SATELLITE EVIDENCE FOR GENERATION OF VLF EMISSIONS AT MEDIUM LATITUDE BY TRANSVERSE RESONANCE INSTABILITY

During the great magnetic storm of 1967 May 25/26 the orbital plane of the Ariel III satellite (altitude 500-600 km, orbital inclination 80°) lay in the dawn/dusk sector of the magnetosphere. By means of the on-board tape recorder it was possible to determine in some detail, the storm-time evolution of the VLF activity both in intensity and distribution (in invariant latitude). The results are interpreted in terms of generation by the transverse resonance plasma instability. It is shown that the emissions are generated in the vicinity of the geomagnetic equator and exhibit a maximum or saturation intensity close to that predicted by Kennel and Petschek (1966). The maximum intensity of the zone occurs on an L-shell where the minimum gyrofrequency (at the geomagnetic equator) is somewhat greater than the frequency of observation. In the morning sector the VLF emissions stay near to the saturation intensity throughout the solar wind enhancement; in the evening sector they attain saturation intensity towards the end of the storm, during the deflation of the magnetosphere and the decay of the asymmetric ring current. © 1969.