Impact Hα line polarization and return current

Abstract

Electron beams, bombarding the dense chromospheric layers during solar flares, carry electric currents which need to be neutralized by so-called return currents. Return currents are formed by background plasma electrons having an anisotropic velocity distribution. Thus they can generate impact H α line polarization. First, a numerical method of computation of the impact H α line polarization for an arbitrary electron distribution function is presented. Then the polarization due to return current electrons associated with beam electrons is computed. For low electron beam fluxes, the return current is low and the polarization is only due to the electron beam, i.e. it is perpendicular to the electron beam direction and it reaches -8.0%. Increasing the return current and the beam flux leads to a change of orientation of the polarization by 90° and the polarization degree can even reach a maximum of +22.4%. But this change and the maximum of the polarization require very high electron beam fluxes of $1.79 \times 10^{12}$ and $4.8 \times 10^{12}$ ergs cm -2 s -1 , respectively. Therefore plasma processes, which can reduce the high-energy flux requirement for the polarization change observations, are briefly discussed.