Graham Stewart Kerr

Physical processes in solar flares span many orders of magnitude in spatiotemporal scales, so a single unified model accounting for all relevant processes remains computationally intractable. Instead, specialized codes have been developed, targeting specific domains that tackle specific aspects of the solar flare problem. Here, we present a comprehensive framework that links three models to capture flare initiation and energy release, the subsequent particle acceleration, the propagation and thermalization of those particles, and ultimately the heating of the lower solar atmosphere. We use the 2.5D ARMS magnetohydrodynamics (MHD) code to model stressing of the magnetic field in a simulated active region, the formation of a current sheet, magnetic reconnection, and the evolution of newly formed flare loops. We use the conditions in the ARMS current sheet to initialize the kglobal model, which predicts time-dependent nonthermal electron and proton distributions. Finally, we use RADYN+FP to inject these distributions into a newly reconnected ARMS loop and compute subsequent particle transport, atmospheric heating, and radiative emissions. This is the first detailed presentation of a RADYN+FP simulation tracking nonthermal electrons and protons in the same loop. As well as discussing the interesting physics that takes place, we note how this results in white-light flare emission height consistent with observations. This work is a first step toward building a fully 3D flare modeling framework.

Wave–plasma interactions and energy transport are fundamental processes in stellar atmospheres, shaping elemental composition through the first ionization potential (FIP) and inverse FIP (IFIP) effects. Although stellar measurements provide global evidence of abundance anomalies, the Sun offers a unique local laboratory in which to resolve how these processes operate on small spatial and temporal scales. In this overview, we summarize the current state of knowledge of the IFIP effect, its observational signatures and the theoretical framework that underpins it. We then present new insights from a detailed case study of active region (AR) NOAA AR 11967, where combined Hinode/extreme-ultraviolet imaging spectrometer (EIS), IRIS, SDO/AIA and Fermi/GBM observations and IRIS2+ inversions indicate that torsional Alfvén waves generated below the chromospheric fractionation region can account for the highly localized IFIP effect plasma observed on the Sun. This analysis highlights the possible role of sub-chromospheric processes in the formation of the IFIP effect on the Sun and informs pathways for understanding similar mechanisms in the coronae of active M dwarfs. This article is part of the Theo Murphy meeting issue ‘Solar atmospheric abundances in space and time’.

The origin of the extreme broadening observed in chromospheric metal lines during solar and stellar flares—particularly Mg ii h and k and Ca ii H and K—remains poorly understood. These lines often display Lorentzian-like wings whose widths exceed standard Stark broadening predictions by factors of ∼30, with no known collisional mechanism capable of producing such enhancements. We posit that magnetic fields are responsible for this additional broadening, due to the increase of magnetic activity during flares. A magnetic field distribution of the form P ( B ) ∝ B ⁻³ reproduces the observed Mg ii profile wings while leaving H i Balmer lines and optically thin transitions largely unaffected. To explain the broadening using magnetic fields, the high- B tail can extend up to 10 ⁶ G with extremely low probabilities where the filling factors are ≲10 ⁻⁶ . We propose that observations of flares using spectropolarimetry can verify whether the anomalous broadening is from magnetic structures in flare ribbons.

Solar flares release a tremendous amount of magnetic energy that subsequently manifests in several forms; the bulk of this energy is transported through the Sun’s atmosphere and explosively heats the chromosphere. While hard X-ray observations have pointed to flare-accelerated electrons as a primary means by which energy is transported following flares, alternative processes undoubtedly act alongside, or even instead of, those energetic electrons. To shed light on this we analysed flare-optimized, high-cadence Solar Orbiter observations. Footpoints from two flare ribbons were observed by the Spectral Imaging of the Coronal Environment (SPICE) instrument. Curiously, those footpoints exhibited contrasting behaviour: one had short-lived yet strong decreases in the Lyman β/Lyman γ line intensity ratio, whereas the other exhibited a more prolonged, moderate dip in that ratio. These observations were compared to synthetic spectra from radiation hydrodynamic simulations of flares driven by various energy transport mechanisms. This revealed that one footpoint was driven by energetic particle precipitation, while the other was driven by enhanced thermal heat flux. The implication is that energetic particles do not dominate along the entirety of flare ribbons. Critically, we must now focus on understanding where, when and why different mechanisms dominate in solar flare energy transport.

