Regulation of temperature anisotropy for solar wind protons and alpha particles by collisions and instabilities

Context. Determining which mechanisms regulate proton and alpha particle temperature anisotropies in the solar wind is an outstanding problem in collisionless plasma systems. For decades, the occurrence distributions of the various charged particle species measured in the near-Earth solar wind have been known to be characterized by peculiar rhombic shapes in ( β ∥ , T ⊥ / T ∥ ) phase space, where β ∥ is the ratio of parallel (with respect to the ambient magnetic field) plasma thermal pressure and the ambient magnetic field energy density and T ⊥, ∥ are temperatures in the perpendicular or parallel directions. Despite this fact, a convincing explanation for the physical mechanisms producing the low- β edges had not been forthcoming until recently. Aims. Recent works have provided plausible explanations for the origin of these distributions by invoking the combined effects of collisions and instability excitation; however, the initial applications were limited to proton and electron plasmas. In the present paper, the same coupled mechanism is extended to include alpha particles (He ++ ), which dynamically couple to the protons. Methods. We performed an ensemble simulation based upon the collisional relaxation equation that couples the protons and alpha particle dynamics in the low-beta regime. We also carried out another ensemble simulation based on the instability-induced quasi-linear relaxation equation for the high-beta regime. Results. We find that the combined effects provide a satisfactory first-order explanation of the observed temperature distribution, resolving one of the long-standing problems in contemporary heliospheric physics. Conclusions. The findings of the present study demonstrate that the collisional relaxation is adequate to describe the existence of an outer boundary associated with the proton and alpha particle occurrence distribution in the low-beta regime. For the high-beta regime, it is known that the instability-induced relaxation is important, and the present ensemble simulation confirms this notion.