Rare examples of early VLF events observed in association with ISUAL-detected gigantic jets

Abstract

[1] We examine narrowband VLF observations and investigate the association of early VLF perturbations with gigantic jets recorded by the Imager of Sprites and Upper Atmospheric Lightnings (ISUAL) instrument aboard FORMOSAT-2. From its inception in 2004 to April 2013, the ISUAL instrument has recorded 90 gigantic jets using a triggered camera. Stanford VLF receivers located around the world are used to detect perturbations to VLF transmitter signals associated with lightning. While nine gigantic jet events occurred within 100 km of a VLF transmitter-receiver great circle path, only four early VLF events were detected in association with three ISUAL gigantic jets. One of these is a moderate event of 0.4 dB amplitude change, and the others are very small. The recovery time of these events are less than a couple of minutes and so do not constitute the "long recovery" early VLF events that have been postulated to be associated with gigantic jets. We speculate on possible explanations for the lack of other events on monitored paths, including a lack of significant ionization produced in the D region ionosphere by the gigantic jet event, weak transmitter signals recorded by the receivers, or mode effects on transmitter paths.

TLEs and early VLF events: Simulating the important impact of transmitter‐disturbance‐receiver geometry

“…Cotts and Inan [2007] initially suggested that LOREs may be a signature of gigantic jets (GJs) [Pasko and George, 2002], which are direct cloud-to-ionosphere electrical discharges (upside-down lightning), due to the fact that ionization at low altitudes may lead to longer recovery times. On the other hand, Marshall et al [2014] studied the association between a series of GJs observed by the ISUAL instrument aboard FORMOSAT-2 and VLF signal perturbations and found only a few events. Haldoupis et al [2012] and Haldoupis et al [2013] provide some evidence that LOREs may be connected to elves, an expanding disk of electron heating and ionization of the neutral gas that results from the EMP.…”

Photochemical response of the nighttime mesosphere to electric field heating—Recovery of electron density enhancements

“…Electric field intensity changes produced by lightning discharges are capable of directly ionizing the mesosphere (∼50 km up to ∼100 km altitude), sometimes resulting in the scattering of subionospherically propagating very low frequency (VLF, 3-30 kHz) waves [e.g., Inan et al, 1996;Dowden et al, 1997;Hardman et al, 1998;Haldoupis et al, 2006Haldoupis et al, , 2010Marshall et al, 2006Marshall et al, , 2014Cheng et al, 2007]. Recently, a class of these "early VLF" scattering events was identified which exhibit recoveries up to tens of minutes [Cotts and Inan, 2007], much longer than typical recoveries of 60-240 s (onset to zero pertubation) [Sampath et al, 2000].…”

Classifying onset durations of early VLF events: Scattered field analysis and new insights

“…Definitive observations of VLF scattering from gigantic jets (GJs) have been presented by Marshall et al []. However, the 1 s time resolution presented therein does not allow for classification of either the onset delay (i.e., early) or the onset duration (i.e., fast/slow).…”

“…Preliminary observations of early VLF events exhibited high variability in onset duration [ Inan et al , , ], defined as the time between the start of the event and the occurrence of the maximal amplitude/phase perturbation, ranging from ≤20 ms to ∼2 s. Since then, early event onset durations have been categorized as “fast” (onset duration ≤20 ms) [ Inan et al , ] and “slow” (between ∼0.5 and ∼2.5 s) [ Haldoupis et al , , ]. However, some studies after the work of Inan et al [] have utilized the term fast to describe events with slower onset durations of <50, <100, or 1000 ms [ Sampath et al , ; Kumar et al , ; Salut et al , ; Marshall et al , ]. We note that elves, sprite halos, and sprites have been observed with optical durations less than 20 ms [e.g., Stenbaek‐Nielsen et al , ; Stenbaek‐Nielsen and McHarg , ; Newsome and Inan , ].…”