Pressure Anisotropy and By in the Magnetotail Current Sheet

Ions were almost isotropic at the neutral sheet throughout the (-31 < x < -7 R E ), (0 < |y| < 15 R E ) region studied using Geotail data. Pressure anisotropies developed in the middle and outer current sheet, indicating the influence of nonguiding center motion or parallel electric fields. The pressure anisotropies were only large enough to provide from 10% to 30% of the force needed to balance the j x B force. The sign of the measured By was used to find where magnetic field fluctuations and the influence of the IMF By dominate over the long term averaged B y . The fluctuation and IMF effects dominated throughout the current sheet near midnight and near the neutral sheet away from midnight. The average fluctuation plus IMF influence produced a 2 to 3 nT contribution to B y in the low and mid β regions near midnight and a 1 to 2 nT contribution to B y in the highest β region. Since shielding of these contributions to By was found to be weak, this suggests that any IMF effect most likely distorts the current sheet rather than generating a steady field in the plane of the neutral sheet. The lack of shielding also suggests that fluctuations have scale lengths on the order of an ion gyroradius or less.