Between 2017 and 2024, the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory has observed numerous white-light solar flares (WLFs). HMI spectropolarimetric observations of certain WLFs, in particular the X9.3 flare of 2017 September 6, reveal one or more locations within the umbra or along the umbra/penumbra boundary of the flaring active region where the Fe I 6173 Å line briefly goes into full emission, indicating significant heating of the photosphere and lower chromosphere. For five flares featuring Fe I 6173 Å line-core emission, we perform spectropolarimetric analysis using HMI 90 s cadence Stokes data. For all investigated flares, line-core emission is observed to last for a single 90 s frame and is either concurrent with or followed by an increase in the line continuum intensity lasting one to two frames (90–180 s). Additionally, permanent changes to the Stokes Q, U, and/or V profiles were observed, indicating long-lasting nontransient changes to the photospheric magnetic field. These emissions coincided with local maxima in hard X-ray emission observed by Konus-Wind, as well as local maxima in the time derivative of soft X-ray emission observed by GOES 16-18. Comparison of the Fe I 6173 Å line profile synthesis for the ad hoc heating of the initial empirical VAL-S umbra model and quiescent-Sun (VAL-C-like) model indicates that the Fe I 6173 Å line emission in the white-light flare kernels could be explained by the strong heating of initially cool photospheric regions.

The first simultaneous observations of the Fe xviii 974.86 Å and Fe xx 721.56 Å forbidden lines from the Spectral Imaging of the Coronal Environment (SPICE) spectrograph on Solar Orbiter are presented. The lines were observed from the postflare loops of an M2.5 class solar flare that peaked at 23:49 UT on 2024 March 23. The Fe xx/Fe xviii ratio is used to derive a temperature spacetime map for the flaring period, with values ranging from 8 to 20 MK. The map reveals repeated episodes of heating at the SPICE slit location over a 30 minute period. For one location with the brightest emission, the plasma cooled from 10 to 8 MK in 5 minutes, which is longer than the expected conductive cooling time of 170 s, suggesting continued background heating during the cooling period. Doppler shifts of between 0 and +10 km s⁻¹ were obtained with precisions of 1–4 km s⁻¹, but the accuracies are lower due to uncertainties over the absolute wavelength calibration; hence, we cannot conclude there are plasma flows in the flare loops. The widths of the two lines were found to be close to the instrumental widths with no evidence of nonthermal broadening although this result is limited by the instrument resolution. The Fe xviii and Fe xx lines have high signal-to-noise with only a 5 s exposure time, demonstrating that the lines will be valuable for high-cadence flare studies with SPICE.

Recent high-cadence flare campaigns by the Interface Region Imaging Spectrograph (IRIS) have offered new opportunities to study rapid processes characteristic of flare energy release, transport, and deposition. Here, we examine high-cadence chromospheric and transition region spectra acquired by IRIS during a C-class flare from 2022 September 25. Within the flare ribbon, the intensities of the Si iv 1402.77 Å, C ii 1334.53 Å and Mg ii k 2796.35 Å lines peaked at different times, with the transition region Si iv typically peaking before the chromospheric Mg ii line by 1–6 s. To understand the nature of these delays, we probed a grid of radiative hydrodynamic flare simulations heated by electron beams, thermal conduction-only, or Alfvén waves. Electron beam parameters were constrained by hard X-ray observations from the Gamma-ray Burst Monitor (GBM) onboard the Fermi spacecraft. Reproducing light curves where Si iv peaks precede those in Mg ii proved to be a challenge because only a subset of Fermi/GBM-constrained electron beam models were consistent with the observations. Light curves with relative timings consistent with the observations were found in simulations heated by either high-flux electron beams or by Alfvén waves, while the thermal conduction heating does not replicate the observed delays. Our analysis shows how delays between chromospheric and transition region emission pose tight constraints on flare models and properties of energy transport, highlighting the importance of obtaining very high-cadence data sets with IRIS and other observatories.

