Geotail Observations of Low Frequency Waves at Substorm Onset in the Near Magnetotail

K. Sigsbee, C. A. Cattell (School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455)

F. Mozer (Space Sciences Laboratory, University of California, Berkeley, CA 94720)

D. Fairfield (NASA/GSFC, Laboratory for Extraterrestrial Physics, Code 695, Greenbelt, MD 20771)

K. Tsuruda and T. Yamamoto (ISAS, Sagamihara 229, Japan)

S. Kokubun (STELAB, Nagoya University, Toyokawa 442, Japan)

On April 26, 1995 a substorm onset occurred at about 0750 UT as identified by auroral kilometric radiation, ground magnetometers and data from several spacecraft(Fairfield et al., Paper Presented at ICS 3, 1996). During this event, Geotail was in the near equatorial region around 13 Re and 1300 LT. Instruments on board the Geotail spacecraft observed intense DC electric fields of 50 mV/m in conjunction with some of the largest high speed earthward flows ever detected at 13 Re. We will present time-series analysis of low frequency waves observed during the April 26, 1995 substorm event. The observed waves are extremely intense, bursty and often associated with large spiky fields. At frequencies below a few Hz, the dominant power is at frequencies comparable to the lower hybrid frequency. At lower frequencies, a brief compression in the Pi2 range is observed, as well as waves at higher frequencies. Similar waves are observed in association with other onsets in the inner magnetotail. Statistical comparisons to waves observed during other events will be made. The dominant wave modes will be identified and their implications for relevant substorm processes will be discussed.

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