The Sun offers a convenient nearby laboratory to study the physical processes of particle acceleration and impulsive energy release in magnetized plasmas that occur throughout the universe, from planetary magnetospheres to black hole accretion disks. Solar flares are the most powerful explosions in the solar system, releasing up to 10³²-10³³ ergs over only 100-1,000 seconds. These events can accelerate electrons up to hundreds of MeV and can heat plasma to tens of MK, exceeding ~40 MK in the most intense flares. The accelerated electrons and the hot plasma each contain tens of percent of the total flare energy, indicating an intimate link between particle acceleration, plasma heating, and flare energy release.

X-ray emission is the most direct signature of these processes; accelerated electrons emit hard X-ray bremsstrahlung as they collide with the ambient atmosphere, while hot plasma emits soft X-rays from both bremsstrahlung and excitation lines of highly-ionized atoms. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes this emission from ~3 keV to ~17 MeV with unprecedented spectral, spatial, and temporal resolution, providing the most precise measurements of the X-ray flare spectrum and enabling the most accurate characterization of the X-ray-emitting hot and accelerated electron populations.

RHESSI observations show that "super-hot" temperatures exceeding ~30 MK are common in large flares but are achieved almost exclusively by X-class events and appear to be strictly associated with coronal magnetic field strengths exceeding ~170 Gauss; these results suggest a direct link between the magnetic field and heating of super-hot plasma, and that super-hot flares may require a minimum threshold of field strength and overall flare intensity.

Imaging and spectroscopic observations of the 2002 July 23 X4.8 event show that the super-hot plasma is both spectrally and spatially distinct from the usual ~10-20 MK plasma observed in nearly all flares, and is located above rather than at the top of the loop containing the cooler plasma. It exists with high density even during the pre-impulsive phase, which is dominated by coronal non-thermal emission with negligible footpoints, suggesting that particle acceleration and plasma heating are intrinsically related but that, rather than the traditional picture of chromospheric evaporation, the origins of super-hot plasma may be the compression and subsequent thermalization of ambient material accelerated in the reconnection region above the flare loop, a physically-plausible process not detectable with current instruments but potentially observable with future telescopes. Explaining the origins of super-hot plasma would thus ultimately help to understand the mechanisms of particle acceleration and impulsive energy release in solar flares.