Radial Force Balance in Earth's Magnetotail Thin Current Sheets: MMS Observations

Thin current sheets (TCS) in Earth's magnetotail are fundamental to magnetospheric dynamics. A key question concerning static magnetotail TCSs is the mechanism of radial force balance. Using the unprecedented measurements from the Magnetospheric Multiscale mission, we statistically analyze TCS crossing events from 2017 to 2020 to investigate this issue. Our analysis reveals a strong magnetic tension within TCSs, with the radial thermal pressure gradient accounts for only 10%–30% of the required balance. The off-diagonal pressure components (P_i,xz and P_e,xz) are crucial for achieving force balance, contributing ∼55% of the required force in the further-Earth region (−30 R_E < X < −20 R_E, where R_E is Earth's radius), and ∼30% in the near-Earth region (−20 R_E < X < −10 R_E). This work provides the first direct observational evidence demonstrating that particle kinetic effects (ion nongyrotropy and electron pressure anisotropy) play a significant role in the force balance of magnetotail TCSs.