Ion distributions in the Earth's foreshock region: Hybrid-Vlasov simulations and spacecraft observations

We present the ion distribution functions in the terrestrial foreshock, simulated by the hybrid-Vlasov model called Vlasiator and observed by the THEMIS and Cluster spacecraft. In the hybrid-Vlasov description, the ion distribution function is propagated in up to three spatial and three velocity dimensions and electrons are modelled as a massless charge-neutralising fluid. Vlasiator was used to model self-consistently the terrestrial bow shock and foreshock regions in the ecliptic plane (two spatial, three velocity dimensions). The simulations were run for tens of ion gyroperiods over hundreds of ion inertial lengths. Vlasiator provides, for the first time, a large-scale picture of the ion distribution in the foreshock with a quality comparable to or even better than spacecraft data thanks to the uniform velocity space sampling and the absence of statistical noise. This allows us to study the interaction between the backstreaming ions and the solar wind, which can trigger instabilities leading to waves in the foreshock.