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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: Energetics
Up: Phenomenology
Previous: Blue Jets and Blue
Matt Heavner
2002-02-13