VLF Studies During TLE Occurrences in Europe: A Summary of New Findings

Abstract

The paper reviews the past few years' European efforts for characterising the effects of TLEs, in particular sprites and elves, on the lower ionosphere. A mostly experimental approach was applied for the analysis of data collected during the EuroSprite campaigns by optical cameras, very low frequency (VLF, 3-30 kHz) receivers and lightning detection systems. The new findings of these multi-instrumental studies can be summarised as follows: 1) A close relationship between sprites and early VLF perturbations was established which constitutes evidence of upper D-region electron density changes in association with sprites. 2) VLF backscatter from the sprite-affected regions exists but it occurs rarely.3) Long-delayed sprites were present in a large percentage, contrary to previous reports; they occurred in relation to long-lasting continuing currents that contribute to the buildup of sprite-causative quasi-electrostatic fields. 4) Intracloud lightning was found to be the key-factor which determines the sprite morphological features. 5) A new subcategory of VLF events was discovered, termed early/slow, characterised by long onset durations from 100 ms up to ∼2 s. The slow onsets, which were attributed to a gradual ionisation build-up, are driven by a dense sequence of intracloud electromagnetic pulses that accompany the sprite-causative discharge. 6) A D-region chemical model was applied to simulate the measured recovery phases of the early VLF perturbations. This led to estimates about the mean altitude and electron density enhancements of the sprite-related ionospheric perturbations. 7) Early VLF events were identified for the first time to occur in association with elves, providing evidence that corroborates theoretical predictions on lower-ionospheric ionisation production by lightning-emitted electromagnetic pulses.

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

“…The slow onset of early VLF events implies an underlying mechanism which causes a gradual build-up of ionization following a causative CG discharge. Some researchers (Haldoupis et al, 2004Mika et al, 2005;Kumar et al, 2008;Mika and Haldoupis, 2008) studied nighttime early/slow events and showed that the gradual onset of such perturbations is linked to complex and dynamic lightning activity, composed of a few CG return strokes and clusters of IC discharges, which produce primary and secondary ionizations, respectively. The secondary ionization is likely to be responsible for a gradual build up of ionization and hence the slow onset.…”

Physics of Electric Discharges in Atmospheric Gases: An Informal Introduction

“…Other effects are the excitation of several kinds of waves, the generation of radio waves and modification of propagation conditions. These have been observed to occur during and in the wake of thunderstorms and in coincidence with sprites and jets (Bosinger and Shalimov 2008, Mika and Haldoupis 2008, Simões et al 2008). Low frequency electromagnetic radiation (VLF, ELF, ULF, Schumann resonances) which accompanies TLEs is well understood (Rodger 1999, Reising et al 1996, Cummer and Inan 1997, Füllekrug et al 2001, Füllekrug et al 2006a).…”

“…[The processes of the generation and separation of charges which are required before a discharge can be generated are reviewed in this issue (Roussel-Dupré et al 2008, Yair 2008 and also form the content of Chapters 2 and 6. Some of the theory and the observation of such effects as blue jets and sprites are summarized in Chapter 7 (Mishin andMilikh 2008, Mika andHaldoupis 2008).] Because of the importance of such effects the physics of gaseous discharges is far from being exhausted, in particular in view of the newly discovered fine structuring of streamers (Ebert et al 2006).…”

“…Lightning discharges have been observed to be accompanied by radiation in the optical, UV, X rays and even up to the gamma ray energy range and, also in the ELF and ULF radio wave ranges (Simões et al 2008, Bosinger and Shalimov 2008, Mika and Haldoupis 2008. Moreover, spacecraft observations of other planets (Jupiter, Saturn) provide evidence of short duration radio emissions in connection with lightning on those planets (Fischer et al 2008).…”

On ULF Signatures of Lightning Discharges

“…It is related to sprites and similar transient luminous events (TLE, see Mika and Haldoupis 2008). Since ground-based observations of sprites always require a clear sky and considerable instrumental investments, one was looking for related measurements, which could serve as a proxy for the appearance of sprites (e.g.…”