ISEE 1 observations of VLF line radiation in the Earth's magnetosphere

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Abstract
A search has been carried out for magnetospheric line radiation in the VLF data acquired during the period October 1977–August 1979 by the Stanford University VLF receiver on the ISEE 1 satellite, while the satellite position varied between L = 2 and L = 8 and the satellite longitude lay in the range 50°W to 110°W. This magnetopsheric region encompassed the magnetic field lines linking three stations, Eights, Siple and Roberval, from which most ground‐based data on VLF line radiation has been obtained and also included one of the main regions in which the occurrence of VLF chorus has been actively linked to power line radiation. Line radiation was detected on only 5 of 90 orbits, and all examples occurred at frequencies below 4 kHz. The most clearly defined example of line radiation was acquired near L = 5 under conditions of very low magnetic activity during which the plasma‐pause was located at L ∼ 7. In this particular case, the strongest lines in the spectrum exhibited a frequency change of approximately 22 Hz/min over a 9‐min period, while weaker lines changed frequency as slowly as 6 Hz/min. The line radiation was detected during a period when whistler mode echoing was quite pronounced on lower L shells and may actually have been a scattered component of line radiation echoing between hemispheres on a magnetic shell of lower L value. In all instances the S/N ratio or the absolute intensity of the lines was quite low. Thus, in agreement with earlier work, we conclude that very little of the background VLF wave energy in the outer magnetosphere is contained directly in VLF line radiation. However, the catalytic role of line radiation in controlling VLF wave‐particle interactions remains to be assessed.