IRREGULAR, LONG-PERIOD BOUNDARY OSCILLATIONS BEYOND SIMILAR-TO-100 R(E) - GEOTAIL PLASMA OBSERVATIONS

Near the tail boundary beyond about 100 Re, GEOTAIL often measures irr egular, long-period oscillations in plasma velocity and density. Flow speed and density oscillate between magnetosheath values and values an order of magnitude less. The oscillations can persist for days. A typ ical oscillation lasts 100 minutes, but the range is large. The oscill ations are highly asymmetric in that the increasing phase of the oscil lation is an order of magnitude faster than the decreasing phase. This asymmetry shows that they are a distinct class of oscillations, not p reviously explicitly reported, and that they are not mere consequences of tail flapping in a variable solar wind. The changes in flow direct ion through an oscillation imply that the oscillation results from a m otion of the boundary toward and away from the spacecraft with an ampl itude between 5 and 10 R(e). A consideration of options suggests that the most plausible cause of these oscillations is the 'breathing' of t he magnetotail that attends the substorm cycle.