From June 2008 to January 2016 nearly 800 elves have been recorded by a low-light camera in northeastern Spain. Elves occur in this region mainly over the lower-topped cold airmass maritime thunderstorms, peaking from November to January. Cloud-to-ground strokes still produce elves when maritime winter storms are carried inland, suggesting the cold season thunderstorm charge configuration favors strokes with large electromagnetic pulses.
Altitudes of 389 elves were determined using optical data combined with a lightning location network. The overall median altitude was 87.1 km, near the typical OH airglow height, but average heights during individual nights ranged between 83 and 93 km. The lower elve nights (~84 km) occurred during slightly elevated geomagnetic conditions (Kp>3-, ap-index >10). Elve altitude often shifts by several kilometers during the night, apparently in response to changing background conditions in the upper mesosphere.
CitacióVan Der Velde, O., Montaña, J. Statistics and variability of the altitude of elves. "Geophysical research letters", 07 Maig 2016, vol. 43, p. 5467-5474.