Hα-line impact polarisation by electron beams in solar flares

The Hα-line intensity, linear and circular impact polarisation were calculated during the impulsive phase of flares using the density matrix approach. Depolarising effects of diffusive radiation and collisions with thermal electrons are considered for a 3 levels Hydrogen model atom with Zeeman splitting in weak and intermediate magnetic fields. Electron beam energy and angular distributions in depth are found from the time-dependent anisotropic kinetic solutions in presence of the induced electric field of return current and converging magnetic field. The circular Hα-line polarisation is found to be negligible. The Hα-line profiles of linear polarisation are shown to be affected by electron beams in the line cores whereas wings are fully depolarised by thermal electrons. The full Hα-line linear polarisation, caused by weak or moderate electron beams, is shown to be 2− 20% and either positive or negative depending on time after the beam onset and on depth in the atmosphere. The plane of polarisation in the Hα-line is normally perpendicular to the direction of electron beam propagation. Depending on a viewing angle it can be either parallel or perpendicular to the flare-to-solar centre direction. The simulations are used to explain some observational features of the Hα-line polarisation.