Hard X rays of relativistic electrons accelerated in solar flares

Abstract

The direction and polarization degree of hard X rays (HXRs) in solar flares are studied. The continuous injection of relativistic electrons, which is implemented in powerful flares, is considered. The stationary relativistic kinetic equation is studied by using the method of expansion in terms of the Legendre polynomial and by integrating the equations for the expansion coefficients. The HXR characteristics are calculated using the bremsstrahlung relativistic cross-section for different angular and energetic electron distributions in the acceleration region. A high linear polarization degree of HXRs (∼35%) has been obtained for narrow (∼cos6θ) beams of electrons with a soft spectrum (∼E −6); the polarization degree decreases with increasing quanta energy, whereas the directivity of a high-energy emission increases. This effect is absent for a nonrelativistic approximation. The considered model is applied to one of the most powerful flares in cycle 23, registered on October 28, 2003. The measured polarization degree values at relativistic energies (0.2–0.4 and 0.4–1 MeV) agree with the results achieved in the considered model when the electron energy spectrum index (δ = 2.5), angular distribution part (∼cos6θ), and the spectrum cutoff energy (E max = 1.3 MeV) were specified.