Solar flares can release energy up to 10³²–10³³ ergs in just some minutes.  Magnetic energy released in the corona accelerates electrons, which either escape outward or else stream down flare loops to the chromosphere.  These energetic electrons emit bremsstrahlung HXR radiation which is brightest at the footpoints of the flare loop.

However, coronal sources are typically 10 to 100 times fainter.  Currently, the most sensitive solar HXR images are made by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI).  RHESSI’s indirect imaging is intrinsically limited in sensitivity and dynamic range, so coronal sources can rarely be studied in detail.

The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA Low Cost Access to Space sounding rocket payload for studying faint hard X-ray (HXR) emission from the quiet Sun and flares.

Solar HXR measurements are important for investigating flare particle acceleration. However, imaging faint nonthermal HXR requires a larger sensitivity and dynamic range than is currently available. The focusing optics of FOXSI will achieve a sensitivity 100 times better than that of current solar X-ray instruments (i.e. RHESSI) at energies around 10 keV. FOXSI uses nested-shell, grazing-angle optics and fine-pitch silicon strip detectors to achieve a 0.5 keV energy resolution, an angular resolution of 10 arcseconds (FWHM), and an energy range of 4-15 keV. As a sounding rocket, FOXSI investigates the nonthermal energy content in quiet-Sun nanoflares, which are thought to play an important role in coronal heating. FOXSI is a pathfinder for future solar HXR observing missions, particularly those interested in imaging faint HXR emission from particle acceleration regions in the corona.

FOXSI flew for the first time on November 2, 2012, successfully imaging a solar microflare and searching for faint HXR emission from the quiet Sun.