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Ann. Geophys., 24, 2179-2189, 2006
www.ann-geophys.net/24/2179/2006/
© European Geosciences Union 2006
Early VLF perturbations observed in association with elves
Á. Mika1, C. Haldoupis1, T. Neubert2, H. T. Su3, R. R. Hsu3, R. J. Steiner1, and R. A. Marshall4
1Department of Physics, University of Crete, Heraklion, Greece
2Danish National Space Center, Copenhagen, Denmark
3Department of Physics, National Cheng Kung University, Tainan, Taiwan
4Space, Telecommunications, and Radioscience Laboratory, Stanford University, Stanford, USA
Abstract. VLF remote sensing is used to detect lower-ionospheric electron density changes
associated with a certain type of transient luminous events known as elves.
Both ground- and satellite-based observations of elves are analysed in relation to VLF data
acquired at various receiver sites in Europe, the United States and Antarctica.
Ground-based observations were performed during the EuroSprite2003 campaign,
when five elves were captured by low-light cameras located in the
Pyrenees. Analysis of VLF recordings from Crete shows early
VLF perturbations accompanying all of the elves.
A large dataset consisting of elves captured by the ISUAL
(Imager of Sprites and Upper Atmospheric Lightning) payload on Taiwan's
FORMOSAT-2 satellite over Europe and North America has also
been analysed.
Early/fast VLF perturbations were found to accompany some of the
elves observed over Europe. However, no VLF perturbations were detected in
relation to the elves observed by ISUAL over North America.
The present analysis – based on the largest database of optical elve
observations used for VLF studies so far – constitutes evidence
of processes initiated by the lightning EMP (electromagnetic pulse) causing
electron density changes in the lower ionosphere in line with theoretical
predictions.
It also proves that sub-ionospheric electron density changes associated with elves can intrude to lower heights and thus perturb
VLF transmissions.
The possibility of VLF detection, however, depends on several factors,
e.g., the distance of the elve from the receiver and the transmitter–receiver great
circle path (GCP), the altitude of the ionised region and the characteristics of the VLF transmitter, as well
as the EMP energy, which occasionally may be sufficient to cause optical emissions but not ionisation.
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