Narrow‐band plasmapause hiss observed by ISIS satellites

Abstract

Typical frequency‐time spectra of narrow‐band hiss obtained near plasmapause latitude from ISIS VLF electric field data (50 Hz to 30 kHz) telemetered at Syowa Station, Antarctica, under quiet conditions, are similar to those of narrow‐band 5‐kHz hiss observed at mid‐latitudes and low‐latitudes on the ground. This hiss in the topside ionosphere first appeared as a thin, bar‐shaped emission around 5 kHz at geomagnetic invariant latitude of about 56° and grew into a wider band between 3 and 6 kHz at invariant latitudes around 60° which is the average plasmapause latitude. Then, at higher latitudes, the band narrowed somewhat and disappeared around 65°. The hiss had no lower‐frequency cutoff, and its center frequency remained approximately constant at 5 kHz, between about 56° and 65°. This narrow‐band hiss is completely different from the electrostatic LHR hiss which 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 plasmapause latitude (60°), hereafter we will call this hiss narrow‐band plasmapause hiss. Latitude and local time distributions of the occurrence rate for narrow‐band plasmapause hiss were obtained by analyzing six‐frequency narrow‐band data processed from the wideband VLF signals received at Syowa Station from ISIS 1 and ISIS 2 during 507 passes between December 1976 and January 1983. Although when taken over the whole latitude range, 55° to 64°, the occurrence rate of narrow‐band plasmapause hiss under various geomagnetic conditions (Kp = 0–6) is 0.83, the rate at any given latitude is below 0.35. Thus it is rather a rare phenomenon as compared with LHR hiss. The peak occurrence rate lies at 58° to 59°, under geomagnetically quiet and moderate conditions (Kp = 0–3) and extends over the somewhat broader range of 56° to 60° under disturbed conditions (Kp = 4–6). The occurrence rate under disturbed conditions is about one third that under quiet and moderate 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 under disturbed conditions. The similarity between frequency‐time spectra of narrow‐band hiss observed by the ISIS satellites and on the ground suggests that the narrow‐band hiss generated in the magnetosphere propagates toward the Earth in the whistler mode. The peak occurrence rates of the hiss at invariant latitude 60° (average plasmapause latitude) in the late evening sector under quiet and moderate conditions, and in the midnight sector under disturbed conditions, suggest that narrow‐band plasmapause hiss is generated by cyclotron resonant instability of energetic electrons which are injected from the magnetotail and drift eastward around the equatorial plasmapause.