Xiaoyan Xie

We investigate the thermal properties of a solar flare by the observations of the soft X-ray Telescope on board Hinode and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. Our analysis reveals a tenuous but hot plasma cloud located above the flare loops. The flare loops and plasma cloud exhibit independent temperature profiles as a function of time, implying that different heating mechanisms are present in these structures. We interpret the plasma cloud as an eruptive filament directly heated in the corona via magnetic reconnection that occurs when a rising filament interacts with this eruptive filament. Previous disk-integrated solar soft X-ray spectral measurements of this flare using the Miniature X-ray Solar Spectrometer CubeSat-1 (MinXSS-1) inferred coronal abundances at the flare peak. However, MinXSS synthetic spectra from AIA observations show that the plasma cloud is too tenuous to be detected in MinXSS-1; thus, the coronal abundances of low first ionization potential elements must be in emission from the flare loops. Furthermore, we demonstrate the nonnegligible differences in differential emission measures between coronal and photospheric abundance models. These results highlight the importance of instruments capable of simultaneous spectral and spatial diagnostics during large-scale solar events. Such coordinated measurements would enable more accurate thermal and compositional analyses and deeper insight into the underlying physical mechanisms.

We investigate the aftermath of a giant quiescent solar filament eruption on 2023 December 24. One feature of the eruption is an extensive fan above the filament channel that is about 3 times as wide as similar structures that appear above active regions (ARs) during solar flares. The fan contains numerous supra-arcade downflows (SADs), and we investigate the largest SADs with continuous Hinode X-ray Telescope observations. The measured maximum width of the SADs in this event is at least 3 times the maximum width of SADs observed in AR flares, whereas the velocities of the largest SADs are similar to the typical values of AR SADs. The kinetic characteristics of the largest SADs observed in this event align with previous model predictions, where SADs originate from the nonlinear development of Rayleigh–Taylor-type instabilities. In this scenario, the larger system size allows the existence of larger-scale instabilities, while the development of the velocities of these instabilities is expected to be independent of the system size. Compared to AR flares, the temperature and emission measure in this event are lower, and there is less overall radiation, resulting in no evident Geostationary Operational Environmental Satellite signature. Similar to those in AR flares, SADs show lower temperatures compared to the surrounding fan plasma. Our observations show that SADs are present in a wide variety of eruptions. The reconnection mechanisms present in quiescent filament eruptions are similar to those driving more compact eruptions originating from ARs.

We investigate the thermal properties of a solar flare by the observations of soft X-ray Telescope (XRT) on board Hinode and the Atmospheric Image Assembly on board the Solar Dynamics Observatory (SDO/AIA). Our analysis reveals a tenuous but hot plasma cloud located above the flare loops. The flare loops and plasma cloud exhibit independent temperature profiles as a function of time, implying different heating mechanisms are present in these structures. We interpret the plasma cloud as an eruptive filament directly heated in the corona via magnetic reconnection that occurs when a rising filament interacts with this eruptive filament. Previous disk-integrated solar soft X-ray spectral measurements of this flare using the Miniature X-ray Solar Spectrometer CubeSat-1 (MinXSS-1) inferred coronal abundances at the flare peak. However, MinXSS synthetic spectra from AIA observations show that the plasma cloud is too tenuous to be detected in MinXSS-1 and thus the coronal abundances of low FIP elements must be in emission from the flare loops. Furthermore, we demonstrate the non-negligible differences in differential emission measures (DEMs) between coronal and photospheric abundance models. These results highlight the importance of instruments capable of simultaneous spectral and spatial diagnostics during large-scale solar events. Such coordinated measurements would enable more accurate thermal and compositional analyses and deeper insight into the underlying physical mechanisms.

Recent observations and simulations indicate that solar flares undergo extremely complex 3D evolution, making 3D particle transport models essential for understanding electron acceleration and interpreting flare emissions. In this study, we investigate this problem by solving Parker’s transport equation with 3D MHD simulations of solar flares. By examining energy conversion in the 3D system, we evaluate the roles of different acceleration mechanisms, including reconnection current sheet (CS), termination shock (TS), and supra-arcade downflows (SADs). We find that large-amplitude turbulent fluctuations are generated and sustained in the 3D system. The model results demonstrate that a significant number of electrons are accelerated to hundreds of keV and even a few MeV, forming power-law energy spectra. These energetic particles are widely distributed, with concentrations at the TS and in the flare looptop region, consistent with results derived from recent hard X-ray (HXR) and microwave (MW) observations. By selectively turning particle acceleration on or off in specific regions, we find that the CS and SADs effectively accelerate electrons to several hundred keV, while the TS enables further acceleration to MeV. However, no single mechanism can independently account for the significant number of energetic electrons observed. Instead, the mechanisms work synergistically to produce a large population of accelerated electrons. Our model provides spatially and temporally resolved electron distributions in the whole flare region and at the flare footpoints, enabling synthetic HXR and MW emission modeling for comparison with observations. These results offer important insights into electron acceleration and transport in 3D solar flare regions.

We investigate the aftermath of a giant quiescent solar filament eruption on December 24, 2023. One feature of the eruption is an extensive fan above the filament channel that is about three times as wide as similar structures that appear above active regions (ARs) during solar flares. The fan contains numerous supra-arcade downflows (SADs), and we investigate the largest SADs with continuous Hinode X-ray Telescope (XRT) observations. The measured maximum width of the SADs in this event is at least three times the maximum width of SADs observed in AR flares, whereas the velocities of the largest SADs are similar to the typical values of AR SADs. The kinetic characteristics of the largest SADs observed in this event align with previous model predictions, where SADs originate from the non-linear development of Rayleigh-Taylor type instabilities. In this scenario, the larger system size allows the existence of larger-scale instabilities, while the development of the velocities of these instabilities is expected to be independent of the system size. Compared to AR flares, the temperature and emission measure in this event are lower, and there is less overall radiation, resulting in no evident Geostationary Operational Environmental Satellite (GOES) signature. Similar to those in AR flares, SADs show lower temperatures compared to the surrounding fan plasma. Our observations show that SADs are present in a wide variety of eruptions. The reconnection mechanisms present in quiescent filament eruptions are similar to those driving more compact eruptions originating from ARs.

