NARROW-BAND PLASMAPAUSE HISS OBSERVED BY ISIS SATELLITES

Typical frequency-time spectra of nar-row-band hiss obtained near plas mapause latitude from ISIS VLF electric field data (50 Hz to 30 kHz) t elemetered at Syowa Station. Antarctica, under quiet conditions, are s imilar to those of narrow-band 5-kHz hiss observed at mid-latitudes an d low-latitudes on the ground. This hiss in the topside ionosphere fir st appeared as a thin, bar-shaped emission around 5 kHz at geomagnetic invariant latitude of about 56-degrees and grew into a wider band bet ween 3 and 6 kHz at invariant latitudes around 60-degrees which is the average plasmapause latitude. Then, at higher latitudes, the band nar rowed somewhat and disappeared around 65-degrees. The hiss had no lowe r-frequency cutoff, and its center frequency remained approximately co nstant at 5 kHz, between about 56-degrees and 65-degrees. This narrow- band hiss is completely different from the electrostatic LHR hiss whic h is also observed above the ionosphere with an electric antenna, but which has latitude-dependent lower-frequency cutoff. Since the narrow- band 5-kHz hiss occurs often around plasma-pause latitude (60-degrees) , hereafter we will call this hiss narrow-band plasmapause hiss. Latit ude and local time distributions of the occurrence rate for narrow-ban d plasmapause hiss were obtained by analyzing six-frequency narrow-ban d data processed from the wideband VLF signals received at Syowa Stati on from ISIS 1 and ISIS 2 during 507 passes between December 1976 and January 1983. Although when taken over the whole latitude range, 55-de grees to 64-degrees, the occurrence rate of narrow-band plasmapause hi ss under various geomagnetic conditions (Kp = 0-6) is 0.83, the rate a t any given latitude is below 0.35. Thus it is rather a rare phenomeno n as compared with LHR hiss. The peak occurrence rate lies at 58-degre es to 59-degrees, under geomagnetically quiet and moderate conditions (Kp = 0-3) and extends over the somewhat broader range of 56-degrees t o 60-degrees under disturbed conditions (Kp = 4-6). The occurrence rat e under disturbed conditions is about one third that under quiet and m oderate conditions. The magnetic local time (MLT) distribution of the occurrence rate has a significant maximum at 1800-2100 hours MLT under quiet and moderate conditions, and a maximum at 0000-0300 hours MLT u nder disturbed conditions. The similarity between frequency-time spect ra of narrow-band hiss observed by the ISIS satellites and on the grou nd suggests that the narrow-band hiss generated in the magnetosphere p ropagates toward the Earth in the whistler mode. The peak occurrence r ates of the hiss at invariant latitude 60-degrees(average plasmapause latitude) in the late evening sector under quiet and moderate conditio ns, and in the midnight sector under disturbed conditions, suggest tha t narrow-band plasmapause hiss is generated by cyclotron resonant inst ability of energetic electrons which are injected from the magnetotail and drift eastward around the equatorial plasmapause.