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arXiv · 2603.25443

The Effect of Expansion and Instabilities in the Thermodynamic Regulation of the Young Solar Wind Plasma

Matilde Coello-Guzmán·Víctor A. Pinto·Roberto E. Navarro·Pablo S. Moya
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Abstract

Using Parker Solar Probe measurements of the solar wind, we demonstrate that $\beta_{\parallel}$ is the main driver that determines which instabilities limit proton temperature anisotropy. At radial distances from 10 to 30 solar radii, $\beta_{\parallel}<1$ drives electromagnetic ion-cyclotron and parallel firehose instabilities, in contrast to the situation at 1 astronomical unit, where, due to most $\beta_{\parallel}>1$, mirror and oblique firehose modes are dominant instead. Furthermore, we show that the temperature anisotropy radially evolves following the semi-empirical anti-correlation $T_\perp/T_\parallel\sim\beta_\parallel^{-0.55}$, consistent with observations at larger distances from the Sun.

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Matilde Coello-Guzmán, Víctor A. Pinto, Roberto E. Navarro, Pablo S. Moya. 2026-03-26. The Effect of Expansion and Instabilities in the Thermodynamic Regulation of the Young Solar Wind Plasma. https://arxiv.org/abs/2603.25443

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