New Methods for Solar Hard X-ray Imaging Analysis with the FOXSI Sounding Rocket Experiment

Spectroscopic imaging in the hard X-ray (HXR) regime is essential for investigating the sites of energy release and transfer in solar flares, as HXRs provide insight into the acceleration of electrons and plasma heating. The Focusing Optics X-ray Solar Imager ( FOXSI ) sounding rocket experiment is the first solar-dedicated direct imager for HXRs and offers unprecedented capabilities for high-energy imaging spectroscopy of flares. Through use of grazing incidence optics, FOXSI achieves greater sensitivity and imaging dynamic range than its esteemed predecessor, the Reuven Ramaty High-Energy Solar Spectroscopic Imager ( RHESSI ), which was limited by the high background inherent to its indirect imaging technique. Additionally, FOXSI improves upon the Nuclear Spectroscopic Telescope Array ( NuSTAR ) with an angular resolution at scales relevant for studying flares and a higher detector throughput for measuring solar sources. Here we demonstrate the power of these enhanced capabilities through imaging analyses of two sub-A class microflares observed by the second FOXSI sounding rocket ( FOXSI-2 ) on December 11, 2014. We apply a custom FOXSI image deconvolution method using a maximum likelihood procedure to explore the changing morphology of these small scale-events and compare the results to features observed in contemporaneous data from the Solar Dynamics Observatory Atmospheric Imaging Assembly ( SDO /AIA). Additionally, we perform imaging spectroscopy to study the spatial complexity of energy release in these microflares and assess whether they are more similar in evolution to large flares or nanoflares.