Energy Release and Transport in Super-Hot Solar Flares

Abstract:

Solar flares efficiently convert the magnetic energy stored in the Sun's complex coronal magnetic field into the kinetic energies of hot plasma, accelerated particles, and bulk flows. In intense flares, up to 10^32-33 ergs can go into heating plasma to tens of MK, accelerating electrons to hundreds of MeV and ions to tens of GeV, and ejecting 10^9-10 kg of coronal material into the heliosphere at thousands of km/s. However, the exact physical mechanisms behind these phenomena are poorly understood. For example, while "super-hot" (T > 30 MK) plasma temperatures appear to be common in the most intense, X-class flares, how that plasma is so efficiently heated remains unknown. Current studies favor an in situ heating process for super-hot plasma, versus chromospheric evaporation for cooler plasma, although the specific mechanism is under debate. X-class flares are also often associated with enhanced photospheric/chromospheric white light emission, which is itself poorly understood, and with fast (>1000 km/s) CMEs; super-hot flares are more commonly observed in eruptive two-ribbon arcade flares than in highly-confined events. These phenomena may well have common underlying drivers.

We discuss the current understanding of super-hot plasma in solar flares, its formation, and evolution, based on observations from RHESSI, SDO/EVE, SDO/AIA, and other instruments. We discuss the energetics of these events and their relationship to white light enhancement and fast CMEs. We explore the possibility of energy deposition by accelerated ions as a common driver for super-hot plasmas and white light enhancement, and discuss future instrumentation -- both for CubeSats and Explorers -- that will provide a deeper understanding of these phenomena and their interrelationships.