Tailward flows with positive BZ in the near‐Earth plasma sheet

Abstract

The present study examines tailward flows in the central plasma sheet using Geotail measurements at X ≥ −31 RE. A focus is placed on the midnight near‐Earth 8 > X ≥ −15 RE and ∣Y∣ < 5 RE) region. It is found that the BZ component is predominantly positive when the X component of the perpendicular flow velocity is negative and that a substantial portion (37%) of earthward magnetic flux transport is canceled by tailward flows. This ratio is larger than farther down the tail (−15 > X ≥ −31 RE), 25%, suggesting that there is a cause of tailward flows that works favorably in the near‐Earth region. The tailward flow velocity occasionally exceeds 200 km/s. The results of a superposed epoch analysis and case studies of such fast tailward flows are summarized as follows: (1) the typical duration of a fast tailward flow is 1 min; (2) EY is negative, suggesting that the fast tailward flow is an electric drift; (3) the local magnetic configuration tends to become more dipolar; (4) the ion temperature and density increases and decreases, respectively, and there is no significant change in ion pressure; (5) a fast tailward flow is often preceded by a fast earthward flow; and (6) the Y component of the flow velocity is generally smaller than . It is also found that the geosynchronous magnetic field on the night side rarely changes during these fast tailward flows. The rebound of fast earthward flows is suggested as the most plausible cause of fast tailward flows.