Sudden Pressure Enhancement and Tailward Retreat in the Near-Earth Plasma Sheet: THEMIS Observation and MHD Simulation

Abstract:

Sudden enhancement of the plasma pressure in the near-Earth plasma sheet is one of the common manifestations of the substorms, and is thought to play an important role in relevant disturbances in the magnetosphere and ionosphere. On 1 March 2008 four of the THEMIS (Time History of Events and Macroscale Interactions during Substorms) probes observed the sudden enhancement of the plasma pressure around 15:40 UT. The four probes were almost aligned along the Sun-Earth line, which was suitable for investigating spatial-temporal evolution of the near-Earth plasma sheet around the substorm onset. The four probes were located off the equatorial plane, according to a magnetic field model. The plasma pressure suddenly increased at the inner most probe first (at ~7.2 Re), followed by the outer probes (at ~7.5, ~8.3, and ~10.4 Re), that could be seen as a tailward propagation (or retreat) of high-pressure region (HPR). After comparing with results of a global magnetohydrodynamics (MHD) simulation, we found that only the tailward propagation of the HPR could be seen at off-equator. Near the equatorial plane, the HPR propagates earthward from the magnetotail region, then it retreats tailward. In the course of the tailward propagation, the HPR also propagates away from the equatorial plane. As a consequence, the inner most probe observed the pressure enhancement first, followed by the outer probes. The propagation of the HPR in the Z_GSM direction is understood to be a combination of the convergence of the plasma flow (the divergence of bulk velocity along the Z_GSM axis), and the pressure gradient force.