OGO 3 observations of ELF noise in the magnetosphere: 1. Spatial extent and frequency of occurrence

Abstract

The magnetic noise in the magnetosphere in the frequency range from 10 to 800 Hz has been extensively measured by the spectrum analyzers of the search coil magnetometer on OGO 3. This paper is a statistical study of the spatial extent and frequency of occurrence of noise at the higher end of this passband, at which frequencies noise above the detector thresholds is most common within the magnetosphere. Steady noise and noise bursts are found to constitute two distinct populations. Both the local-time and magnetic latitude distribution of both classes of signals are investigated. When the magnetic latitude distributions are extrapolated downward to 1000-km altitudes, the results are consistent with previous satellite observations at these low altitudes. However, the equatorial distributions cannot be inferred by simply projecting the magnetic noise measured at low altitudes onto the equator along flux tubes. The in situ measurements cannot determine the exact location of the source of all the noise observed. However, it is found that steady noise is definitely generated near 45° magnetic latitude on the dayside of the magnetosphere for L values from 6 to 10 and that bursts are generated near the equator above L = 6 from 0400 to 1800 local time. The latter observation can be used to explain the generation of both auroral microbursts and chorus as seen on the ground by means of whistler mode wave growth at the equator supported by a pitch-angle anisotropy maintained by the loss cone.