Work

Solar-wind proton heating and its effect on the temperature anisotropy evolution between 0.05 and 1 au

Year: 2026

Type: article

Abstract: This study focused on the radial evolution of the solar-wind proton adiabatic invariants and temperature anisotropies in the inner heliosphere. More specifically, we studied in situ measurements provided by the Parker Solar Probe, between 0.05 au and 0.25 au from the Sun, and the Solar Orbiter spacecraft between 0.3 au and 1 au. Throughout the studied range of radial distances, we observed a significant average heating in the direction perpendicular to the local magnetic field for both fast and slow solar-wind populations. On the other hand, there is no clear deviation from adiabaticity in the parallel direction regardless of the wind speed. The perpendicular heating is enough to significantly reduce the generation of the temperature anisotropy expected from a double adiabatic evolution. Despite the heating, an important portion of the solar wind (especially the slower wind streams) develops substantial anisotropies with higher parallel temperatures, which eventually become constrained by kinetic fire-hose instabilities.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202662003

Authors: Etienne Berriot, Arnaud Zaslavsky, Olga Alexandrova, Pascal Démoulin, Petr Hellinger, Lorenzo Matteini