ELF noise bands associated with auroral electron precipitation.

Observation of a new type of ELF noise band that is closely associated with low-energy auroral electron precipitation. These observations have been made at relatively low altitudes (less than 3000 km) with the polar-orbiting satellite Injun 5. The noise bands typically have a center frequency of 100 to 300 Hz and often appear to consist of many nearly monochromatic bursts, typically of a few seconds' duration, superimposed to produce the observed noise band. These ELF noise bands are observed only in a relatively narrow range of latitudes (a few degrees) in the auroral zone and are almost always associated with intense fluxes of precipitating electrons with energies from a few hundred electron volts to several kiloelectron volts. On the dayside of the magnetosphere the region where the ELF noise bands and the associated low-energy electron precipitation are observed has been identified as the polar cusp. In considering the possible explanations of these ELF noise bands, it is noted that the spectral characteristics of this noise are very similar to a type of narrowband electromagnetic noise called 'lion's roar,' which has been observed at much higher altitudes in the magnetosheath with the satellite Ogo 5. It is suggested that the ELF noise bands observed at low altitudes with Injun 5 are caused by lion's roar emissions that have propagated down 'open' magnetic-field lines to low altitudes from the magnetosheath region.