A RHESSI search for chromospheric evaporation in super-hot flares

Chromospheric evaporation (CE) - thought to occur when downward-accelerated coronal electrons impact the denser chromosphere, heating the ambient material which then rises to fill the flaring loop - has often been suggested as the primary source of hot thermal looptop plasma. Evidence for CE is given by crystal spectrometer observations of blueshifted spectral lines from footpoints. RHESSI observations show that peak flare temperatures generally occur soon after the hard X-ray (HXR; >20~keV) peak, when the emission measure is still only ~20% of its later peak value. Imaging places the thermal plasma at the looptop even for large (GOES X-class) flares that exceed super-hot (Te > 30~MK) temperatures. In such flares, non-thermal energy deposition can exceed ~1029~erg/s, which for typical electron energies, chromospheric densities, and footpoint areas should yield CE temperatures of ~10-20~MK with emission measures of ~1050~cm-3. Such bright thermal emission has never been observed from footpoints, suggesting that while CE may contribute the bulk loop material, heating occurs primarily in the corona. We present an analysis of selected RHESSI M- and X-class flares, employing imaging spectroscopy to determine the temperature and emission measure (or limits thereof) of spatially-separated footpoint and looptop sources. We compare the time evolution of the thermal footpoint signatures to HXR lightcurves and to the looptop thermal component. We calculate the energy contained in non-thermal electrons, the thermal energies at the looptop and footpoints, and discuss the implications for heating by non-thermal electrons and the contribution of CE to the thermal flare plasma.