Fluctuations of the heliospheric magnetic field and the anisotropy of pickup ions

Abstract

Pitch angle distribution of interstellar pickup ions at high heliographic latitude is studied. Based on high time resolution magnetic field observations by Ulysses during the south polar pass in 1994, we have determined the angular distribution of particles at the moment of ionization. Due to large deviations of the field from Parker's spiral, it was found that 13% of the freshly ionized particles stream away of the Sun in the solar wind frame. We have solved the transport equation of the pickup particles which included an anisotropic source term. Our model calculation shows that the effect of the source on the particle anisotropy is the dominant one, as opposed to the effect of the density gradient. Matching the results of the model with particle anisotropy observation by SWIGS aboard Ulysses, we have estimated a scattering mean free path of interstellar pickup protons λ = 0.25AU.