Subionospheric early VLF signal perturbations observed in one‐to‐one association with sprites

Abstract

[1] Observations on the night of 21 July 2003 of the ionospheric effects of a thunderstorm in central France are reported. From 0200 to 0315 UT, a camera system in the Pyrenees Mountains captured 28 sprites, triggered by +CG lightning as observed by the French METEORAGE lightning detection system. A narrowband VLF receiver located on Crete, at $2200 km southeast of the storm, observed subionospheric VLF signals from six ground-based transmitters. The amplitude of one of the VLF signals, originating at a transmitter located $150 km west of the storm and passing through the storm region, exhibited rapid onset perturbations occurring in a nearly one-to-one relationship with the optical sprites. These ''early'' VLF events are consistent with a process of narrow-angle forward scattering from a volume of enhanced ionization above the storm with lateral sizes larger than the VLF radio wavelength. The many +CG and ÀCG discharges that did not produce sprites were also found to not be associated with detectable VLF amplitude perturbations, even though some of these discharges reached relatively large peak currents. The rapid onsets of several of the sprite-related VLF perturbations were followed by relatively long onset durations, ranging from $0.5 to 2.5 s, indicating that these events were early but not ''fast.'' These ''early/slow'' events may suggest a slow process of ionization build-up in the lower ionosphere, following intense lightning discharges that also lead to sprites. A limited number of early VLF perturbation events were also associated with whistler-induced electron precipitation events, or classic Trimpi perturbations, undoubtedly produced by the precipitation of electrons due to whistlermode waves injected into the magnetosphere by the same lightning flash that led to the production of the sprite.

Early VLF perturbations observed in association with elves

“…It is now believed that sprites in the lower ionosphere associate closely with early/fast VLF perturbations, although these perturbations may not be unique to them. This relation was hinted by early measurements over the United States, reported by Inan et al (1996b) and Dowden et al (1996), while more recent observations in Europe, reported by Haldoupis et al (2004) and Mika et al (2005), suggested a nearly one-to-one relationship.…”

“…The perturbation magnitudes varied from 0.15 to 0.3 dB and the onset durations (i.e. the time needed for the maximum perturbation to be reached) were in the range of 20 ms−2 s, including both early/fast and early/slow (e.g., see Haldoupis et al, 2004) type events, while the relaxation times were ∼2−3min. Example perturbations accompanying the elve event discussed previously in Fig.…”

Lightning-associated VLF perturbations observed at low latitude: Occurrence and scattering characteristics

“…Armstrong (1983) rst reported the VLF perturbations that occurred within 100 ms (i.e., within the time resolution of the instrument) of the causative lightning discharges, which were thus termed "early" or "early/fast" events by Inan et al (1988). Early VLF events are caused by scattering from the localized regions of the conductivity enhancements in the lower ionosphere associated with lightnings, producing Transient Luminous Events (TLEs) such as sprites, halos and elves (e.g., Dowden et al, 1994;Inan et al, 1996a;Sampath et al, 2000;Rodger, 2003;Haldoupis et al, 2004;Mika et al, 2005Mika et al, , 2006Marshall et al, 2006;Kumar et al, 2008;Inan et al, 2010). However, from a comparison of early/fast VLF events with sprite observations in the USA on three dates (during 1995-2000) with high sprite activity, it was concluded that early/fast VLF events and sprites were not well correlated as many sprites occurred even without any early/fast event and vice versa .…”

Massive disturbance of the daytime lower ionosphere by the giant γ‐ray flare from magnetar SGR 1806–20

“…[2] VLF remote sensing is a sensitive technique to detect transient disturbances of the nighttime lower ionosphere ($40 to 90 km altitude), resulting from high energy auroral precipitation [e.g., Potemra and Rosenbert, 1973;Cummer et al, 1997], lightning-induced electron precipitation [e.g., Inan and Carpenter 1987], electromagnetic and quasielectrostatic coupling produced by lightning discharges (e.g., sprites and elves) Moore et al, 2003;Haldoupis et al, 2004;Cheng and Cummer, 2005], cosmic gamma-ray bursts (GRBs) [Fishman and Inan, 1988], and g-ray flares from a magnetar [Inan et al, 1999]. VLF detection of daytime ionospheric disturbances is less common, but include solar X-ray flares [Mitra, 1974].…”