RHESSI Observation of Chromospheric Evaporation

Abstract

We present analyses of the spatial and spectral evolution of hard X-rayemission observed by RHESSI during the impulsive phase of an M1.7 flareon 2003 November 13. In general, as expected, the loop top (LT) sourcedominates at low energies, while the footpoint (FP) sources dominate thehigh-energy emission. At intermediate energies, both the LT and FPs maybe seen, but during certain intervals emission from the legs of the loopdominates, in contrast to the commonly observed LT and FP emission. Thehard X-ray emission tends to rise above the FPs and eventually mergeinto a single LT source. This evolution starts at low energies andproceeds to higher energies. The spectrum of the resultant LT sourcebecomes more and more dominated by a thermal component with anincreasing emission measure as the flare proceeds. The soft and hardX-rays show a Neupert-type behavior. With a nonthermal bremsstrahlungmodel, the brightness profile along the loop is used to determine thedensity profile and its evolution, which reveals a gradual increase ofthe gas density in the loop. These results are evidence forchromospheric evaporation and are consistent with the qualitativefeatures of hydrodynamic simulations of this phenomenon. However, someobserved source morphologies and their evolution cannot be accounted forby previous simulations. Therefore, simulations with more realisticphysical conditions are required to explain the results and the particleacceleration and plasma heating processes.