Lightning‐induced electron precipitation events observed at L ∼ 2.4 as phase and amplitude perturbations on subionospheric VLF signals

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Abstract
Lightning‐induced electron precipitation (LEP) events are studied using the Trimpi effect, in which the precipitation‐induced ionization enhancements in the lower ionosphere (D region) give rise to rapid perturbations of subionospheric VLF signals. In 1983, the phase and amplitude of signals from the NPM transmitter in Hawaii (23.4 kHz) and the Omega transmitter in Argentina (12.9 kHz) were measured at Palmer, Antarctica (L ∼ 2.4), together with the magnetospheric whistler background. The long baseline and over‐sea great circle paths from these two sources make it possible for the observed perturbations to be interpreted using a single waveguide mode theory. Analytical expressions are used to relate the magnitude of the phase perturbations to differential changes in ionospheric reflection height along a segment of the propagation path. The predicted relationship between relative perturbation sizes on the two different signals is compared with measurements. From this information, the whistler‐induced flux levels are inferred to be in the 10−4 − 10−2 erg cm−2 s−1 range and the precipitation regions are inferred to be roughly “circular” in shape, rather than elongated along L shells. Measured amplitude changes tended to be small (∼ 0.5 dB) and negative, as expected from a single‐mode theory, but the ratios of simultaneous amplitude and phase perturbations were slightly larger than the theory predicts, probably due to the effects of an additional mode(s). An assessment of the relative detectability of amplitude versus phase perturbations favors phase perturbations by ∼ 10 dB, irrespective of the detection scheme used.