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Elves

A third phenomena observed above thunderstorms are elves (Emissions of Light and VLF perturbations due to EMP Sources (Fukunishi et al.(1995))), very brief (less than 1 ms) red emissions at the bottom of the ionosphere (at the upper altitudes of sprites). Optical ``airglow enhancements'' associated with lightning were first reported based on shuttle observations (Boeck et al.(1992)) and possibly from sounding rockets (Li et al.(1991)). The phenomena were named elves after further documentation (Desroschers et al.(1995),Lyons and Nelson(1995)) and photometer observations determined the brief (~1 ms) lifetime of elves (Fukunishi et al.(1996)). Strong Joule heating of the base of the ionosphere by tropospheric lightning EMP was predicted earlier (Inan et al.(1991)), and the possibility of optical emissions were first investigated by (Taranenko et al.(1993b)) stimulated by the observations of lightning associated airglow enhancements by (Boeck et al.(1992)). The simple model for elves is the interaction between the (spherically expanding) EMP from a cloud-to-ground lightning discharge and the (planar) lower ionosphere (Inan et al.(1996b)). The radially expanding disc (intersection of the expanding sphere and plane) was measured using a ``fly's-eye'' array of seventeen photometers operating at 40 kHz (Inan et al.(1997)).

Elves occur above thunderstorms, both in conjunction with sprites and alone. Elves observed in the intensified color camera are red, and spectral observations of elves identify the main emissions to be N2(1PG) (Heavner(2000)). Filtered observations of N2(1PG) emission confirm the identification of elves emissions (personal communications, Mike Taylor). To date, triangulation analysis of elves has not been performed. Single station analysis using National Lightning Detection Network lightning location estimates the altitude of elves between 75-95 km, the typical vertical thickness is several km, and the horizontal extent as much as 530 km (Dial and Taylor(1999)). Elves are observed above both positive and negative CG lightning (Barrington-Leigh and Inan(1999)).


next up previous
Next: Energetics Up: Phenomenology Previous: Blue Jets and Blue
Matt Heavner 2002-02-13