Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmae24b..01g&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #AE24B-01
Other
3324 Lightning, 3346 Planetary Meteorology (5445, 5739)
Scientific paper
A broad overview is given of lightning in planetary atmospheres. Searches for lightning using spacecraft-borne instrumentation have now been conducted at almost all of the planets in the solar system, the exceptions being Mercury, which has no appreciable atmosphere, and Pluto which has not yet been visited by a spacecraft. The techniques used include (1) imaging observations to detect optical flashes produced by lightning; (2) high-frequency radio measurements to detect the impulsive broadband radio bursts, called spherics, produced by lightning discharges; and (3) low-frequency plasma wave measurements to detect the whistling tones, called whistlers, produced by lightning. Using these techniques, lightning has been reported at five planets other than Earth. These are: Venus, Jupiter, Saturn, Uranus, and Neptune. Of these, the existence of lightning at Venus is doubtful, and the evidence of lightning at Neptune is at best marginal. Jupiter and Saturn have by far the most intense and well documented lightning activity. During the Voyager 1 flyby of Jupiter, whistlers and intense optical flashes, comparable to those from terrestrial superbolts, were observed by the plasma wave and optical imaging instruments. However, no impulsive high-frequency radio bursts were observed. Two factors may be responsible for the absence of high-frequency radio signals: (1) the very strong magnetic field of Jupiter, which blocks the escape of the extra-ordinary mode; and (2) the relatively high electron collision frequency in the ionosphere, which increases the absorption of radio waves. During the Voyager 1 and 2 flybys of Saturn many very strong high-frequency radio bursts, called Saturn Electrostatic Discharges (SEDs), were detected. Although the origin of these impulsive radio bursts was initially uncertain, strong evidence now exists that SEDs are produced by lightning. Recent optical imaging and radio measurements from the Cassini spacecraft clearly show that SEDs originate from reappearing convective storms that are carried around by Saturn's rotation. One whistler has been observed at Saturn, but no optical flashes have been observed, possibly due to the enhanced background caused by ring shine, or due to extinction caused by clouds or haze in the atmosphere. During the Voyager 2 flyby of Uranus, numerous high- frequency radio bursts, called Uranian Electrostatic Discharges (UEDs), were observed that are comparable to those at Saturn, although at a much lower rate. No optical flashes or whistlers were observed at Uranus. The available data shows that the lightning discharges at Jupiter, Saturn, Uranus, and maybe Neptune, are much more intense than terrestrial lightning, by factors of 10{^1} to 10{4}, depending on the method of comparison. Since the electrical breakdown field increases with pressure, the high intensities suggest that lightning at the giant planets, and the convective storms that cause the discharges, occur deep in the atmosphere, at pressures considerably greater than for terrestrial lightning.
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