ATMOSPHERIC PHYSICS:
Nota Bene: Catching Sprites by Radio

Sprites are upper atmospheric optical phenomena associated with lightning (1, 2). They have been implicated in electrochemical processes in the upper atmosphere (3) and transfer large amounts of charge between different atmospheric regions (4). Further interest comes from a growing body of evidence for perturbation of natural lightning by pollutants, with a recent study showing that smoke advected from southern Mexico into the United States led to large amounts of lightning with positive polarity, as well as an unusually high number of sprites (5).

 The first images of sprites were serendipitously recorded only in 1990 (2), although anecdotal reports preceded this discovery. Sprites are barely detectable by the human eye because they are short-lived, faint compared with cloud-to-ground and intracloud lightning, and often obstructed by clouds. Associated with about one in every 100 lightning strikes--usually strong strikes with positive polarity--sprites occur at altitudes of about 40 to 90 kilometers above thunderstorms and reveal complex structures when viewed with an intensified television camera (see the figure).

CREDIT: S. REISING; SOURCE: STANFORD/LOCKHEED-MARTIN

Many aspects of sprite formation and their effect on the global atmospheric environment remain poorly understood. Purely visual observations of sprites are now complemented by other techniques, such as detailed optical spectra (6). A recent paper (7) shows that radio atmospherics or sferics, which are impulsive electromagnetic signals produced by lightning discharges, may enable determination of the global rate of sprite occurrence and the characteristics of sprite-producing storms by remote sensing. Sferics recorded for lightning associated with sprites have a long-lived tail (see the figure, right panel), whereas a normal lightning strike does not (8). Reising et al. (7) now demonstrate that a longer lasting current exists in lightning strikes that lead to sprites and that sprites themselves radiate low-frequency waves of similar strength to those emitted by the lightning. These features can be used over distances of up to 12,000 kilometers for sprite detection and characterization independently from optical measurements. The method potentially allows low-cost global detection and monitoring of sprites from just a few monitoring stations, and may help to quantify the amount of global ionization and heating in the middle and upper atmosphere due to sprites.

References and Notes

1. Information on sprites and other upper atmospheric processes associated with lightning can be found at http://sprite.gi.alaska.edu/html/sprites.htm. See also S. B. Mende, D. D. Sentman, E. M. Wescott, Sci. Am. 277, 56 (August 1997) (available at http://www.sciam.com/0897issue/0897mende.html).
2. R. D. Franz, R. J. Nemzek, J. R. Winckler, Science 249, 48 (1990).
3. B. D. Green et al., Geophys. Rev. Lett. 23, 2161 (1996).
4. V. P. Pasko, U. S. Inan, T. F. Bell, S. C. Reising, ibid. 25, 3493 (1998).
5. W. A. Lyons et al., Science 282, 77 (1998).
6. D. L. Hampton, M. J. Heavner, E. M. Wescott, D. D. Sentman, Geophys. Res. Lett. 23, 82 (1996); S. B. Mende, R. L. Rairden, G. R. Swenson, W. A. Lyons, ibid. 22, 2633 (1995)
7. S. C. Reising, U. S. Inan, T. F. Bell, ibid. 26, 987 (1999).
8. ------, W. A. Lyons, ibid. 23, 3639 (1996).