On the origin of reverse polarity TCRs

Reverse polarity, or South-then-North (SN) bipolar, traveling compression r egions (SN TCRs) are often observed in the Earth's magnetotail lobes. These events have been interpreted as either slowly earthward propagating "proto -plasmoids" during extremely quiet geomagnetic conditions, or due to pressu re pulses in the solar wind or magnetosheath compressing the magnetotail. T his study presents a survey of 21 IMP 8 observations of SN TCRs and the cor responding solar wind pressure conditions as measured by WIND. We found tha t solar wind or magnetosheath pressure pulses nicely explain most (17), tho ugh not all, of the SN TCR observations. Therefore, it appears that both ex planations previously given are needed to explain SN TCRs. We also found th at most of these events occurred during northward Interplanetary Magnetic F ield (IMF) conditions. This suggests that the magnetotail may respond diffe rently to solar wind dynamic pressure pulses for different orientations of the IMF.