Generation Mechanism of ESW Based on Geotail Plasma Wave Observation, Plasma Observation, and Particle Simulation

H. Matsumoto, L. A. Frank, Y. Omura, H. Kojima, W. R. Paterson, M. Tsutsui, R. R. Anderson, S. Horiyama, S. Kokubun, T. Yamamoto.
Geophys. Res. Lett., submitted for publication, 1997.

Data on electrostatic waves and plasma particles in the deep magnetotail (X -209RE) respectively obtained by Plasma Wave Instrument (PWI) and Comprehensive Plasma Wave Instrument (CPI) onboard the GEOTAIL spacecraft were used for the present study. When GEOTAIL experienced multiple crossings of the plasma sheet boundary layer (PSBL) because of the flapping of the magnetotail after a substorm, broadband electrostatic noise (BEN) and Langmuir waves (LW) were observed alternatively. BEN has very fine structures in the dynamic frequency spectra, and their waveforms are in fact a series of electrostatic solitary waves (ESW). The LW are observed when the velocity distribution function of thermal electrons are relatively cold, while the ESW are observed in association with the hot thermal electrons covering the velocity range of the drift velocity of the assumed electron beam. These plasma conditions are in agreement with ESW generation model based on particle simulations. The present study confirms that the ESW are generated by the bump-on-tail instability in the PSBL, and that the ESW travel along the ambient magnetic field being embedded in the high energy tail of the velocity distribution function of the thermal electrons.