FOXSI-4: Instrument Upgrades for a Proposed Fourth Focusing Optics X-Ray Solar Imager Sounding Rocket Experiment

Observations of the Sun in hard X-rays can provide insight into many solar phenomena, including the mechanisms behind energy release and transport in flares. The indirect imaging methods used by RHESSI , the previous state-of-the-art solar hard X-ray instrument, however, were fundamentally limited in sensitivity and imaging dynamic range . By instead using the direct imaging technique of focusing hard X-rays, the structure and evolution of faint coronal sources, including microflares, active regions, and flare acceleration sites, can be investigated in greater depth. FOXSI ( Focusing Optics X-ray Solar Imager ), a hard X-ray instrument flown on three sounding rocket campaigns to date, seeks to achieve these improved capabilities by using focusing optics for solar observations in the 4-20 keV range. A fourth FOXSI sounding rocket experiment, FOXSI-4, has been proposed for launch in 2023, and the planned instrument upgrades are presented here. One main focus of FOXSI-4 will be to improve the angular resolution for solar hard X-ray instruments through the use of high-resolution optics, with the goal of reaching a half power diameter (HPD) of ∼5". Higher resolution will allow for individual footpoints and acceleration sites to be distinguished during a flare, which would benefit studies of the chromospheric response and provide constraints on flare acceleration mechanisms. FOXSI-4 will ideally be part of a flare campaign, in which multiple solar sounding rockets would be launched one after another to demonstrate the strength of new solar technologies when utilized in conjunction for flare observations.