WEAK PITCH-ANGLE SCATTERING OF FEW MV RIGIDITY IONS FROM MEASUREMENTS OF ANISOTROPIES IN THE DISTRIBUTION FUNCTION OF INTERSTELLAR PICKUP H+

The distribution function of interstellar pickup hydrogen is measured with the SWIGS instrument on Ulysses at high heliographic latitudes, over a broad range of velocities, including those below that of the solar wind. It is found that these few MV rigidity protons exhibit a pronounced inward radial streaming in the frame of the solar wind, suggesting that neither instabilities generated by the pickup ions nor ambient turbulence is effective in scattering and isotropizing these particles. These results have implications for theories for the instabilities associated with pickup ions and for the propagation of low rigidity particles in the solar wind.