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This report contains the results of our VLF program which started in 1961. One year’s data obtained in 1963 using a rubidium vapor oscillator as the College frequency standard forms the major part of the data discussed. The VLF variations, primarily perturbations of the phase of the signal, are divided into two categories, 1) those effects known to have been associated with a particular type of event, and 2) those which occur regularly with no apparent connections to geophysical events of a disturbance nature. The first category contains VLF phase anomalies produced by solar flare x-rays, solar flare cosmic rays (these produce the polar cap disturbance, PCD), solar eclipses, and nuclear explosions. Solar flare x-rays produce large amounts of D region ionization. The probability that a given flare produces a VLF sudden phase anomaly (SPA) varies linearly with the optical area of the flare. The SPA magnitude depends upon path length and solar zenith angle, but other factors which have not been determined also enter in strongly. Time differences of a minute or more are observed between SPA onsets on different frequencies over the same path, or over different paths. The SPA decay rate is exponential on long paths, and the lower frequency signal over a given path appears to recover faster. This is consistent with the notion that the lower frequency is reflected lower in the ionosphere. Polar cap disturbances (PCD), produced by low energy cosmic rays, are readily detectable on transpolar VLF signals. Of seven small PCD events since November 1961, we observed four of the five which occurred when our equipment was operating. The polar D region was depressed greatly during this period, even during darkness, and took several days to recover in all the cases noted. The PCD effects were very widespread, undoubtedly covering the entire polar cap. A large scale electron precipitation between the latitudes of 40 and 60 degrees, geomagnetic, on October 1, 1961, produced a markedly abnormal diurnal phase shift on the NBA-College signal. Similar precipitation data is not available for other times so we are not sure that all such abnormal diurnal phase traces can be attributed to widespread particle precipitations. The July 20, 1963, solar eclipse produced a phase retardation that was roughly proportional to the obscured area of the sun as averaged over each VLF path. At first and fourth contact the phase appeared to be decreased. Small phase perturbations were superimposed on the phase anomaly. These appear to be the result of mode interference at the point of totality. Nuclear tests of sufficient yield and altitude produce a definite ‘signature’ on VLF records for paths sufficiently close to the burst. The phase records obtained during the US and USSR high altitude tests of 1962 are shown and discussed briefly. Under the second category we include the regularly-occurring phase anomalies, occurring primarily at night, which are observed at College. Evidence that these variations are the result of phase interference is presented. Using an analysis of the sunrise pattern, we show that second mode energy must normally propagate strongly to 5,000 km and significantly to well over 8,000 km at night. The variations we observed in the VLF phase and amplitude are consistent only with the notion of wave interference. The production mechanism of the phase variations for each propagating mode is still unknown. The phase variations appear to be produced in a relatively small region of the ionosphere; therefore, height changes cannot produce the effects we see. We propose a model in which appreciable mode conversion or coupling between ordinarily independently propagating waves occurs at some sort of D region disturbance. Such coupling appears to be the mechanism for the production of the sunrise-sunset pattern; in that case mode conversion takes place near the sun line in the D region. We suggest that gradients similar to those produced by the sun line might be produced by travelling ionospheric disturbances. The interference between waves produced by coupling at such disturbance lines and those normally propagating could easily produce the large phase changes we observe. This mechanism does not require widespread changes in the D region height, and is therefore in agreement with observations. Some events do produce widespread D region height changes so this mechanism is not being suggested to explain all VLF variations. No significant statistical relationship between geomagnetic disturbances and VLF phase variations was found. A discussion of the present theories about magnetic disturbances shows that none would be expected if no data selection process is used. Our finding is therefore reasonable, in spite of the reports of connections between VLF and geomagnetic effects. Mode interference at great distances greatly complicates the use of VLF signals for ionospheric research purposes. We conclude that the long-distance VLF method is a good detection system for certain events such as cosmic-ray flares, solar-flare-induced ionospheric disturbances, and high altitude nuclear explosions. It cannot presently be used to determine quantitatively the properties of the upper atmosphere or of the various D region disturbance phenomena, so, by itself, it is not a good research tool. In conjunction with other methods it is quite useful.

Publication Date

7-17-1964

Keywords

VLF radio wave propagation, Ionospheric radio wave propagation

Handle

http://hdl.handle.net/11122/15422

High latitude VLF propagation

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