Artificially stimulated very-low-frequency radiation from the ionosphere

In the course of experimental studies of whistlers and related ionospheric effects conducted by Stanford University, a new phenomenon has been discovered. It is the triggering of VLF emissions in the ionosphere by whistler-mode signals transmitted from VLF stations operated by the U.S. Navy. Although much remains to be learned about this remarkable phenomenon, certain important features are already sufficiently clear to warrant reporting at this time. Evidence of triggered emissions was first found on the IGY-IGC synoptic tape recordings made at Wellington, New Zealand, on November 28, 1959, at 0835 UT [Helliwell, 1962]. A spectrum analysis, from 15 to 35 kc/s, of a short section of that record is shown in Figure 1. The triggering signals, Morse code transmissions from station NPG (Jim Creek, Washington), can be seen as a broken line at 18.60 kc/s on the spectrogram. The emissions consist mainly of rising tones (called ‘risers’) lasting about 0.1 or 0.2 sec. Analysis of the starting time of each of these risers with respect to the transmitted Morse code characters showed that the risers associated with 96 per cent of the dashes and only 4 per cent of the dots.