Observation and Modeling of High-temperature Solar Active Region Emission during the High-resolution Coronal Imager Flight of 2018 May 29

Abstract

Excellent coordinated observations of NOAA active region 12712 were obtained during the flight of the Highresolution Coronal Imager (Hi-C) sounding rocket on 2018 May 29. This region displayed a typical active region core structure with relatively short, high-temperature loops crossing the polarity inversion line and bright "moss" located at the footpoints of these loops. The differential emission measure (DEM) in the active region core is very sharply peaked at about 4 MK. Further, there is little evidence for impulsive heating events in the moss, even at the high spatial resolution and cadence of Hi-C. This suggests that active region core heating is occurring at a high frequency and keeping the loops close to equilibrium. To create a time-dependent simulation of the active region core, we combine nonlinear force-free extrapolations of the measured magnetic field with a heating rate that is dependent on the field strength and loop length and has a Poisson waiting time distribution. We use the approximate solutions to the hydrodynamic loop equations to simulate the full ensemble of active region core loops for a range of heating parameters. In all cases, we find that high-frequency heating provides the best match to the observed DEM. For selected field lines, we solve the full hydrodynamic loop equations, including radiative transfer in the chromosphere, to simulate transition region and chromospheric emission. We find that for heating scenarios consistent with the DEM, classical signatures of energy release, such as transition region brightenings and chromospheric evaporation, are weak, suggesting that they would be difficult to detect.

NuSTAR Observation of Energy Release in 11 Solar Microflares

“…This could imply the observation of emission from cospatial thermal components or, in the nonthermal interpretation, a situation in which loop-top flare-accelerated electrons are thermalized before reaching the loop footpoints (as was concluded in Glesener et al 2020). A similar lack of spatial complexity was also seen in the NuSTAR microflare examined in Glesener et al (2017), while, contrastingly, FOXSI HXR microflare emission was shown to be spatially complex in Vievering (2019). Further studies involving a greater number of HXR microflares of A class and below are necessary to determine the relative incidence of these two contrasting results and investigate if they are connected to other microflare properties.…”

Multi-Scale Variability of Coronal Loops Set by Thermal Non-Equilibrium and Instability as a Probe for Coronal Heating

“…Hydrodynamic models suggest a steady (high-frequency) heating (Winebarger et al, 2011) with loops close to equilibrium, satisfying the RTV scaling law (Rosner et al, 1978;Martens, 2010). This is supported partly by observations displaying very little intensity variability in the moss region (Warren et al, 2020), while other observations of active region cores provide evidence of strong variability, linked to either coronal nanoflares or magnetic flux cancellation events (Testa et al, 2014;Chitta et al, 2018;Tiwari et al, 2019).…”

The source of the major solar energetic particle events from super active region 11944

“…The red contours mostly surround the bright moss emission in the AR core. Moss corresponds to the footpoints of high temperature (∼4 MK) loops ( 32 ). It means that plasma with the highest Si/S abundance ratios, a signature that is detected by Wind in the SEP events, is produced at the footpoints of the highest temperature loops in the AR.…”