The solar corona, at temperatures of ~1-2 megaKelvin (MK), is significantly hotter, by about 2 orders of magnitude, than the chromosphere and photosphere. Temperatures above active regions are typically even higher, ~4-6 MK, while during solar flares, plasma temperatures can rise up to 30-50 MK, possibly hotter. Exactly how this plasma is heated, during flares or during quiescent periods, remains the subject of much debate. Standard flare models typically consider plasma heating from “chromospheric evaporation,” where chromospheric plasma is heated by collisional energy losses from accelerated particles and then “evaporates” into the corona, but recent results suggest a significant in situ mechanism directly heating coronal plasma. For the quiescent corona, a leading candidate for heating above active regions is the nanoflare model, whereby many tiny impulsive, flare-like events occur in rapid succession, although because these events are unresolved, it is not known where in the atmosphere the heating occurs. Moreover, while the Sun’s magnetic field is commonly accepted to be the energy source for both flares and nanoflares, the specific connection between these two opposite regimes of energy release has not been well established.
We discuss recent results from RHESSI, using soft and hard X-ray observations to examine thermal plasma in intense solar flares, especially ones that reach temperatures >30 MK. We also present observations of both the quiet and active non-flaring Sun from a new soft X-ray spectrometer flown on two SDO/EVE sounding rocket missions, and scheduled to fly on an upcoming CubeSat mission. We discuss the implications of these results for plasma heating, both during flares and for quiescent times.