On the use of VLF measurements for obtaining information on the lower ionosphere (especially during solar flares)

Three types of VLF propagation measurements which can be used to determine some characteristics of the lower ionosphere are described. These methods are: 1) determination of the field strength (and ideally the phase) of a continuous wave transmitter as a function of distance, 2) determination of the relative phase and amplitude of a broadband signal, such as an atmospheric, and 3) observing simultaneously the phase and amplitude of a continuous wave signal at a single point remote from the transmitter. The latter method is discussed in detail. Although it cannot alone yield much information about the normal steady-state ionosphere, it can be used to investigate changes in the electron density distribution. In this paper phase and amplitude variations observed during solar flares are used to deduce the corresponding D-region changes, both for a sharply bounded and for an exponentially varying model of the ionosphere. It is further suggested that the exponential model can be used to explain VLF observations made during Polar Cap Absorption events, as well as the somewhat anomalous diurnal variation of VLF signals observed on equatorial paths.