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

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.

Accepted in ApJL

References in corpus (7)

• Whistler mode waves and the electron heat flux in the solar wind: Cluster observations
• Statistical Study of Whistler Waves in the Solar Wind at 1 AU
• Scattering of Strahl Electrons in the Solar Wind between 0.3 and 1 au: Helios Observations
• Alternative high plasma beta regimes of electron heat-flux instabilities in the solar wind
• On the role of solar wind expansion as a source of whistler waves: scattering of suprathermal electrons and heat flux regulation in the inner heliosphere
• On quasi-parallel whistler waves in the solar wind
• Stability of superthermal strahl electrons in the solar wind

Cited by in corpus (3)

• Whistler waves generated inside magnetic dips in the young solar wind: observations of the Search-Coil Magnetometer on board Parker Solar Probe
• Quantifying the diffusion of suprathermal electrons by whistler waves between 0.2 and 1 AU with Solar Orbiter and Parker Solar Probe
• The Wave Energy Density and Growth Rate for the Resonant Instability in Relativistic Plasmas