Characteristics of localized ionospheric disturbances inferred from VLF measurements at two closely spaced receivers

The very low frequency (VLF) NPM signal from Hawaii was recorded at two closely spaced (∼50 km) receivers (located at Palmer and Faraday stations) on the Antarctic Peninsula. Measurements of characteristic amplitude and phase signatures of lightning-induced electron precipitation (LEP) events were made on three different days in March 1992. Both amplitude and, for the first time, phase measurements are quantitatively interpreted using a three-dimensional model of VLF propagation in the Earth-ionosphere waveguide in the presence of lower ionospheric disturbances. This is the first time such a study has been undertaken with mirrored precipitation. Differences between the amplitude and phase changes at the two sites are accounted for by the location of the LEP ionospheric disturbance transverse to the VLF signal propagation paths. The change in these differences is explained by the horizontal movement of the disturbance region and, therefore, the causative whistler duct footprint across the transmitter-receiver paths. Trends in the amplitude and phase changes on a timescale of order 1 hour are found to be encompassed by the modeling of the passage of the day-night terminator along the paths.