Ion distributions in the Earth's foreshock: Hybrid-Vlasov simulation and THEMIS observations

We present the ion distribution functions in the ion foreshock upstream of the terrestrial bow shock obtained with Vlasiator, a new hybrid‐Vlasov simulation geared toward large‐scale simulations of the Earth's magnetosphere (http://vlasiator.fmi.fi). They are compared with the distribution functions measured by the multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The known types of ion distributions in the foreshock are well reproduced by the hybrid‐Vlasov model. We show that Vlasiator reproduces the decrease of the backstreaming beam speed with increasing distance from the foreshock edge, as well as the beam speed increase and density decrease with increasing radial distance from the bow shock, which have been reported before and are visible in the THEMIS data presented here. We also discuss the process by which wave‐particle interactions cause intermediate foreshock distributions to lose their gyrotropy. This paper demonstrates the strength of the hybrid‐Vlasov approach which lies in producing uniformly sampled ion distribution functions with good resolution in velocity space, at every spatial grid point of the simulation and at any instant. The limitations of the hybrid‐Vlasov approach are also discussed. Key Points Ion foreshock distributions from global hybrid‐Vlasov magnetosphere simulations Presentation and comparison with THEMIS data The hybrid‐Vlasov model produces ion distributions of unprecedented quality