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VLF remote sensing is used to detect lower-ionospheric electron density changes\udassociated with a certain type of transient luminous events known as elves.\udBoth ground- and satellite-based observations of elves are analysed in relation to VLF data\udacquired at various receiver sites in Europe, the United States and Antarctica.\udGround-based observations were performed during the EuroSprite2003 campaign,\udwhen five elves were captured by low-light cameras located in the\udPyrenees. Analysis of VLF recordings from Crete shows early\udVLF perturbations accompanying all of the elves.\udA large dataset consisting of elves captured by the ISUAL\ud(Imager of Sprites and Upper Atmospheric Lightning) payload on Taiwan's\udFORMOSAT-2 satellite over Europe and North America has also\udbeen analysed.\udEarly/fast VLF perturbations were found to accompany some of the\udelves observed over Europe. However, no VLF perturbations were detected in\udrelation to the elves observed by ISUAL over North America.\udThe present analysis – based on the largest database of optical elve\udobservations used for VLF studies so far – constitutes evidence\udof processes initiated by the lightning EMP (electromagnetic pulse) causing\udelectron density changes in the lower ionosphere in line with theoretical\udpredictions.\udIt also proves that sub-ionospheric electron density changes associated with elves can intrude to lower heights and thus perturb\udVLF transmissions.\udThe possibility of VLF detection, however, depends on several factors,\ude.g., the distance of the elve from the receiver and the transmitter–receiver great\udcircle path (GCP), the altitude of the ionised region and the characteristics of the VLF transmitter, as well\udas the EMP energy, which occasionally may be sufficient to cause optical emissions but not ionisation

Early VLF perturbations observed in association with elves