Electrostatic noise bands associated with the electron gyrofrequency and plasma frequency in the outer magnetosphere

Abstract

Naturally occurring noise bands near the electron plasma frequency are frequently detected by the University of Iowa plasma wave experiment on the Imp 6 satellite in the region from just inside the plasmapause to radial distances of about 10 earth radii in the outer magnetosphere. The electric field strength of these noise bands is usually small with typical broad band electric field strengths of about 2 microvolts per m. A magnetic field has been detected only in a few unusually intense cases, and in these cases the magnetic field energy density is several orders of magnitude smaller than the electric field energy density. The bands are observed at all magnetic latitudes covered by the Imp 6 orbit and appear to be a permanent feature of the outer magnetosphere. They are found at all local times and occur least frequently in the quadrant from 18 to 24 hours. The bands appear to consist of two distinct spectral types called diffuse and narrow band. In both types the center frequency of the noise band is bounded by consecutive harmonics of the electron gyrofrequency, and these noise bands occur most often between harmonics that are near the local electron plasma frequency.