RHESSI Observations of Super-Hot (T > 30 MK) Plasma in Large Solar Flares

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The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) has observed over 500 M- and X-class flares since 2002, and has shown that super-hot (T > 30 MK) plasmas are relatively common among large events. However, the origins of such high-temperature plasmas have been largely undetermined. With a spectral resolution of 1 keV FWHM and an imaging resolution of 2 arcsec, RHESSI observations of flare X-ray emission down to 3 keV enable precise measurements, both spectrally and spatially, of the thermal bremsstrahlung continuum as well as the Fe and Fe/Ni line complexes at 6.7 and 8 keV. We present a new, detailed analysis of the 2002-July-23 X4.8 event which reveals the simultaneous presence of two thermal plasmas throughout the flare: a super-hot component that peaks at 44 MK, and the usual GOES-temperature plasma that peaks at 25 MK. New imaging methods show that the two plasmas are spatially distinct. The morphology and temporal evolution of the two plasmas suggests that the super-hot component is heated directly by the reconnection outflows, while the cool plasma results from chromospheric evaporation driven by the accelerated particles. We also present limited results from a statistical survey of super-hot plasmas in other RHESSI-observed M- and X-class flares, which shows that super-hot temperatures are achieved predominantly in X-class flares; the distribution suggests a relationship between flare magnitude, peak temperature, and energy density of the thermal plasma.

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