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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

Abstract

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

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

Abstract

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

Paper Structure

This paper contains 5 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Data Selection
  3. Radial Trends
  4. The role of expansion
  5. Summary and discussion

Figures (4)

  • Figure 1: Bi-dimensional log-log histograms from PSP measurements across encounters 1-21, aggregated by distance. In each panel, the colorbar represents the net number of measurements counted in equal-sized bins for (a) the proton $\beta_\parallel$, (b) their anisotropy $T_\perp/T_\parallel$, and (c) the proton gyrofrequency in Hertz.
  • Figure 2: Color scale plot of the number of measurements of $(\beta_{\parallel},T_{\perp}/T_{\parallel})$, with bins in log-log scale. All valid measurements from encounters 1-21 are used. Instability thresholds for the EMIC, mirror, parallel firehose, and oblique firehose instabilities are shown with solid, dash-dotted, dashed, and dotted lines, respectively, with contours $\gamma_{\text{max}}/\Omega=10^{-3}$ in gray, and $10^{-2}$ in black.
  • Figure 3: Measurements of $(\beta_{\parallel}, T_{\perp}/T_{\parallel})$ filtered by radial distance. $N$ is the total number of measurements in each panel. The shared colorbar represents the counts per bin in log-scale. Only the EMIC and parallel firehose instability thresholds are shown for $\gamma_{\text{max}}/\Omega=10^{-3}$ (gray lines) and $10^{-2}$ (black lines).
  • Figure 4: Centroids for each panel in Figure \ref{['fig:ba_by_distance']}, corresponding to bins with over 525 measurements. The red line is Marsch2004 relation $T_{\perp}/T_{\parallel}\sim\beta_{\parallel}^{-0.55}$.