Chapter 7 - An Interpretation of Lofti–I V.L.F. Observations

Abstract

During February and March of 1961, the Naval Research Laboratory LOFTI–I satellite received very low frequency signals transmitted into the ionosphere from U.S. Navy stations NBA in the Canal Zone and NPG in Jim Creek, Washington, The signals were sometimes observed to have been delayed by as much as 600 milliseconds. It was possible to deduce the thickness (scale height) of the ionosphere above the height of maximum electron density. The values correspond roughly to those previously found from rocket measurements. Simultaneous daytime reception of both NPG and NBA showed an interesting difference in attenuation of the signal strengths such that the signal from NBA was more heavily attenuated. The difference can be explained by taking into account the tilt of the earth's magnetic field and the arrival of the NBA signal from the south and the NPG signal from the north. In general, the whistler mode signals observed in LOFTI–I show no evidence of having been trapped in columns of enhanced ionization. This is in contrast to the characteristics of naturally occurring whistlers observed on the ground, which are best explained by assuming that they have been trapped in such columns. The difference may be explained by noting that whistler energy not trapped in columns of enhanced ionization is not observed on the ground, presumably as a result of total internal reflection at the lower boundary of the ionosphere.