Evidence for nongyrotropic alpha particle and proton distribution functions: TAUS solar wind measurements

The goal of this paper is to present convincing evidence for the occurrence of nongyrotropic alpha particle and proton velocity distribution functions in the solar wind plasma. We analyze the three-dimensional velocity distribution functions obtained by measurements with high time 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 during an event which occurred after a discontinuity in the interplanetary magnetic field. It was characterized by a spike in the density and temperature ratios, with n(alpha)/n(p) and T-alpha/T-p reaching values of about 0.3 and 12, respectively. In a detailed analysis of the distributions, we show that the nongyrotropic features are associated with sizable fractions of the alpha particle distribution functions. Furthermore, it is also shown that similar nongyrotropic features are present in solar wind proton distributions. The nongyrotropic particles of both species in the distributions can be characterized as particles being gyro-phase-bunched in velocity space. We indicate that there might be a close relation between the nongyrotropy of the distribution and the large temperature anisotropies (with T-perpendicular to > T-parallel to) observed in these events. We also point out some differences in the symmetry properties with respect to the local magnetic field direction between the nongyrotropic proton distribution functions found in the undisturbed solar wind and the nongyrotropic distributions originating at interplanetary shocks.