August 13, 2010, 12:00 pm - 1:00 pm

August 13, 2010, 1:00 pm - 2:00 pm

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

Amir Caspi (University of California, Berkeley, Space Sciences Laboratory)

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; they can accelerate electrons up to hundreds of MeV and heat plasma to tens of MK, exceeding ~40 MK in the most intense flares. Observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) show that "super-hot" temperatures exceeding ~30 MK are achieved almost exclusively by X-class events and appear to be strictly associated with coronal magnetic field strengths exceeding ~200 Gauss, suggesting a direct link between the magnetic field and heating of super-hot plasma, and potentially a minimum threshold of field strength and overall flare intensity required to achieve super-hot temperatures. Images and spectra of the 2002 July 23 X4.8 event show that the super-hot plasma is distinct from the usual ~10-20 MK plasma observed in nearly all flares. 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.