TAUS measurements of non-gyrotropic distribution functions of solar wind alpha particles

Abstract

We analyse three-dimensional distribution functions of solar wind alpha particles in velocity space and study their symmetry properties relative to the interplanetary magnetic field. We present convincing evidence for the occurrence of non-gyrotropic alpha-particle velocity distribution functions in the solar wind plasma. The three-dimensional distributions are obtained by measurements with high resolution (8 s for alpha particles and 2 s for protons) of the plasma experiment TAUS on the PHOBOS II mission to planet Mars. The measurements were made at heliocentric distances near 1.3 AU. In a detailed analysis of the distributions we show that non-gyrotropic features are related to a sizable fraction of phase space density in the alpha-particle distributions. These non-gyrotropic particles can be characterised as being gyro-phase bunched in velocity space. There are indications of a close relation between the non-gyrotropy of the core particles of a distribution and the large core temperature anisotropies (with T⊥>T∥), which are observed in these events.