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Super-hot (T > 30 MK) Thermal Plasma in Solar Flares

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 10^32-10^33 ergs over only 100-1,000 seconds, accelerating electrons up to hundreds of MeV and heating plasma to tens of MK. 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. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observes the X-ray emission from these processes from ~3 keV to ~17 MeV with unprecedented spectral, spatial, and temporal resolution. RHESSI observations show that "super-hot" (>30 MK) plasma temperatures are achieved almost exclusively by intense, GOES X-class flares and appear to be strictly associated with coronal magnetic field strengths exceeding ~170 Gauss, suggesting a direct link between the magnetic field and heating of super-hot plasma. Images and spectra of the 2002 July 23 X4.8 event reveal that the super-hot plasma is both spectrally and spatially distinct from the commonly-observed ~10-20 MK plasma, and is located above the cooler source. It exists with high density even during the pre-impulsive phase, which is dominated by coronal non-thermal emission with negligible footpoints, suggesting 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.