Continuum radiation associated with low-energy electrons in the outer radiation zone

Abstract

A weak nonthermal continuum radiation is generated by the earth's magnetosphere in the frequency range from about 500 Hz to greater than 100 kHz. During magnetically disturbed periods the intensity of this continuum radiation increases significantly, by as much as 20 dB during large disturbances. In this paper we present a series of observations obtained by the Hawkeye 1 and Imp 8 spacecraft during a period of greatly enhanced continuum radiation intensity which occurred from October 14–21, 1974. The enhanced continuum radiation intensities observed during this event are found to be closely correlated with the injection of very intense fluxes of energetic, ∼1–30 keV, electrons into the outer radiation zone. Direction-finding measurements of the continuum radiation observed during this event show that the radiation is primarily coming from the dawn side of the magnetosphere, in agreement with the observed dawn-dusk asymmetry in the 1- to 30-keV electron distribution. These results suggest that the continuum radiation may be generated by a coherent plasma instability involving relatively low-energy, ∼1 to 30 keV, electrons rather than by gyrosynchrotron radiation from very energetic, 200 keV to 1 MeV, electrons as has been previously suggested.