The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere

TL;DR

This study investigates the stability of the electron strahl in the solar wind, finding it generally stable against the oblique FM/W instability in the inner heliosphere, with sporadic and short-lived potential excitations.

Contribution

It provides observational evidence that the electron strahl remains stable against the oblique FM/W instability in the inner heliosphere, challenging previous assumptions about its role in scattering.

Findings

Electron strahl is mostly stable against oblique FM/W instability in the inner heliosphere.

The instability, if excited, occurs sporadically and on short timescales.

Alternative mechanisms may be responsible for electron strahl scattering in the solar wind.

Abstract

We analyze the micro-kinetic stability of the electron strahl in the solar wind depending on heliocentric distance. The oblique fast-magnetosonic/whistler (FM/W) instability has emerged in the literature as a key candidate mechanism for the effective scattering of the electron strahl into the electron halo population. Using data from Parker Solar Probe (PSP) and Helios, we compare the measured strahl properties with the analytical thresholds for the oblique FM/W instability in the low- and high-$\beta_{\parallel c}$ regimes, where $\beta_{\parallel c}$ is the ratio of the core parallel thermal pressure to the magnetic pressure. Our PSP and Helios data show that the electron strahl is on average stable against the oblique FM/W instability in the inner heliosphere. Our analysis suggests that the instability, if at all, can only be excited sporadically and on short timescales. We discuss the caveats of our analysis and potential alternative explanations for the observed scattering of the electron strahl in the solar wind. Furthermore, we recommend the numerical evaluation of the stability of individual distributions in the future to account for any uncertainties in the validity of the analytical expressions for the instability thresholds.