The VLF fingerprint of elves: Step‐like and long‐recovery early VLF perturbations caused by powerful ±CG lightning EM pulses

Abstract

[1] Subionospheric VLF recordings are investigated in relation with intense cloud-to-ground (CG) lightning data. Lightning impacts the lower ionosphere via heating and ionization changes which produce VLF signal perturbations known as early VLF events. Typically, early events recover in about 100 s, but a small subclass does not recover for many minutes, known as long-recovery early events (LORE). In this study, we identify LORE as a distinct category of early VLF events, whose signature may occur either on its own or alongside the short-lived typical early VLF event. Since LORE onsets coincide with powerful lightning strokes of either polarity (±), we infer that they are due to long-lasting ionization changes in the uppermost D region ionosphere caused by electromagnetic pulses emitted by strong ± CG lightning peak currents of typically > 250 kA, which are also known to generate elves. The LORE perturbations are detected when the discharge is located within~250 km from the great circle path of a VLF transmitter-receiver link. The probability of occurrence increases with stroke intensity and approaches unity for discharges with peak currents ≥~300 kA. LOREs are nighttime phenomena that occur preferentially, at least in the present regional data set, during winter when strong ± CG discharges are more frequent and intense. The evidence suggests LORE as a distinct signature representing the VLF fingerprint of elves, a fact which, although was predicted by theory, it escaped identification in the long-going VLF research of lightning effects in the lower ionosphere.Citation: Haldoupis, C., M. Cohen, E. Arnone, B. Cotts, and S. Dietrich (2013), The VLF fingerprint of elves:Step-like and long-recovery early VLF perturbations caused by powerful ±CG lightning EM pulses,

Remote sensing and modeling of lightning caused long recovery events within the lower ionosphere using VLF/LF radio wave propagation

“…Anyway our example endorses the observation of Salut et al (2012) that long recovery events are attributed predominantly to lightning discharges occurring over the sea. So with regard to long recovery events it is especially promising to analyze propagation paths crossing regions with frequent and strong thunderstorm activity over the sea, for example the Mediterraneans during winter time, see Haldoupis et al (2013), or the Indian ocean, as reported by Salut et al (2012).…”

“…Early VLF perturbations associated with elves have been detected successfully on several occasions, Inan et al ( , 2010; Mika et al (2006). It is proposed, that LORES are caused by large and long-lived electron density enhancements in the upper D-region ionosphere caused by intense positive or negative cloud to ground (CG+/−) lightning discharges, Haldoupis et al (2012Haldoupis et al ( , 2013.…”

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, in addition to the observations of large elves and elve‐sprite pairs in association with LOREs reported by Haldoupis et al [], led Haldoupis et al [] to conclude that LOREs were produced by ionospheric conductivity changes associated with elves, due to the polarity‐invariant theory of elve production by the lightning electromagnetic pulse [e.g., Barrington‐Leigh and Inan , ; Newsome and Inan , ]. Their argument, however, disregards evidence that −CGs account for nearly half, if not more, of all sprite halos [e.g., Newsome and Inan , ; Williams et al , ] and that rare circumstances of “negative” sprites have been observed [e.g., Barrington‐Leigh et al , ; Taylor et al , ; Li et al , ].…”

“…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. 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.…”