Generation of solar Hα impact polarization by fragmented evaporative upflows

In this paper a novel mechanism is proposed for the generation of Hα impact polarization observed during some solar flares. Rather than being generated by the primary particle beams transporting energy from the chromosphere to the corona, we suggest that following heating, the solar chromosphere evaporates in a fragmented manner, and that impact excitations in the regions of interaction of hot evaporating and cool nonevaporating material locally generates impact-polarized Hα emission. This thermal upflow model is more consistent with the large areas and times over which polarization is observed than are beam models. A simple model for the process is given, and the resulting polarization is calculated and compared with observations, under two assumptions about the number density of neutral particles in the interaction regions.