Explorer 45 and Imp 6 observations in the magnetosphere of injected waves from the Siple Station VLF transmitter

Abstract

This paper reports the first results of a Stanford University-University of Iowa joint experiment in which VLF waves from the Siple Station transmitter in Antarctica are injected into the magnetosphere along the earth's magnetic field lines and are detected near the magnetic equatorial plane by the Explorer 45 and Imp 6 spacecraft. The purpose of this experiment is to conduct a controlled in situ study of VLF wave-particle interactions, in particular, to determine the propagation characteristics of the injected waves in the magnetosphere, to determine the regions where VLF emissions are produced, and to determine the effective volume of the magnetosphere illuminated by the Siple transmitter. During the first 3 months of the joint experiment, transmissions to the two spacecraft were attempted on 25 separate occasions when the satellites were within ±30° longitude of the magnetic field line linking Siple Station and its conjugate point at Roberval, Canada. Measurable signals were produced during 10 of these 25 passes, and these signals were observed as much as 20°E and 15°W of the Siple longitude and on various L shells between 2.9 and 5. On one pass a strong, well-defined signal was present over a 6000-km-long orbital section between L ∼ 4 and L ∼ 5, indicating that the transmitter can illuminate a large volume of the magnetosphere. Emissions stimulated by the transmitter pulses were observed on three passes, and there was evidence on one of these passes that emission stimulation and/or wave entrainment effects were taking place far from the magnetic equator. Signal amplitude measurements were available on only one pass, but these were obtained over a wide region. Amplitudes varied from 10−5 γ outside the plasmapause at L = 5 to 10−4 γ just inside the plasmapause at L ∼ 4. On the basis of these first results of the Stanford University-University of Iowa joint experiment we conclude that important possibilities exist for meaningful in situ high-altitude satellite wave measurements during VLF wave injection experiments.