ELECTRON-BEAM DYNAMICS AND HARD X-RAY BREMSSTRAHLUNG POLARIZATION IN A FLARING LOOP WITH RETURN CURRENT AND CONVERGING MAGNETIC-FIELD

In a kinetic approach the electron beam dynamics and its effect on the hard X-ray bremsstrahlung emission are investigated in flaring loops with the atmospheres taken from previously calculated hydrodynamical models. The electron beam is assumed to have a power law distribution in energy and to precipitate from the top of the loop in the corona into the chromosphere. The time dependent kinetic equation was solved numerically, taking into account the anisotropy of electrons scattering, for the following channels of energy loss and pitch angle change: Coulomb collisions and collisions with neutral atoms, Ohmic dissipation and magnetic field convergence. The evolution with depth and in time of the electron beam energy distributed function, as well as the X-ray bremsstrahlung emission functions, as well as the X-ray bremsstrahlung emission and polarization were evaluated and compared with observations. The electron beam distribution functions are shown to be strongly dependent on depth, energy and pitch angle cosine, in relation to the initial beam parameters at the injection site, and weakly dependent on the magnetic field convergence. The last, along with the induced electrical field, produces a preferential scattering along the field lines which is rather important for the electron beam precipitation at lower chromospheric levels. The temporal hard X-ray bremsstrahlung emission profiles are symmetrical ones and resemble observations for events with timescales around a few seconds. Polarization varies noticeably with emergent photon energy below 40 keV, being up to 30% and down to - 10% for different angles of view; these variations cover the range of observed magnitudes. The harder X-ray radiation has almost constant polarization with increasing photon energy, for a fixed angle of view.