Statistical mechanics of the electrons in the solar wind: stability and instability of whistler waves in the inner heliosphere

The electrons in the solar wind often exhibit non-equilibrium velocity distribution functions. Observed non-equilibrium electron features in the inner heliosphere include a field-aligned beam (called "strahl"), a suprathermal halo population, a sunward deficit in the distribution, and temperature anisotropy. These features are the result of a complex interplay between global expansion effects, collisions, and local interactions between the particles and the electromagnetic fields. Global effects create, for example, the strahl via the mirror force in the decreasing magnetic field and the sunward deficit via reflection effects in the interplanetary electrostatic potential. Local wave-particle interactions such as instabilities and wave damping change the shape of these signatures and thus the overall properties and moments of the electron distribution. We discuss the formation of the relevant features in the electron distribution and analyse their impact on the linear stability of whistler waves in the inner heliosphere. We then present results from our numerical ALPS code that is capable of evaluating the linear stability of plasma with arbitrary background distributions. With results from our ALPS code, we show that the strahl-core-deficit configuration near the Sun drives oblique whistler waves unstable. However, it leads to enhanced damping of parallel whistler waves compared to a Maxwellian configuration. As the distribution evolves, the sunward deficit fills with electrons, at which point the plasma becomes unstable and drives parallel whistler waves. Our results highlight the need to treat electrons statistically as a globally inhomogeneous plasma component and to account for the detailed shape of their distribution in the evaluation of the plasma's linear stability.