Statistical Investigation of the Effect of Coulomb Collisions on Thermodynamic Evolution of the Alfvénic Slow Wind

다국어 초록 (Multilingual Abstract)

Coulomb collisions are believed to be a fundamental process governing the thermodynamic equilibrium of solar wind plasmas, yet their role within the Alfvénic slow solar wind remains poorly understood. In this study, we carry out a statistical analysis of proton core parameters measured by the Helios spacecraft to examine how its temperature anisotropy (<i>T</i>_⊥/<i>T</i>_∥) varies with collisional age for three types of solar wind, namely, fast wind, Alfvénic slow wind, and non-Alfvénic slow wind. Here, <i>T</i>_⊥ and <i>T</i>_∥ denote the proton temperatures perpendicular and parallel to the ambient magnetic field. Consistent with previous findings, we confirm that Coulomb collisions play a negligible role in the fast wind but strongly regulate the thermodynamics of non-Alfvénic slow wind. Remarkably, however, the Alfvénic slow wind displays two distinct regimes: a weakly collisional regime characterized by significant temperature anisotropy, <i>T</i>_⊥/<i>T</i>_∥ > 1, and a collisionally regulated regime characterized by <i>T</i>_⊥/<i>T</i>_∥ ≈ 1. In addition, a close examination of the radial evolution of several well-defined Alfvénic slow wind streams indicates that their thermodynamic behavior is strongly governed by several key plasma parameters, notably the solar wind speed. This in turn implies that the origin of the streams—either the cores of coronal holes or their over-expanded edges—determines the dominant processes governing the dynamics and thermodynamics of the Alfvénic slow wind.