Global hybrid simulation of hot flow anomalies near the bow shock and in the magnetosheath

Abstract

A two-dimensional global hybrid simulation is carried out to examine the generation of hot flow anomalies (HFAs) by interaction of the Earth's bow shock (BS), magnetosheath, and magnetosphere with an interplanetary tangential discontinuity (TD). By including the dayside magnetosphere in the simulation domain, this new study focuses on the formation and internal structure of strong HFAs, both near the bow shock and in the magnetosheath, and their effects on the dayside magnetopause. The simulation is performed for directional TDs, which carry a magnetic field orientation change and propagate to the BS from various directions. In the cases in which the magnetic field orientations across the TD result in a normal component of the upstream motional electric field pointing toward the TD, a strong HFA can be generated by coherent, gyrating reflected ion beams near the BS. In the center of the upstream part of the HFA, a low plasma density and a low magnetic field strength are present. The plasma is heated to a nearly isotropic temperature that is much greater than that in the downstream magnetosheath. The bulk flow speed is greatly reduced, and a significant sunward flow deflection is present in the HFA. The HFA bulges into the solar wind due to an enhancement of the total pressure in the hot cavity. As a result, the local BS and the magnetosheath may expand into the upstream solar wind by several Earth radii. A significant enhancement in the density and magnetic field strength flanks the bulge. The structure of the magnetosheath HFA, which encompasses the transmitted tangential discontinuity, is similar to that for the upstream HFA. Nevertheless, relative to the magnetosheath total pressure perpendicular to the HFA boundaries, a low pressure region is present in the magnetosheath HFA due to pressure balance with the upstream HFA. The magnetopause can expand sunward by several Earth radii as the HFA passes by. In the simulation, the size of the diamagnetic cavity increases with the rotation angle Δ Φ of the magnetic field through the TD, which corresponds to the magnitude of the normal electric field. The maximum strength of the HFA occurs when the angle of the propagation direction of the TD relative to the Sun–Earth line, γ, is nearly 80°. In addition, spatial structures with alternate temperature increases and decreases develop at the quasi-parallel shock. These structures are elongated along field lines, both upstream and downstream of the bow shock. Those with temperature increases and density, magnetic field, and the flow speed decreases appear like weak HFAs, but without strong flow deflections.