Onset and Evolution of the Oblique, Resonant Electron Firehose Instability in the Expanding Solar Wind Plasma

A double adiabatically expanding solar wind would quickly develop large parallel to perpendicular temperature anisotropies in electrons and ions that are not observed. One reason is that firehose instabilities would be triggered, leading to an ongoing driving/saturation evolution mechanism. We verify this assumption here for the first time for the electron distribution function and the electron firehose instability (EFI), using fully kinetic simulations with the Expanding Box Model. This allows the self-consistent study of onset and evolution of the oblique, resonant EFI in an expanding solar wind. We characterize how the competition between EFI and adiabatic expansion plays out in high- and low-beta cases, in high- and low-speed solar wind streams. We observe that, even when competing against expansion, the EFI results in perpendicular heating and parallel cooling. These two concurrent processes effectively limit the expansion-induced increase in temperature anisotropy and parallel electron beta. We show that the EFI goes through cycles of stabilization and destabilization: when higher wave number EFI modes saturate, lower wave number modes are destabilized by the effects of the expansion. We show how resonant wave/particle interaction modifies the electron velocity distribution function after the onset of the EFI. The simulations are performed with the fully kinetic, semi-implicit expanding box code EB-iPic3D.