• Martinez-Sykora, Juan (2022). Modeling of small-scale phenomena. Full text in Research Archive
• Martinez-Sykora, Juan (2022). RMHD simulations of the chromosphere with NEQ ionization and ion-neutral interaction effects. Full text in Research Archive
• Martinez-Sykora, Juan (2022). New insights into the transition region from IRIS observations and numerical simulations that include non-equilibrium ionization. Full text in Research Archive
• Martinez-Sykora, Juan (2022). Multilfuid Alfven wave simulations to understand the chemical fractionation in the chromosphere and the role of the NEQ ionization. Full text in Research Archive
• Martinez-Sykora, Juan (2019). Ebysus: Multi-fluid & multi-species numerical code. Full text in Research Archive
• Martinez-Sykora, Juan (2019). Beyond single fluid or two fluids: Multi-component (multi-species) and multi fluid effects in the solar atmosphere. Full text in Research Archive
• Martinez-Sykora, Juan (2019). Ebysus: a multi-fluid and multi-species numerical code: on coupling between ionized species. Full text in Research Archive
• Sykora, Juan Martinez (2009). Flux emergence from the convection zone to the corona. Universitetet i Oslo. Full text in Research Archive

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