A search for geographic control on the occurrence of magnetospheric ELF emissions

Abstract

Search coil magnetometer data obtained directly in the magnetosphere on Ogo 5 and Ogo 6 have been studied extensively for possible evidence of man‐induced stimulation of the two dominant electromagnetic emissions, chorus and plasmaspheric hiss. Analysis of chorus obtained during the first year of operation of Ogo 5 exhibits considerable variability with geographic longitude. However, there is little or no correspondence with the longitudinal dependance of Ogo 3 chorus previously reported by Luette et al. (1977). This discrepancy is explained in terms of the persistent nature of chorus which can lead to oversampling of the data. When oversampling is eliminated, the longitudinal variability of Ogo 5 chorus is adequately explained by normal statistical fluctuations. A search was also made for enhancements, onsets or quantum jumps in chorus emissions at frequencies related to harmonics of the earth's transmission lines. No significant effect was found. An independent study of plasmaspheric hiss detected on the polar‐orbiting satellite Ogo 6 indicates that this emission is also essentially uniformly distributed in dipole longitude. The only exception to this was a modest and probably significant (4.5σ) enhancement in the occurrence of weak emissions in the 340°–360° dipole longitude region during very quiet geomagnetic conditions. The source of this enhancement could be man induced, or it might simply be related to the expected enhanced level of sferic activity associated with thunderstorm activity over the North American continent. However, because this feature is present only when ELF hiss is weak, it is clear that it will not contribute significantly to the non adiabatic dynamics of radiation belt electrons. We conclude that the totality of satellite data analyzed to date is consistent with a predominantly natural origin for chorus and plasmaspheric hiss. While we do not dispute the existence of triggered waves, it is difficult, in view of their low overall occurrence, to imagine that they play any major role in the non adiabatic dynamics of geomagnetically trapped electrons.