Combined Whistler Heat Flux and Anisotropy Instabilities in Solar Wind

Whistler heat flux and electron temperature anisotropy instabilities are spontaneously generated in the expanding solar wind. The present study investigates the interplay between the two unstable modes and how each contributes to the dynamical evolution of macroscopic quantities. Whistler heat flux instability is excited in the forward direction along the heat flow, whereas anisotropy instability propagates in both backward and forward directions. Velocity moment-based quasi-linear theory is employed in order to investigate the saturation behavior of these instabilities, and it is shown that under certain conditions, the forward propagating whistler heat flux instability dominates, while for other conditions, especially when the temperature anisotropies are sufficiently high, it is shown that the backward mode may also contribute to the dynamical process, and even dominate. These saturation behaviors cannot be predicted by linear growth rate alone and that they imply that in the global-kinetic models of the solar wind, bidirectional nature of these instabilities must be carefully taken into account.