Recent observations and simulations indicate that solar flares undergo extremely complex three-dimensional (3D) evolution, making 3D particle transport models essential for understanding electron acceleration and interpreting flare emissions. In this study, we investigate this problem by solving Parker's transport equation with 3D MHD simulations of solar flares. By examining energy conversion in the 3D system, we evaluate the roles of different acceleration mechanisms, including reconnection current sheet (CS), termination shock (TS), and supra-arcade downflows (SADs). We find that large-amplitude turbulent fluctuations are generated and sustained in the 3D system. The model results demonstrate that a significant number of electrons are accelerated to hundreds of keV and even a few MeV, forming power-law energy spectra. These energetic particles are widely distributed, with concentrations at the TS and in the flare looptop region, consistent with results derived from recent hard X-ray (HXR) and microwave (MW) observations. By selectively turning particle acceleration on or off in specific regions, we find that the CS and SADs effectively accelerate electrons to several hundred keV, while the TS enables further acceleration to MeV. However, no single mechanism can independently account for the significant number of energetic electrons observed. Instead, the mechanisms work synergistically to produce a large population of accelerated electrons. Our model provides spatially and temporally resolved electron distributions in the whole flare region and at the flare footpoints, enabling synthetic HXR and MW emission modeling for comparison with observations. These results offer important insights into electron acceleration and transport in 3D solar flare regions.

Solar flare above-the-loop-top (ALT) regions are vital for understanding solar eruptions and fundamental processes in plasma physics. Recent advances in three-dimensional (3D) magnetohydrodynamic (MHD) simulations have revealed unprecedented details on turbulent flows and MHD instabilities in flare ALT regions. Here, for the first time, we examine the observable anisotropic properties of turbulent flows in ALT by applying a flow-tracking algorithm on narrow-band extreme-ultraviolet images that are observed from the face-on viewing perspective. First, the results quantitatively confirm the previous observation that vertical motions dominate and that the anisotropic flows are widely distributed in the entire ALT region with the contribution from both upflows and downflows. Second, the anisotropy shows height-dependent features, with the most substantial anisotropy appearing at a certain middle height in ALT, which agrees well with the MHD modeling results where turbulent flows are caused by Rayleigh–Taylor-type instabilities in the ALT region. Finally, our finding suggests that supra-arcade downflows (SADs), the most prominently visible dynamical structures in ALT regions, are only one aspect of turbulent flows. Among these turbulent flows, we also report the antisunward-moving underdense flows that might develop due to MHD instabilities, as suggested by previous 3D flare models. Our results indicate that the entire flare fan displays group behavior of turbulent flows where the observational bright spikes and relatively dark SADs exhibit similar anisotropic characteristics.

Solar flare above-the-loop-top (ALT) regions are vital for understanding solar eruptions and fundamental processes in plasma physics. Recent advances in 3D MHD simulations have revealed unprecedented details on turbulent flows and MHD instabilities in flare ALT regions. Here, for the first time, we examine the observable anisotropic properties of turbulent flows in ALT by applying a flow-tracking algorithm on narrow-band Extreme Ultraviolet (EUV) images that are observed from the face-on viewing perspective. First, the results quantitatively confirm the previous observation that vertical motions dominate and that the anisotropic flows are widely distributed in the entire ALT region with the contribution from both upflows and downflows. Second, the anisotropy shows height-dependent features, with the most substantial anisotropy appearing at a certain middle height in ALT, which agrees well with the MHD modeling results where turbulent flows are caused by Rayleigh-Taylor-type instabilities in the ALT region. Finally, our finding suggests that supra-arcade downflows (SADs), the most prominently visible dynamical structures in ALT regions, are only one aspect of turbulent flows. Among these turbulent flows, we also report the anti-sunward-moving underdense flows that might develop due to MHD instabilities, as suggested by previous three-dimensional flare models. Our results indicate that the entire flare fan displays group behavior of turbulent flows where the observational bright spikes and relatively dark SADs exhibit similar anisotropic characteristics.

We deliberately select three flares to investigate heating effects of supra-arcade downflows (SADs) on the surrounding fan plasma. Prior work found in one flare that the plasma around most SADs tends to heat up or stay the same temperature, accompanied by discernible signatures of the adiabatic heating due to plasma compression as well as viscous heating due to viscous motions of plasma. We extend this work to more flares and find that the heating effects of the SADs are also present in these events. The adiabatic heating is dominant over the viscous heating in each event. The adiabatic heating in the two M1.3 flares, being on the order of about 0.02–0.18 MK s⁻¹, is fairly comparable. In the more energetic X1.7 flare, the adiabatic heating is on the order of 0.02–0.3 MK s⁻¹, where we observe a more pronounced temperature increase during which dozens of SADs descend through the fan. As SADs constantly contribute to the heating of the surrounding fan plasma, the areas where SADs travel through tend to cool much slower than the areas without SADs, and the plasma of higher temperature ends up concentrating in areas where SADs frequently travel through. We also find that the cooling rate of areas without SADs is ∼1000 K s⁻¹, much slower than would be expected from normal conductive cooling. Instead, the cooling rate can be interpreted nicely by a process where conductive cooling is suppressed by turbulence.