On the characteristics of VLF emissions in the upper ionosphere and on the ground

Results are presented of an experimental study of the propagation of VLF emissions in the upper atmosphere and on the ground based on the simultaneous detection of VLF emissions on board a satellite and at two widely separated ground stations. Data obtained from low-orbiting Intercosmos satellites at frequencies between 0.2 and 16 kHz were compared with recordings of chorus or structured hiss made at the midlatitude Panska Ves station (L = 2.1) and the auroral zone station at Lovozero (L = 5). VLF emission spectra recorded simultaneously at both ground stations during magnetically disturbed periods are found to be similar, indicating a common source which is observed to extend from L = 2.2 to L = 5.9. Comparison of ground and satellite observations reveals the best correspondence between them to be at L = 2.8-4.1, the region of the emission maximum. Results can be interpreted in terms of the ducting of VLF emissions near the plasmapause following generation in the equatorial magnetosphere, suggesting the existence of an ionospheric zone in which VLF emissions may propagate to the earth's surface, and subsequent propagation in the earth-ionosphere waveguide.