CHARACTERISTICS OF LOCALIZED IONOSPHERIC DISTURBANCES INFERRED FROM VLF MEASUREMENTS AT 2 CLOSELY SPACED RECEIVERS

The very low frequency (VLF) NPM signal from Hawaii was recorded at tw o closely spaced (similar to 50 km) receivers (located at Palmer and F araday stations) on the Antarctic Peninsula. Measurements of character istic amplitude and phase signatures of lightning-induced electron pre cipitation (LEP) events were made on three different days in March 199 2. Both amplitude and, for the first time, phase measurements are quan titatively interpreted using a three-dimensional model of VLF propagat ion in the Earth-ionosphere waveguide in the presence of lower ionosph eric disturbances. This is the first time such a study has been undert aken with mirrored precipitation. Differences between the amplitude an d phase changes at the two sites are accounted for by the location of the LEP ionospheric disturbance transverse to the VLF signal propagati on paths. The change in these differences is explained by the horizont al movement of the disturbance region and, therefore, the causative wh istler duct footprint across the transmitter-receiver paths. Trends in the amplitude and phase changes on a timescale of order 1 hour are fo und to be encompassed by the modeling of the passage of the day-night terminator along the paths.