Local time variation of ELF emissions during periods of substorm activity

The relationship between naturally generated electromagnetic ELF emissions and magnetospheric substorm activity has been studied by using spectrum channel data from the OGO 6 search coil magnetometer. Statistics are presented on the probability of observing emissions for all local times during different phases of the substorm activity. Our statistical results exhibit a correlation between the onset of the onset of the substorm expansive phase and enhancements of ELF emissions in the postmidnight and early morning sectors; it is believed that these emissions are predominantly ELF chorus which is generated through cyclotron resonance with anisotropic electrons injected into the nightside outer radiation zone at the substorm onset. The most intense and persistent ELF emissions are observed during daytime conditions, when wave intensities exceeding 4.5 x 10-4 γ Hz-1/2 occur on more than 50% of the OGO 6 orbits analyzed; this result even applies during periods of extreme magnetic quiescence. These dayside ELF emissions are thought to be predominantly plasmaspheric hiss. In the postnoon and early evening sector, such emissions decrease during the substorm activity but subsequently exhibit a pronounced enhancement for a period of over 10 hours after the substorm activity has subsided. During this postsubstorm period the peak signals also shift to progressively later local times at a rate consistent with the azimuthal drift time for medium-energy (approx. = 30 keV) electrons injected into the nightside outer radiation zone (L approx. =5) during the substorm expansive phase. This behavior is interpreted in terms of electron cyclotron resonant generation of plasmaspheric hiss just within the plasmapause