Ion-Driven Instabilities in the Inner Heliosphere I: Statistical Trends

TL;DR

This study analyzes the occurrence and trends of plasma instabilities in the inner heliosphere using linear theory and observational data, revealing how instability activity varies with distance from the Sun and plasma parameters.

Contribution

It applies the Nyquist criterion to a large dataset of proton and alpha particle VDFs, providing the first statistical analysis of instability trends in the inner heliosphere.

Findings

Instability fraction decreases linearly with distance from the Sun.

More extreme instability trends are observed with increasing Coulomb number.

Instability growth rates decline significantly with Coulomb number.

Abstract

Instabilities described by linear theory characterize an important form of wave-particle interaction in the solar wind. We diagnose unstable behavior of solar wind plasma between 0.3 and 1 au via the Nyquist criterion, applying it to fits of $\sim1.5$M proton and $\alpha$ particle Velocity Distribution Functions (VDFs) observed by \emph{Helios I} and \emph{II}. The variation of the fraction of unstable intervals with radial distance from the Sun is linear, signaling a gradual decline in the activity of unstable modes. When calculated as functions of the solar wind velocity and Coulomb number, we obtain more extreme, exponential trends in the regions where collisions appear to have a notable influence on the VDF. Instability growth rates demonstrate similar behavior, and significantly decrease with Coulomb number. We find that, for a non-negligible fraction of observations, the proton beam or secondary component might not be detected due to instrument resolution limitations, and demonstrate that the impact of this issue does not affect the main conclusions of this work.