IMPACT POLARIZATION OF THE H-ALPHA LINE OF HYDROGEN IN SOLAR-FLARES -CALCULATIONS OF ELECTRON AND PROTON COLLISIONAL ANISOTROPIC PROCESSESBETWEEN THE ZEEMAN EXCITED SUBLEVELS

Linear polarization oriented towards the center of the disk has been o bserved in the Her line of hydrogen in solar flares (Henoux et al. 199 0). An accurate polarimetric diagnostic is highly desirable for obtain ing quantitative informations on the vertical energetic beams of charg ed particles which are suspected to be the cause of that polarization. This would be of fundamental importance for the understanding of the physics and evolution of the flaring phenomenon. As a matter of fact, electrons and protons of the flare medium are probably not isotropic, otherwise H alpha would be quite depolarized, owing to the relatively high electron density (about 10(12) cm(-3)). In the present paper, a m ethod for calculating anisotropic collisional processes between the ex cited Zeeman sublevels with the electrons and protons of the medium is proposed. Numerical results are given for distributions of velocities having a cylindrical symmetry around the vertical. These results will be used in a forthcoming paper where the rates equations leading to p opulations and alignments of the excited levels of hydrogen will be so lved in the physical conditions of solar flares.