Free Energy Sources of Ion-scale Waves Observed by Parker Solar Probe

TL;DR

This study uses Parker Solar Probe data to statistically analyze ion-scale waves, revealing their connection to plasma microinstabilities, turbulent dissipation, and specific free energy sources in the solar wind.

Contribution

It provides the first comprehensive statistical survey linking ion-scale wave types to their free energy sources and plasma conditions in the inner heliosphere.

Findings

Left-handed waves increase closer to the Sun, reaching ~30% occurrence.

Turbulent spectra are steeper during ion cyclotron wave storms.

Fast magnetosonic waves correlate with enhanced proton heat flux.

Abstract

Parker Solar Probe (PSP) observes abundant circularly polarized ion-scale waves throughout the inner heliosphere. These waves are a signature of the interplay between plasma microinstabilities and turbulent dissipation. We perform a mission-wide statistical survey of ion-scale waves observed by PSP, investigating if the waves correspond to specific free energy sources in the measured proton velocity distributions. We find that left-handed waves (LHWs) are frequently observed, with the fraction of time they are observed increasing closer to the Sun, reaching $\sim$30\%. Right-handed waves (RHWs) are less frequently observed, with the associated time fraction decreasing closer to the Sun. The observed LHWs are generally consistent with parallel propagating ion cyclotron wave (ICW) storms that occur continuously for extended periods of time. Turbulent energy spectra are consistently steeper when LHW storms are observed; these wave storms mediate the spatial transport of the free energy associated with temperature anisotropy. The observed RHWs are generally consistent with oblique and parallel fast magnetosonic waves (FMWs), and their observation is well correlated with enhanced proton parallel heat flux, which quantifies the presence of secondary proton populations. Using observations and the SAVIC machine learning instability identification algorithm, we identify a threshold on the proton heat flux beyond which FMWs are likely to be driven unstable by the proton beams. We are thus able to associate trends in the observed ion-scale waves with known sources of free energy for Encounters 3 through 24 of the PSP's prime science phase.