The first simultaneous observations of the Fe XVIII 974.86 Å and Fe XX 721.56 Å forbidden lines from the Spectral Imaging of the Coronal Environment (SPICE) spectrograph on Solar Orbiter are presented. The lines were observed from the post-flare loops of an M2.5 class solar flare that peaked at 23:49 UT on 2024 March 23. The Fe XX/Fe XVIII ratio is used to derive a temperature space-time map for the flaring period, with values ranging from 8 to 20 MK. The map reveals repeated episodes of heating at the SPICE slit location over a 30 min period. For one location with the brightest emission, the plasma cooled from 10 MK to 8 MK in 5 min which is longer than the expected conductive cooling time of 170 s, suggesting continued background heating during the cooling period. Doppler shifts of between 0 and +10 km/s were obtained with precisions of 1-4 km/s, but the accuracies are lower due to uncertainties over the absolute wavelength calibration hence we can not conclude there are plasma flows in the flare loops. The widths of the two lines were found to be close to the instrumental widths with no evidence of non-thermal broadening, although this result is limited by the instrument resolution. The Fe XVIII and Fe XX lines have high signal-to-noise with only a 5 s exposure time, demonstrating that the lines will be valuable for high-cadence flare studies with SPICE.

Recent high-cadence flare campaigns by the Interface Region Imaging Spectrograph (IRIS) have offered new opportunities to study rapid processes characteristic of flare energy release, transport, and deposition. Here, we examine high-cadence chromospheric and transition region spectra acquired by IRIS during a C-class flare from 2022 September 25. Within the flare ribbon, the intensities of the Si IV 1402.77, C II 1334.53 and Mg II k 2796.35 lines peaked at different times, with the transition region Si IV typically peaking before the chromospheric Mg II line by 1 - 6 seconds. To understand the nature of these delays, we probed a grid of radiative hydrodynamic flare simulations heated by electron beams, thermal conduction-only, or Alfvén waves. Electron beam parameters were constrained by hard X-ray observations from the Gamma-ray Burst Monitor (GBM) onboard the Fermi spacecraft. Reproducing lightcurves where Si IV peaks precede those in Mg II proved to be a challenge, as only a subset of Fermi/GBM-constrained electron beam models were consistent with the observations. Lightcurves with relative timings consistent with the observations were found in simulations heated by either high-flux electron beams or by Alfvén waves, while the thermal conduction heating does not replicate the observed delays. Our analysis shows how delays between chromospheric and transition region emission pose tight constraints on flare models and properties of energy transport, highlighting the importance of obtaining very high-cadence datasets with IRIS and other observatories.

Solar flare ribbon fronts appear ahead of the bright structures that normally characterize solar flares, and can persist for an extended period of time in spatially localized patches before transitioning to “regular” bright ribbons. They likely represent the initial onset of flare energy deposition into the chromosphere. Chromospheric spectra (e.g., He i 10830 Å and the Mg ii near-UV lines) from ribbon fronts exhibit properties rather different to typical flare behavior. In prior numerical modeling efforts we were unable to reproduce the long lifetime of ribbon fronts. Here we present a series of numerical experiments that are rather simple but which have important implications. We inject a very low flux of nonthermal electrons (F = 5 × 10⁸ erg s⁻¹ cm⁻²) into the chromosphere for 100 s before ramping up to standard flare energy fluxes (F = 10¹⁰⁻¹¹ erg s⁻¹ cm⁻²). Synthetic spectra not only sustained their ribbon-front-like properties for significantly longer: in the case of harder nonthermal electron spectra, the ribbon front behavior persisted for the entirety of this weak-heating phase. Lengthening or shortening the duration of the weak-heating phase commensurately lengthened or shortened the ribbon front lifetimes. Ribbon fronts transitioned to regular bright ribbons when the upper chromosphere became sufficiently hot and dense, which happened faster for softer nonthermal electron spectra. Thus, the lifetime of flare ribbon fronts are a direct measure of the duration over which a relatively low flux of high-energy electrons precipitates to the chromosphere prior to the bombardment of a much larger energy flux.