Shaping of the magnetotail from the mantle: Global and local structuring

This paper discusses kinetic modeling of the properties of magnetotail formation from a plasma mantle source and develops a unified view of the structure of the central part of the magnetotail plasma sheet as well as the structure of its boundary layer. Trajectories of mantle protons in the presence of a uniform dawn‐dusk electric field have been traced using the Tsyganenko magnetic field model for quiet periods of magnetospheric activity. The most important portion of the particle trajectories is each particle's first interaction with the sharp reversal of the magnetic field in the tail midplane, because this interaction results in particle energization and chaotic scattering. The closer this interaction takes place to the X line, the larger a particle's energy will become. The intensity of the chaotic scattering in the tail midplane depends also on the position in the tail at which it occurs. The energization and scattering result in a significant restructuring of the tail ion distributions, both in space and in velocity coordinates. Our model shows the evolution of the global structure of the tail with a clearly defined central plasma sheet and plasma sheet boundary layer developing from its beginnings as a plasma “nucleus” in the distant tail current sheet. This large‐scale restructuring is accompanied by the creation of small‐scale features in the particle distribution functions. For example, the model not only correctly reproduces the spatial distribution and velocity dispersion of the fast ion beams moving both earthward and tailward in the plasma sheet boundary layer, but also indicates that these beams should be highly structured spatially into 5‐6 smaller beamlets with distinct velocities. In addition the model shows that complementary ring distribution structures should also exist in the central plasma sheet. Our analysis indicates that the ion distribution functions in the central plasma sheet should take a rather specific form in velocity space with loss regions oriented predominantly orthogonal to the magnetic field. Our results also emphasize the importance of counterstreaming populations, not only in the boundary layer, but also in the central part of the plasma sheet. Analytical calculations indicate that the properties of chaotic scattering in the magnetotail under realistic conditions ( x dependence of the normal magnetic field and dawn‐dusk electric field) are quite different from those predicted by earlier simple x independent models. Finally the model results are compared with recent observations of ion distribution functions and their moments for various regions of the magnetotail, and quantitative estimates from the model are shown to be in good agreement with observations. Small‐scale structuring and the presence of counterstreaming are also discussed, as well as their possible importance in explaining the observed intermittency in the plasma sheet bulk flows.