Resolution dependence of magnetosheath waves in global hybrid-Vlasov simulations

Abstract. Plasma waves are ubiquitous in the Earth's magnetosheath. The most commonly observed waves arise from instabilities generated by temperature anisotropy of the ions, such as the mirror and proton cyclotron instabilities. We investigate here the spatial resolution dependence of the mirror and proton cyclotron instabilities in a global hybrid-Vlasov simulation using the Vlasiator model. We compare the proton velocity distribution functions, power spectra and growth rates of the instabilities in a set of simulations with three different spatial resolutions but otherwise identical set-up. We find that the proton cyclotron instability is absent at the lowest resolution and that only the mirror instability remains, which leads to an increased temperature anisotropy in the simulation. We conclude that the proton cyclotron instability is resolved well enough at the highest spatial resolution and that an increase of resolution does not improve the resolution of the instability enough to justify this increase at the cost of numerical resources in future simulations. We also find that a resolution around 0.6 the inertial length in the solar wind, presents an acceptable minimum spatial resolution in which the proton cyclotron is still correctly resolved. These results should be taken into consideration regarding the optimal grid spacing for the modelling of magnetosheath waves, within available computational resources.