How to measure pitch-angle diffusion coefficient at θ ~ 90°

It is well known that the quasilinear theory of particle pitch-angle (θ) scattering by magnetohydrodynamic turbulence results in the peculiarities at θ = 90°. We propose a simple method of measuring of the pitch-angle diffusion coefficient in this range. It is based on the anisotropy detection of the back-scattered flare particles. This possibility relies on the mutual geometrical location of the particle source (flare site), part of the Archemedean spiral where the particles propagate and the measuring device. The most reliably this scheme must work for the neutron-decay protons originated from the behind-limb flares. In this case only the protons scattered at θ = 90° must reach the probe while the direct proton flux will be significantly depressed. The time profile of the particle anisotropy in such a geometry (at known source properties) allows one to choose an adequate model of the particle scattering at the pitch-angle domain θ = 90°.