The plasma depletion layer

The plasma depletion layer (PDL) is a layer on the sunward side of the magnetopause with lower plasma density and higher magnetic field strength compared to their corresponding upstream magnetosheath values. The PDL usually occurs during northward interplanetary magnetic field (IMF) conditions when there is low magnetic shear across the magnetopause. The PDL study has attracted a lot of attention because it affects the coupling of mass, momentum, and energy between the solar wind and the magnetosphere. Theoretical, observational, and numerical studies have made great progress in understanding this structure, however still many problems exist. Are MHD effects and pressure isotropy sufficient to describe the plasma depletion layer? Is the PDL a stable or transient structure? How does the PDL dynamically respond to transient solar wind conditions? What is the spatial extension and global geometry of the PDL? What is responsible for the formation of the PDL? How does a flux tube get depleted in the magnetosheath? Does slow mode front exist in the magnetosheath? What is the exact role of the slow mode waves for the PDL? How is the PDL dependent on solar wind conditions and the Earth dipole tilt? The purpose of this dissertation study is to solve these problems in a systematic way. The UCLA global model is used in the study as an important tool, together with PDL observations and theoretical analysis. First I confirm the validity of the global model in studying the PDL with good fitting between model results and spacecraft observations. MHD effects and pressure isotropy are sufficient to describe the formation of the plasma depletion layer. The PDL is found to be a stable structure under stable solar wind conditions, and dynamically changing solar wind has strong influence on the magnetosheath structures. After model validation, I analyze the forces responsible for the formation of the PDL and find that the combined pressure gradient force and magnetic force is responsible for the formation of the PDL. The PDL extends longitudinally and latitudinally along the magnetopause with varying properties. Flux tube depletion occurs in almost all the subsolar magnetosheath instead of just near the subsolar magnetopause. The bow shock plays an important role in decelerating and depleting flux tube. Near the magnetopause complex pressure gradient force exists which might be responsible for the complex PDL observations. A new method is introduced to calculate the slow mode front in the magnetosheath and the existence of the slow mode front in the magnetosheath for certain, but not all solar wind conditions, is confirmed. However, the PDL does not necessary correspond to the slow mode front and the slow mode front plays no discernible role for shaping the plasma and field in the magnetosheath. Finally, different degrees of dependence of the PDL and the slow mode front on the solar wind magnetosonic Mach number, IMF Bz, the IMF tilt and clock angles, and the Earth dipole tilt are obtained.