Global hybrid simulations: Foreshock waves and cavitons under radial interplanetary magnetic field geometry

Abstract

We use global hybrid (kinetic ions and fluid electrons) simulations to study the solar wind interaction with the magnetosphere for a radial (θvB = 0) interplanetary magnetic field (IMF) geometry. Global hybrid simulations provide a collective picture of processes taking place in the foreshock, bow shock, and magnetosheath. Because ions are treated as particles, these codes also give information on ion‐scale microphysics. Under radial IMF geometry, the foreshock forms in front of the dayside magnetosphere, and the plasma convecting downstream is very perturbed. The foreshock is permeated by weakly compressive ultralow frequency (ULF) waves that propagate at angles up to 30° to the ambient field. These weakly compressive waves are generated by field‐aligned ions. Wave‐Particle interaction results in ion scattering, while wave amplitude grows and fluctuations become compressive as they approach the shock. While weakly compressive waves are dominant far from the shock, a second population of ULF fluctuations arises close to it. These fast magnetosonic waves propagate at large angles to the magnetic field and are linearly polarized. We find that, in addition to the ULF waves, large depressions in density and magnetic field magnitude form near the shock. These density cavitons or depressions are bounded by enhanced magnetic fields, and inside them, hot diffuse ions are present. Foreshock density cavitons in our simulations are embedded in regions with ULF activity, which is in contrast to the isolated character of “foreshock cavities” reported previously. We show that density cavitons form due to the nonlinear interaction of the two types of waves present in the foreshock. Wave interaction also modifies the characteristics of weakly compressive waves. A comparison of our results with Cluster observations reveals that the characteristics of weakly compressive waves in our simulation resemble the properties of right‐handed 30‐s waves found in the terrestrial foreshock. Likewise, we show the existence of density cavitons, or depressions, in regions of Earth's foreshock permeated by ULF waves. The properties of these observed Cluster cavitons are similar to the ones we find in the simulations.