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 occur rence 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 tim e resolution (8 s for alpha particles and 2 s for protons) of the plas ma experiment TAUS on the PHOBOS II mission to planet Mars. The measur ements were made at heliocentric distances near 1.3 AU during an event which occurred after a discontinuity in the interplanetary magnetic f ield. It was characterized by a spike in the density and temperature r atios, 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 frac tions of the alpha particle distribution functions. Furthermore, it is also shown that similar nongyrotropic features are present in solar w ind proton distributions. The nongyrotropic particles of both species in the distributions can be characterized as particles being gyro-phas e-bunched in velocity space. We indicate that there might be a close r elation between the nongyrotropy of the distribution and the large tem perature anisotropies (with T-perpendicular to > T-parallel to) observ ed 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 undistur bed solar wind and the nongyrotropic distributions originating at inte rplanetary shocks.