Equatorial hiss in the topside ionosphere

Abstract

Equatorial hiss seen in the topside ionosphere by the Dartmouth experiment on Ogo 6, using an electric dipole antenna, is described. The observations are consistent with wave normal positions intermediate between the geomagnetic field and the resonance cone. Unlike that seen by Gurnett (1968) on Injun 3, using a loop antenna, the hiss is seen over the full range of altitudes from perigee to apogee (400–1100 km). There is a significant local time dependence, with concentration in the dawn and dusk sectors. Simultaneous occurrence with spread F would appear to be no more than coincidental. The possibility of spacecraft effects has been discounted. A search by Hoffman (1977) for equatorial electron fluxes such as those found by Heikkila (1971) has been negative. Thus the underlying cause of the hiss remains unknown. The hiss occurs in two distinct forms, ‘spiky’ and ‘nonspiky.’ No parameter has been found which clearly determines which type will occur, although there is some local time dependence. The hiss is most intense at the lowest frequency observed (20 Hz), but the spiky form may extend above 15 kHz and occasionally even to 200 kHz. Although it is largely confined to the equatorial zone, two instances were found when this hiss extended to middle latitudes and two when it occurred in the polar cap. In one of the latter it coexisted with the usual auroral hiss. A reassessment of synoptic recordings made at Huancayo during the International Geophysical Year has reaffirmed the absence of hiss, as well as whistlers, on the ground at the magnetic dip equator.