Solar Flare Effects and VLF Radio Wave Observations of the Lower Ionosphere

Abstract

The observed changes of phase and signal amplitude of VLF radio waves, propagated over distances varying between 1,000 and 10,000 km, which accompany solar flares have been studied for the period 1961 to 1964. In general, the phase of the received signal advances and the signal amplitude increases during the onset of a solar flare. However, observations of the GBR transmissions at Rome (1,500‐km path) show that for this particular path the phase of the received signal retards in contrast to all other paths studied. Assuming that the conductivity of the lower ionosphere varies exponentially with height under both quiet and disturbed conditions, it is possible to evaluate the waveguide mode characteristics of VLF propagation over large distances. Using this theoretical approach it has been shown that the observations can only be interpreted if both the D region electron density profile is lowered and the gradient of ionization density increased. The magnitude of the flare‐produced changes in ionization density profiles of the lower ionosphere have been evaluated analytically from solar X‐ray data. These profile changes are consistent with the lowering of the D region and increase in ionization gradient that are required to explain the VLF radio wave observations.