Pitch Angle Scattering of Solar Flare Electrons in the Interplanetary Medium: Observations and Modelling

Abstract:

In the past decade, analysis of near-relativistic (~27 keV - 300 keV) electron events at 1 AU have highlighted two transport effects which require explanation. Firstly, several events feature delayed electron arrival with respect to solar radio and hard x-ray emission, and secondly, the peak-flux spectrum of electrons at 1 AU does not match the predicted spectrum from hard x-ray observations. We analyse several near-relativistic electron events observed via both RHESSI hard x-ray observations at the Sun and in-situ measurements from the Wind/3DP detector at 1 AU. Numerical simulations of electron transport outwards from the Sun are made, which take the electron injection time and peak-flux spectrum from RHESSI data, and the flux subsequently passing 1 AU is calculated. We consider the effects of adiabatic focusing and pitch angle diffusion on the particle transport, and a momentum and distance dependent form of the parallel mean free path for electrons is employed. The simulated lightcurves, peak-flux spectrum, pitch angle distribution, and delay times are then compared with Wind observations. Wefind that, for higher energy electrons (>40 keV), the simulated flux matches well with observations, showing that stochastic pitch angle scattering is able to explain apparent delayed particle injection at the Sun. The lower energy observations, however, remain unmatched by models, which predict much more impulsive events at Earth than are observed. We also find that pitch angle scattering is too weak to vary the peak-flux spectrum sufficiently, thus requiring further exploration.