Properties of current sheet thinning at x～?10 to ?12 R_E

摘要

We report on Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations of current sheet thinning in Earth's magnetotail at around x=?10 to ?12 Earth radii. The THEMIS spacecraft configuration in October–December 2015 allows us to construct both gradients that contribute to the cross-tail current density j_y =μ_0~(?1) (σB_x∕σz ? σB_z∕σx) (GSM coordinates). For 17 events when the spacecraft observed a gradual B_z decrease and jy increase, we find the following average scaling relations: for the current density j_y ～ B_1~(?7∕4) , for the lobe magnetic field B_L～ B_z~(?1∕4) , and for the plasma density ni ～ B_z~(?3∕4) . We show that the temperature of ions and electrons decreases and the plasma pressure gradient σp∕σx rapidly increases during current sheet thinning. The scale L_x =(σ ln p∕σx)~(?1) decreases a few thousand kilometers. We also consider current carriers in thinning current sheets: both ion and electron current-dominated current sheets, preferentially located near dusk and midnight, respectively, are found.