Global View of Current Sheet Thinning: Plasma Pressure Gradients and Large-Scale Currents

Abstract

The formation of a thin magnetotail current sheet is a key process in magnetic energy accumulation before magnetic reconnection during substorms. Although the 3-D configuration of a thinning current sheet has been well reproduced in many numerical simulations, observational details of this configuration have been less thoroughly studied. To investigate the global and local 3-D structure of a thinning current sheet, we use a data set collected by three spacecraft of the Time History of Events and Macroscale Interactions during Substorms mission, two spacecraft of the Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun mission, and the Geostationary Operational Environmental Satellite 15. We demonstrate that near-Earth current sheet thinning is accompanied by the formation of equatorial plasma pressure gradients directed from the flanks toward midnight. These gradients are associated with an intensification of the dawn-dusk current (during current sheet thinning) and field-aligned currents of opposite polarity at the dawn and dusk flanks. Simultaneous Time History of Events and Macroscale Interactions during Substorms and Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun observations show that current sheet thinning occupies a large portion of the tail, from the near-Earth region to lunar orbit. An increase in the equatorial plasma pressure (and the lobe magnetic field pressure) is provided by a cold plasma density increase in the near-Earth tail and likely by a plasma temperature increase in the distant tail. We discuss plasma convection patterns that are consistent with observed properties of the current sheet thinning.