On the relationship between lightning peak current and Early VLF perturbations

Abstract

Lightning strokes are known to cause direct heating and ionization of the D region, some of which are detected via scattering of VLF transmitter signals and are known as Early VLF events. The disturbed ionosphere typically recovers in many tens of seconds. New experimental evidence is presented demonstrating that the scattering pattern and onset amplitude of Early VLF events are strongly related to both the magnitude and polarity of causative lightning peak current. Observations of Early VLF events at nine Stanford VLF receiver sites across the continental United States are combined with lightning geolocation data from the National Lightning Detection Network (NLDN). During January and March 2011, NLDN recorded 7769 intense lightning discharges with high peak currents (>100 kA) generating 1250 detected Early VLF events. We show that the size of the scattered field due to the ionospheric disturbance increases with the peak current intensity of the causative lightning discharge. The most intense peak currents of >+200 and <−250 kA disturb VLF transmitter signals as far as ∼400 km away from the lightning stroke. Early VLF event detection probability also increases rapidly with peak current intensity. On the other hand, the observed VLF amplitude change is not significantly dependent on the peak current intensity. Stroke polarity is also important, with positive strokes being ∼5 times more likely to generate Early VLF disturbances than negative strokes of the same intensity. Intense positive cloud‐to‐ground lightning discharges, especially when occurring over the sea, are also more likely to produce Early VLF events with long recovery (many minutes).

Initial breakdown and fast leaders in lightning discharges producing long‐lasting disturbances of the lower ionosphere

“…The association of LOREs with both positive and negative polarity flashes, a significant occurrence rate of LORE causative flashes over oceans, and the observation of LOREs at wide scattering angles are in agreement with past observations [ Cotts and Inan , ; Salut et al , , ; Haldoupis et al , , ]. For the 30 LOREs with associated LLS data, 13 were associated with positive lightning flashes and 17 were associated with negative lightning flashes (see Table ).…”

“…Recent statistical studies of LOREs have identified a number of important lightning discharge properties that are associated with LOREs. Haldoupis et al [] and Salut et al [] report that both the probability of detecting an early VLF event and the expected event recovery duration increase with lightning peak current inferred from lightning locating systems. A large number of LOREs have been associated with lightning flashes occurring over oceans [e.g., Cotts and Inan , ; Salut et al , ], which may in part be due to the higher occurrence of oceanic, negative flashes with large inferred peak currents [ Said et al , ] and large inferred charge moment changes [ Füllekrug et al , ].…”

“…LOREs are associated with both positive (+) and negative (−) cloud‐to‐ground flashes (CGs) [ Haldoupis et al , ; Salut et al , ]. Haldoupis et al [] observed 340 LOREs (>300 s recovery) produced by lightning discharges with peak currents greater than 200 kA and reported that 110 were associated with −CGs, and 230 were associated with +CGs.…”

“…Haldoupis et al [] observed 340 LOREs (>300 s recovery) produced by lightning discharges with peak currents greater than 200 kA and reported that 110 were associated with −CGs, and 230 were associated with +CGs. Conducting a smaller study, Salut et al [] report that for 90 early VLF events with estimated recoveries >200 s, 20 were associated with −CGs and 70 were associated with +CGs. For comparison with typical recovery events, Salut et al [] report that for 1160 early VLF events with estimated recoveries <200 s, 276 were associated with −CGs and 924 were associated with +CGs (i.e., a similar proportion of −CGs producing typical recovery and producing long recovery events).…”

Exploiting LF/MF signals of opportunity for lower ionospheric remote sensing

“…Using Vaisala's National Lightning Detection Network (NLDN) [Cummins and Murphy, 2009], we were able to detect all lightning strokes that occurred near the great circle path of a receiver in Baxley (Georgia) and the NDGPS transmitter in New Bern (North Carolina). Early/fast events typically occur within 50-200 km of the path [Johnson et al, 1999;Salut et al, 2013]. The data have the characteristic features of an early/fast event: a very sudden onset and a much more gradual recovery.…”