Slow‐mode shocks in the magnetotail

Abstract

We have identified slow‐mode shocks between the plasma sheet and lobe in the midtail to distant‐tail regions by using three‐dimensional magnetic field data and three‐dimensional plasma data including density, velocity, temperature, and heat flux of both ions and electrons observed by the GEOTAIL satellite. Analyzing the data obtained between September 14, 1993, and February 16, 1994, we have found 303 plasma sheet‐lobe boundary crossings at distances between ‐XGSE ∼ −30RE and XGSE ∼ −210RE. Thirty‐two out of these 303 boundaries are identified as slow‐mode shocks. We have found back streaming ions on the upstream side of the slow‐mode shocks, which may be important in understanding the dissipation mechanism of the slow shocks in collisionless plasma. We have also found acceleration of cold ions between the upstream and the downstream of the slow‐mode shocks. These cold ions are often observed in the lobe, and they are usually flowing tailward. Upon entering the plasma sheet, they are accelerated and rotate around the magnetic field and at times show ring‐shaped velocity distributions. These ions may reflect the kinetic structure of slow‐mode shocks. Slow shocks are at times observed also on the front side of plasmoids. These slow shocks on the front side of plasmoids have a different orientation from that of the ordinary slow shocks observed at the plasma sheet‐lobe boundaries, which suggests an existence of “heart” ‐shaped plasmoids predicted by a numerical simulation.