Behaviour of Kinetic Electron and Proton Instabilities in the Magnetosheath Modelled with Semi-Implicit Particle-In-Cell

Abstract

Temperature anisotropies are often encountered in space and astrophysical plasmas. A sufficiently large ratio between the electron temperatures perpendicular and parallel with respect to the magnetic field causes whistler and mirror instabilities to grow. Similarly, a sufficiently large perpendicular proton temperature ratio is expected to result in instabilities such as mirror and proton acoustic instabilities. In the parameter regime surrounding Earth’s magnetosheath linear theory predicts that proton cyclotron instability will have the lowest threshold of all anisotropy-driven instabilities. Despite this, unstable mirror modes are observed in apparent contradiction to linear theory. It has been suggested that an electron perpendicular to parallel temperature anisotropy can alter the relative growth rates of mirror and proton acoustic instabilities by boosting the growth rates of mirror modes. Such an anisotropy could also be consumed by (faster) electron instabilities. Given the potential for cross-species interplay, a thorough understanding of the behaviour of the electron whistler instability, which typically grows fastest compared to instabilities in the electron species, is crucial. By leveraging multidimensional kinetic simulations with the Energy Conserving Semi-Implicit Method ECSIM, this work studies the competition of electron instabilities in parameter space typical of Earth’s magnetosheath. In addition, the possibility that residual electron temperature anisotropy left over after the saturation of electron instabilities can affect the proton acoustic to mirror competition is explored. This second part of the study relies on the semi-implicit nature of the ECSIM code, which allows spatial and temporal resolution to be dimensioned on the basis of the smallest and fastest dynamics expected in the system, rather than the Courant condition that one has to respect for stability in explicit PIC codes.