Stability of superthermal strahl electrons in the solar wind*

We present a kinetic stability analysis of the solar wind electron distribution function consisting of the Maxwellian core and the magnetic-field aligned strahl, a superthermal electron beam propagating away from the sun (Schroeder et. al. 2021, submitted to MNRAS). We use an electron strahl distribution obtained as a solution of a weakly collisional drift-kinetic equation in Boldyrev & Horaites 2019, representative of a strahl affected by Coulomb collisions but not by possible broadening from turbulence. This distribution function is essentially non-Maxwellian and varies with the heliospheric distance. The stability analysis is performed with the linear Vlasov-Maxwell solver LEOPARD (Astfalk & Jenko 2017). We find that depending on the heliospheric distance, the core-strahl electron distribution becomes unstable with respect to sunward-propagating kinetic-Alfvén, magnetosonic, and whistler modes, in a broad range of propagation angles. The wavenumbers of the unstable modes are close to the ion inertial scales, and the radial distances at which the instabilities first appear are on the order of 1 AU. However, we have not detected any instabilities driven by resonant wave interactions with the superthermal strahl electrons. Instead, the observed instabilities are triggered by a relative drift between the electron and ion cores necessary to maintain zero electric current in the solar wind frame, indicating that the electron strahl obtained as a solution of the kinetic equation is stable.