Abstract: The electron distribution function in the solar wind consists of three main populations: a thermal core, a suprathermal, quasi-isotropic halo, and a field-aligned beam called “strahl”. In contrast to the protons, the electrons are a sub-sonic particle population, and due to their small mass, they contribute little to the overall momentum flux of the solar wind. However, their unique kinetic properties supply the solar wind with a significant heat flux.

We investigate the regulation of this heat flux by kinetic microinstabilities. I will present a mathematical framework for the description of electron-driven instabilities and discuss the associated physical mechanisms. We find that an instability of the oblique fast-magnetosonic/whistler (FM/W) mode is the best candidate for a microinstability that regulates the strahl heat flux by scattering strahl electrons into the halo population, consistent with spacecraft measurements. We derive approximate analytic expressions for the FM/W instability thresholds and confirm their accuracy through comparison with numerical solutions to the hot-plasma dispersion relation.

The comparison of our theoretical results with a large statistical dataset from the Wind spacecraft confirms the relevance of the oblique FM/W instability for the solar wind. In addition, we find a good agreement between our theoretical results and numerical solutions to the quasilinear diffusion equation. I will present our results in the context of the latest measurements from Parker Solar Probe and Solar Orbiter.