WAVE-FORM ANALYSIS OF THE CONTINUUM RADIATION OBSERVED BY GEOTAIL

The Plasma Wave Instrument (PWI) onboard the GEOTAIL spacecraft has fr equently observed Continuum Radiation (CR) throughout the geomagnetic tail region, including the Magnetosheath (MS), the Lobe, the Plasma Sh eet Boundary Layer (PSBL), and the Plasma Sheet (PS). In addition to t he usual Escaping CR (from 10 kHz to 30 kHz) and the Trapped CR (from about 5 kHz to 10 kHz), CR with the frequency structure extending down to 1 kHz (the local plasma frequency) has often been observed in the Lobe region. Waveforms of the electric field in the frequency range le ss than 4 kHz, which is acquired by Wave Form Capture in the PWI, are used to analyze in detail the frequency-time structure of such low-fre quency CR near the lower cutoff. Two distinct cutoff frequencies modul ated by the antenna spinning indicates that the CR in the Lobe region propagates both in the O mode and in the X mode almost perpendicular t o the earthward geomagnetic field line. The CR in the Lobe region, esp ecially in the vicinity of the PSBL, is sometimes accompanied by inten se electrostatic electron-cyclotron-harmonic (ECH) (n + 1/2) waves. Th ese suggest that such a low-frequency CR in the distant tail region is most Likely to be generated from the ECH waves near the PSBL, and tra pped within the Lobe between the PSBL and the MS.