A Majority of Solar Wind Intervals Support Ion-Driven Instabilities

TL;DR

This study statistically assesses solar wind stability at 1 AU, revealing that over half of the spectra are unstable due to ion-driven effects, with most instabilities being slow-growing and linked to proton beams and temperature anisotropies.

Contribution

It introduces a comprehensive stability assessment method incorporating multiple ion sources of free energy, extending beyond traditional threshold models.

Findings

53.7% of spectra are unstable with drifting bi-Maxwellian ion models

Only 4.5% are unstable to long-wavelength instabilities

Most instabilities grow slower than ion-kinetic timescales

Abstract

We perform a statistical assessment of solar wind stability at 1 AU against ion sources of free energy using Nyquist's instability criterion. In contrast to typically employed threshold models which consider a single free-energy source, this method includes the effects of proton and He$^{2+}$ temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and He$^{2+}$ components on stability. Of 309 randomly selected spectra from the Wind spacecraft, $53.7\%$ are unstable when the ion components are modeled as drifting bi-Maxwellians; only $4.5\%$ of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities occur for spectra where a proton beam is resolved. Nearly all observed instabilities have growth rates $\gamma$ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively-large He$^{2+}$ drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.