All whistlers are not created equally: Scattering of strahl electrons in the solar wind via particle-in-cell simulations

Solar wind observations show that suprathermal electrons ( 70 eV less than or similar to Energy less than or similar to 1 keV) of the magnetic-field-aligned strahl'' component have broader pitch-angle distributions than are predicted by adiabatic theories of solar wind expansion. Magnetosonic-whistler fluctuations propagating toward the Sun at k x B-o = 0 ( where B-o is the background magnetic field) have a strong cyclotron resonance with suprathermal electrons propagating in the anti-Solar direction along B-o. This resonance enables strong pitch-angle scattering; thus whistlers are a likely source of the observed strahl broadening. Particle-in-cell simulations in a magnetized, homogeneous, collisionless plasma of electrons and protons are used to study the response of a strahl-like electron component to whistler fluctuation spectra. If the whistler anisotropy instability is excited via the initial application of T-perpendicular to/T-parallel to k > 1 to the electron core component, the resulting electron scattering leads to strahl pitch-angle distributions which decrease in width as electron energy increases. In contrast, if a power spectrum of whistler fluctuations proportional to k(-3) is initially applied to the simulations, the resulting electron scattering leads to strahl pitch-angle distributions which increase in width as electron energy increases.