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Ashen light is a hypothesised subtle glow that has been claimed to be seen coming from the night side of the planet Venus. Yet some claim it is of doubtful existence. Its causes remain controversial, but it seems that Ashen light is principally a dusk side phenomenon associated with lightning on Venus.
History of observations
Ashen light was first sighted by the astronomer Giovanni Battista Riccioli on January 9, 1643, and he named it "The Ashen Light of Venus." Subsequent claims have been made by various observers including Sir William Herschel, Sir Patrick Moore, Dale P. Cruikshank, and William K. Hartmann. Ashen Light has been most often sighted when Venus was in the evening sky, when the evening terminator of the planet is toward the Earth. Nobody has managed to capture an image of Ashen light.
The Keck telescope on Hawaii reported seeing a subtle green glow and suggested it could be produced as ultraviolet light from the Sun splits molecules of carbon dioxide (CO
2), known to be common in Venus' atmosphere, into carbon monoxide (CO) and oxygen (O
2), but the green light emitted as oxygen recombines to form O
2 is thought too faint to explain the effect, and it is too faint to have been observed with amateur telescopes.
In 1967, Venera 4 found the Venusian magnetic field to be much weaker than that of Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind, rather than by an internal dynamo in the core like the one inside Earth. Venus's small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges.
It was hypothesized in 1957 by Urey and Brewer that CO+, CO2+ and O2- ions produced by the ultraviolet radiation from the Sun were the cause of the glow. In 1969, it was hypothesized that the Ashen light is an auroral phenomena due to solar particle bombardment on the dark side of Venus.
Throughout the 1980s, it was thought that the cause of the glow was lightning on Venus. The Soviet Venera 9 and 10 orbiters obtained optical and electromagnetic evidence of lightning on Venus. Also, the Pioneer Venus Orbiter recorded visible airglow at Venus in 1978 strong enough to saturate its star sensor. The European Space Agency's Venus Express in 2007 detected whistler waves further confirming the occurrence of lightning on Venus. In 1990, Christopher T. Russell and J. L. Phillips gave further support to the lightning hypothesis, stating that if there are several strikes on the night side of the planet, in a sufficiently short period of time, the sequence may give off an overall glow in the skies of Venus.
However, a Monte Carlo simulation program, indicates that the lightning hypothesis as the cause of the glow is incorrect, as not enough light could be transmitted through the atmosphere to be seen from Earth. Observers have speculated it may be illusory, resulting from the physiological effect of observing a bright, crescent-shaped object. Some spacecraft looking for it have not been able to spot it — leading some astronomers to believe that it is just an enduring myth.
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