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

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.