Foreshock cavitons for different interplanetary magnetic field geometries: Simulations and observations

Global hybrid (kinetic ions, fluid electrons) simulations have shown the existence of foreshock cavitons characterized by large depressions in plasma density and magnetic field magnitude, bounded by enhancements in these two parameters. Foreshock cavitons share some characteristics with reported foreshock cavities, but in contrast to the cavities that often appear as isolated structures, cavitons are always found surrounded by a sea of ultra low frequency (ULF) waves. They always occur in regions deep in the foreshock, downstream from the ion and ULF wave boundaries. We perform global hybrid simulations to show that cavitons can form for a variety of interplanetary magnetic field (IMF) geometries and different solar wind Mach numbers. We use Cluster data to show that cavitons are a common foreshock feature, and that they can form for different IMF orientations, consistent with our simulation results. We find that cavitons show density and field decrements as large as 60% of the ambient values. They are immersed in regions where waves show transverse and compressive components and the presence of diffuse ion distributions. We use linear kinetic theory to estimate wave growth for the two types of waves responsible for caviton formation, i.e., the weakly compressive waves and oblique propagating linearly polarized waves. Some of the cavitons observed by Cluster show trains of high‐frequency waves inside them and this is consistent with the predictions of local higher‐resolution hybrid simulations.