Pitch angle distributions of energetic particles near the heliospheric termination shock

Abstract

The period of Voyager observations of energetic particles during its approach to the solar wind termination shock in 2004 provides a wealth of information about the site where the particles are accelerated. A new anisotropic particle transport model, based on an expansion of a focused transport equation in Legendre polynomials, is used here as a tool to analyze the pitch angle distributions of 1 MeV protons observed by Voyager 1 upstream of the shock. Daily average data from the seven sectors of the LECP instrument are represented in the form of an expansion in spherical harmonics up to second order in the coordinate frame aligned with the direction of the interplanetary magnetic field, permitting a direct comparison with our model results. It is found that the observed pitch angle distributions, are consistent with the model predictions for particle population accelerated at a highly oblique shock wave. We derive the obliquity angle of the termination shock from analysis of first order anisotropies. It is demonstrated that the second-order anisotropy is important for correctly interpreting the data. Many of the observed distributions are shown to possess an abnormally large second moment, which may indicate the presence of dual oppositely propagating beams in regions multiply connected to the termination shock by the interplanetary magnetic field lines. Copyright 2008 by the American Geophysical Union.