Whistler-mode emissions on the OGO 1 satellite

Magnetospheric observations of whistler-mode emissions were made by the OGO 1 satellite over the frequency range 0.3 to 100 kHz. Taken during the geomagnetically quiet period from September 1964 through May 1965, the data cover geocentric distances from 2 to 24 RE and geomagnetic latitudes in the range ±50°. Emissions are found within L ∼ 10 at all local times except for L > 5 in the midnight-to-dawn sector, with intensity peaking at L ∼ 4 and L ∼ 9 near 10 hours local mean time. Their spectra resemble those of chorus and mid-latitude hiss observed on the ground. An important new result is the observation that the upper-frequency limit of most of these emissions is proportional to the minimum electron gyrofrequency along the magnetic fieldline passing through the satellite. This is interpreted to mean that the source of these emissions lies close to the equatorial plane. Estimates of the average intensity of these emissions agree with the intensity required to explain the precipitation of electrons through pitch-angle scattering by whistler-mode waves. Emissions observed on the dayside beyond L = 8 are burst-like, and their occurrence tends to decrease with increasing radial distance. The location of the maximum intensity in this region agrees with that found for 1-kev electron fluxes, giving strong support to theories of generation based on electron cyclotron resonance. No emissions were observed beyond the estimated position of the shock boundary.