Stability analysis of core-strahl electron distributions in the solar wind

In this work, we analyse the kinetic stability of a solar wind electron distribution composed of core and strahl subpopulations. The core is modelled by a drifting Maxwellian distribution, while the strahl is modelled by an analytic function recently derived in (Horaites et al. 2018) from the collisional kinetic equation. We perform a numerical linear stability analysis using the LEOPARD solver (Astfalk & Jenko 2017), which allows for arbitrary gyrotropic distribution functions in a magnetized plasma. In contrast with previous reports, we do not find evidence for a whistler instability directly associated with the electron strahl. This may be related to the more realistic shape of the electron strahl distribution function adopted in our work, as compared to previous studies. We, however, find that for typical solar wind conditions, the core-strahl distribution is unstable to the kinetic Alfvén and magnetosonic modes. The maximum growth rates for these instabilities occur at wavenumbers kdi ≲ 1 (where di is the ion inertial length), at moderately oblique angles of propagation, thus providing a potential source of kinetic-scale turbulence. We therefore suggest that if the whistler modes are invoked to explain anomalous scattering of strahl particles, these modes may appear as a result of nonlinear mode coupling and turbulent cascade originating at scales kdi ≲ 1. © 2018 The Author(s).