Plasmaspheric hiss

Abstract

A relatively steady band of ELF hiss has been detected by the Ogo 5 search coil magnetometer on almost every passage through the plasmasphere; except for an anomalous region on the dayside at high geomagnetic latitudes, the emissions terminate abruptly at the plasmapause, and we therefore refer to them as ‘plasmaspheric hiss.’ A preliminary statistical study of the properties of the observed whistler mode turbulence has yielded the following characteristics: the waves are band limited with a sharp lower-frequency cutoff and a more diffuse upper-frequency cutoff; power spectra show a well-defined maximum near a few hundred hertz, the peak intensities generally ranging between 10−7 and 10−5 γ²/Hz; the wave energy is spread over a bandwidth of a few hundred hertz, and corresponding wide band amplitudes are 5–50 mγ; the waves are highly turbulent in nature and show little tendency of definite polarization. The above properties remain essentially constant throughout the plasmasphere. Observed properties of the hiss are consistent with generation at all local times in a restricted L range just within the plasmapause. Waves subsequently propagate on complex paths to fill the plasmasphere. The most probable generation mechanism is cyclotron resonant instability with low-energy electrons that continually diffuse inward from the outer radiation zone. At lower L, hiss resonates with higher-energy electrons, and thus the electrons are scattered in pitch angle and hence lost to the atmosphere throughout the ‘slot’ between the inner and outer radiation belts.