Magnetosheath cavities: Cluster observations and kinetic hybrid model comparisons

Abstract

High energy particles cause depressed magnetic field and density regions in the foreshock region which are called as foreshock cavities. These cavities are formed as a result of the interaction between the backstreaming ions energized at and reflected from the bow shock. This study investigates whether similar structures are present in the magnetosheath region, and if so, what their characteristics and possible sources are using Cluster observations and compare the results from those obtained using kinetic hybrid particle simulations. Our search shows that the magnetic field strength and ion density are depressed up to 50% in the presence of high energetic (>30 keV) ion particle fluxes. Also slight decreases in ion velocity and increases in ion temperature are seen. The depressed magnetic field and density characterize the magnetosheath cavities. Our search also indicates that magnetosheath cavities occur during the low IMF cone angles less than 30°. We run a 2,5-D kinetic-hybrid particle model for radial IMF (cone angle is 0°) and inclined IMF (cone angle is 45°), to understand the response of the magnetosheath under these conditions. The simulation results clearly demonstrate that the magnetosheath cavities form when the IMF cone angle is low and verify Cluster observations. These results from the model suggest that the solar wind carries the foreshock cavities into the magnetosheath under the low IMF cone angle (radial IMF) conditions. In this study, we compare the magnetic field and density structure of the magnetosheath from the model pointing at the similarities and differences. The particles accelerated to high energies at various energy levels as they pass through the Earth's bow shock. Their further transport with the solar wind into the magnetosheath modifies the magnetosheath's magnetic and plasma characteristics. In the model, depressed magnetic field and ion density regions are seen more distinctly toward the mid latitude magnetosheath. Model's magnetosheath cavities display highly structured and turbulent features and they seem to depend on the location in the magnetosheath. These periodic, high amplitude fluctuations within the model magnetosheath cavities indicate wave activity and wave-particle interactions and that they tend to be stronger and are more evident toward the higher latitudes of the magnetosheath.