Abstract
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.
Highlights
Electromagnetic pulses (EMPs) released by lightning discharges into the lower ionosphere can lead to momentary optical emissions called elves
The scope of the present study is to report on the presence or not of elve-related very low frequency (VLF) perturbations, and it was motivated by an early/fast event presented by Mika et al (2005), which accompanied an elve recorded during the EuroSprite2003 campaign
This suggests that the early VLF perturbations seen at Crete were most likely caused by narrow angle forward scattering, in line with theoretical expectations for the scattering pattern of the broad and smoothly varying ionised region which is anticipated to be produced by a lighting EMP (Dowden, 1996)
Summary
Electromagnetic pulses (EMPs) released by lightning discharges into the lower ionosphere can lead to momentary optical emissions called elves (e.g., see reviews by Rowland, 1998; Rodger, 1999) These phenomena, appearing at altitudes between 75 and 105 km, constitute a certain type of transient luminous events (TLEs) in the upper atmosphere. It is believed that sprites in the lower ionosphere associate closely with early/fast VLF perturbations, 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. VLF data used for comparisons with the optical observations were obtained with Stanford University VLF receivers located in Greece, the United States and Antarctica
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
