Geotail observations of spiky electric fields and low‐frequency waves in the plasma sheet and plasma sheet boundary

Electric field data from the Geotail spacecraft provide an opportunity to extend the observations of spiky fields made by ISEE‐1 to a region of the magnetosphere where quasistatic electric field measurements have not previously been made, to examine their possible importance in the dynamics of the middle and distant tail, and to test some hypotheses about their formation. In this paper, examples of large fields in the plasma sheet and its boundary at radial distances up to ∼90 R_E are presented. It is shown that three different types of large electric fields can occur: (1) spiky fields; (2) “DC” fields; and (3) waves at frequencies comparable to the lower hybrid frequency. There is usually a gradation between (1) and (3), and often large electric field spikes are embedded in regions of lower amplitude waves. The waves tend to occur in short (few to 10's of seconds) packets whose start and stop times are not always correlated with changes in the magnetic field and/or density (as indicated by the spacecraft potential). The peak frequency is often less than but comparable to the lower hybrid frequency in agreement with theories of lower hybrid drift waves in the magnetotail. The largest spikes are not always associated with the largest changes in the spacecraft potential and/or magnetic field. It is suggested that the spiky fields may represent the nonlinear development of the waves.