On the possibility of measuring the pitch-angle diffusion coefficient in the vicinity of θ ∼ 90°

Abstract

It is well known that the quasilinear theory of pitch angle (θ) scattering of particles by magnetohydrodynamic turbulence encounters difficulties in the vicinity of θ = 90°. We propose a simple method for measuring the pitch-angle diffusion coefficient in this range that is based on measuring the anisotropy of back-scattered flare particles. This is possible due to the mutual spatial configuration of the flare region, the part of the Archemedean spiral in which the particles propagate, and the measuring device. This method is best suited for protons originating from the decay of solar neutrons in flares close to the limb of the Sun. Only protons back-scattered at angles of around θ = 90 degree will reach the probe in this case, and the direct proton flux will be significantly weakened. Measuring variations in the time of particle anisotropy in this geometry (with a known source) enables us to choose the most appropriate particle scattering model in the range of pitch angles near θ = 90°.