Impulsive plasma waves observed by DE 1 in nightside magnetosphere

Impulsive plasma waves with a frequency range of 1-9 kHz were found in the wide-band electric field data (650 Hz to 10 kHz) from the DE 1 satellite received at the Kashima station in Japan. The impulsive plasma waves were often accompanied by a strong hiss band at about 1 kHz, but not by a chorus. The frequency-time spectra of these waves are very similar to those of electrostatic bursts associated with ELF chorus in the outer dayside magnetosphere, but these impulsive waves were observed at low geomagnetic latitudes in nightside magnetospheric regions much nearer the Earth for geomagnetic quiet and disturbed times. Local plasma densities ({approximately} 1 cm{sup {minus}3}) obtained from the characteristic frequency of VLF emissions associated with the impulsive plasma waves agree with those observed simultaneously by the DE 1 retarding ion mass spectrometer. It is inferred from the local plasma density and the empirical relation of the plasmapause position with Kp that the impulsive plasma waves are occurring just outside the plasmapause. The impulsive plasma waves are discussed in terms of a resistive medium instability caused by Landau resonant trapping of thermal electrons by the associated hiss band at about 1 kHz, since this mechanism seemsmore » to be plausible from the available data and the frequency-time spectra are similar to electrostatic bursts identified previously. Some of the events do not seem to have a strong ELF hiss band accompanying the impulsive plasma waves. It is suggested that these events may be due to electrons bunched by an ELF hiss band in a nearby region or may be caused by space discharges in the vicinity of the long-wire antenna on the DE 1 spacecraft.« less