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

In this work, we analyze the kinetic stability of a solar wind electron distribution composed of core and strahl subpopulations. The core is modeled by a drifting Maxwellian distribution, while the strahl is modeled 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. We do not find evidence for a whistler instability directly associated with the electron strahl. 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 kd_i ~ 1 (d_i is the ion inertial length), at moderately oblique angles of propagation, providing a potential source of kinetic-scale turbulence. We suggest that the whistler modes may appear as a result of nonlinear mode coupling and turbulent cascade originating at scales kd_i ~ 1.