Inferred Linear Stability of Parker Solar Probe Observations using One- and Two-Component Proton Distributions

TL;DR

This study analyzes Parker Solar Probe data to assess wave-particle instabilities in the solar wind, comparing one- and two-component proton models to understand their stability and growth rates near the Sun.

Contribution

It introduces a method to evaluate linear stability of proton distributions using PSP data and compares the effects of different velocity distribution models on instability predictions.

Findings

Both models show susceptibility to resonant instabilities.

Growth rates vary significantly between models.

Instabilities can be rapid enough to influence solar wind dynamics.

Abstract

The hot and diffuse nature of the Sun's extended atmosphere allows it to persist in non-equilibrium states for long enough that wave-particle instabilities can arise and modify the evolution of the expanding solar wind. Determining which instabilities arise, and how significant a role they play in governing the dynamics of the solar wind, has been a decades-long process involving in situ observations at a variety of radial distances. With new measurements from Parker Solar Probe (PSP), we can study what wave modes are driven near the Sun, and calculate what instabilities are predicted for different models of the underlying particle populations. We model two hours-long intervals of PSP/SPAN-i measurements of the proton phase-space density during PSP's fourth perihelion with the Sun using two commonly used descriptions for the underlying velocity distribution. The linear stability and growth rates associated with the two models are calculated and compared. We find that both selected intervals are susceptible to resonant instabilities, though the growth rates and kind of modes driven unstable vary depending on if the protons are modeled using one or two components. In some cases, the predicted growth rates are large enough to compete with other dynamic processes, such as the nonlinear turbulent transfer of energy, in contrast with relatively slower instabilities at larger radial distances from the Sun.