Earthward electric field and its reversal in the near-Earth current sheet

Using Time History of Events and Macroscale Interactions during Substorms observations (radial distance r from 9 to 35 Earth radii, R-E), we investigate ion and electron contributions to the cross-tail current density in the magnetotail current sheet. We analyze plasma pressure measurements (including the contribution from high-energy particles) and estimate the magnitudes of ion and electron diamagnetic drifts. In the downtail, r > 15 R-E, region, ion (electron) diamagnetic drifts are shown to provide more than 50% (less than 25%) of the cross-tail current density at the neutral plane, B-x = 0. Conversely, in the near-Earth region, r <= 15 R-E, the ion (electron) diamagnetic drift contribution to the cross-tail current density is 20% (50%). The directly measured duskward (dawnward) component of the ion (electron) velocity, v(yi) (-v(ye)), where y is the GSM direction, is very small (quite large) in the downtail region but large (small) in the near-Earth region. This systematic discrepancy between the expected values of v(yi), -v(ye) (based on estimates of diamagnetic drifts) and the direct measurements of the velocity, v(yi), -v(ye), is consistent with a contribution to the total velocity by an E x B drift caused by an electric field oriented parallel to the x axis, E-x. To decrease the ion (increase the electron) total drift to agree with the measured flows in the downtail region and increase (decrease) this total drift to match the measurements in the near-Earth region, this E-x would need to be directed earthward at r > 15 RE and tailward at r <= 15 R-E. Such an E-x distribution is consistent with the equatorial projection of the Harang discontinuity.