Leon Golub

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

We present observations of an eruptive solar flare on 2016 January 6 that occurred behind the solar limb from the perspective of the Earth, but was well observed by the Solar-Terrestrial Relations Observatory (or STEREO) and the Solar Extreme Ultraviolet Monitor on NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Light curves showing the evolution of the flare’s irradiance as a function of time taken by MAVEN are well correlated with the brightness evolution of fan structures observed in the Project for On-Board Autonomy 2's Sun Watcher with Active Pixels and Image Processing (SWAP) 174 Å passband, suggesting that the radiance of structures near the flare site was influenced by emission from the flare. Because SWAP did not directly observe the flare itself, this event represents a rare opportunity to study the evolution of emission near a flare without the risk of instrumental scattered light contaminating the observations. We analyze this evolution and implement a simple model to explore the possibility that resonant excitation (RE, also known as resonant scattering) plays an important role in driving coronal extreme-ultraviolet (EUV) emission during flaring events. Our modeling shows that for a large flare, RE could increase emission from nearby structures by about 45%, consistent with our findings that the involved structures observed by SWAP increased in brightness by about 60% during the flare. We conclude that RE may play an important role in driving coronal EUV emission under certain circumstances and should be accounted for in models and emission-based analysis tools.

We present observations of an eruptive solar flare on 2016 January 6 that occurred behind the solar limb from the perspective of the Earth, but was well observed by STEREO and the Solar Extreme Ultraviolet Monitor on the Mars Atmosphere and {Volatile} EvolutioN (MAVEN) mission. Light curves showing the evolution of the flare's irradiance as a function of time taken by MAVEN are well correlated with the brightness evolution of fan structures observed in the PROBA2 SWAP 174 Å passband, suggesting that the radiance of structures near the flare site was influenced by emission from the flare. Because SWAP did not directly observe the flare itself, this event represents a rare opportunity to study the evolution of emission near a flare without the risk of instrumental scattered light contaminating the observations. We analyze this evolution and implement a simple model to explore the possibility that resonant excitation (or resonant scattering) plays an important role in driving coronal EUV emission during flaring events. Our modeling shows that for a large flare, resonant excitation could increase emission from nearby structures by about 45%, consistent with our findings that the involved structures observed by SWAP increased in brightness by about 60% during the flare. We conclude that resonant excitation may play an important role in driving coronal EUV emission under certain circumstances and should be accounted for in models and emission-based analysis tools.

Nanoflares are thought to be one of the prime candidates that can heat the solar corona to its multimillion kelvin temperature. Individual nanoflares are difficult to detect with the present generation of instruments, but their presence can be inferred by comparing simulated nanoflare-heated plasma emissions with the observed emission. Using HYDRAD coronal loop simulations, we model the emission from an X-ray bright point (XBP) observed by the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), along with the nearest available observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) and the X-Ray Telescope (XRT) on board the Hinode observatory. The length and magnetic field strength of the coronal loops are derived from the linear force-free extrapolation of the observed photospheric magnetogram by the Helioseismic and Magnetic Imager on board SDO. Each loop is assumed to be heated by random nanoflares, whose magnitude and frequency are determined by the loop length and magnetic field strength. The simulation results are then compared and matched against the measured intensity from AIA, XRT, and MaGIXS. Our model results indicate the observed emission from the XBP under study could be well matched by a distribution of nanoflares with average delay times 1500–3000 s. Further, we demonstrate the high sensitivity of MaGIXS and XRT for diagnosing the heating frequency using this method, while AIA passbands are found to be the least sensitive.

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