Nonducted coherent VLF waves and associated triggered emissions observed on the ISEE‐1 satellite

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The ISEE‐1 spacecraft has been an important component of VLF wave injection experiments for studying interactions between coherent VLF waves and energetic particles. The coherent waves are injected into the magnetosphere by ground‐based transmitters such as that at Siple Station, Antarctica (76°W, 84°S geographic, L ∼4.1), and those of the worldwide Omega navigation network. During the period October 1977–August 1979, the Stanford VLF receiver on ISEE‐1 acquired data on approximately 90 separate orbits in the plasmasphere in which the satellite longitude lay within 30° of the magnetic field lines linking the North Dakota Omega transmitter (98°W, 46°N geographic, L ∼3.5). During 12 passes the nonducted Omega signals at 10.2 kHz were observed to trigger VLF emissions on magnetic shells ranging from L=1.8–3.8. The emission characteristics often differed significantly from those of emissions triggered by ducted signals and detected at ground stations. In many cases each of the triggered signals consisted of a large array of discrete rising tones whose rate of change of frequency approached 40 kHz/sec, a value much larger than that usually associated with emissions triggered by ducted signals. The emission activity occurred during periods in which signals propagated to the spacecraft on at least two separate ray paths of disparate time delay and with a received signal duration of 2 to 4 times that of the transmitted pulses. Typically, the emission activity took place only along the paths of longer time delay. Unusual emission activity involving signals from the Siple Station transmitter was also observed on two occasions outside the plasmasphere. Our results indicate that the emission generation process can take place under much more general conditions that previously believed.