Subionospheric VLF signatures of nighttime D region perturbations in the vicinity of lightning discharges

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Abstract
A 12‐hour sequence of perturbations of subionospheric VLF signals observed in association with lightning provided preliminary evidence that the ionospheric regions perturbed in these events may be confined to within ∼150 km of the lightning discharges, and that intracloud flashes as well as cloud‐to‐ground lightning may be important in producing the perturbations. High‐resolution analysis of event signatures indicated the presence of two different classes of events. For one set of events, observed during the most active central 6 hours of the observation period, a ∼0.6‐s delay between the causative lightning and VLF event onset and a ∼1‐s onset duration was observed, consistent with previously suggested models of the gyroresonant whistler‐particle interaction that leads to particle precipitation and perturbation of the Earth‐ionosphere waveguide. However, another set of events, observed during the first 2 hours of the observation period, exhibited a very different temporal signature, characterized by a much smaller (<50 ms) delay and sometimes also very short (<50 ms) rise times. Such events are possibly related to previously reported cases of similarly early/fast events and may involve a more direct coupling between the lightning discharge and the lower ionosphere.