Ashen light

Ashen light is a hypothesised subtle glow that has been claimed to be seen on the night side of the planet Venus. The phenomenon has not been scientifically confirmed. If real, it may be associated with lightning, which was confirmed to occur on Venus by the Venus Express mission.

For the similar appearance with the Earth's moon, see Ashen glow.

History of observations

The ashen light was first reported by the astronomer Giovanni Battista Riccioli on 9 January 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.[1][2]

The ashen light has often been sighted when Venus is in the evening sky, when the evening terminator of the planet is toward the Earth.[1][3] Observation attempts were made on 17 July 2001, when a 67% Venus reappeared from behind a 13% moon. None of the observers of this occurrence (including some using 61 cm (24 in) 'Super RADOTS'[4] telescopes) reported seeing the ashen light. Video from the event was captured, but the camera was too insensitive to detect even the earthshine.[5]

A particularly favourable viewing opportunity occurred on 8 October 2015, with a 40% illuminated Venus reappearing from behind the unlit limb of a 15% sunlit Moon. The event was visible in dark skies throughout Central Australia and was recorded by David and Joan Dunham (of the International Occultation Timing Association) using a 10" f/4 Newton telescope with a Watec 120N+ video camera from a location just north of Alice Springs. They also observed the event visually with an 8" Schmidt–Cassegrain telescope. Neither the real-time visual observation nor close visual inspection of the video recording showed any sign of the dark side of Venus.[6] While not conclusive, these observations suggest the ashen light is more likely attributable to telescope optics and eye physiology rather than atmospheric phenomena on Venus.

The Akatsuki spacecraft, by Japan's space agency JAXA, was injected into Venus orbit on 7 December 2015. Part of its scientific payload includes the Lightning and Airglow Camera (LAC) which is looking for lightning in the visible spectrum (552-777 nm). To image lightning, the orbiter has sight of the dark side of Venus for about 30 minutes every 10 days.[7] No lightning has been detected in 16.8 hours of night-side observation (July 2019).[8]

Hypotheses

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). However, the green light emitted as oxygen recombines to form O
2 is thought too faint to explain the effect,[3] and it is too faint to have been observed with amateur telescopes.[9]

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,[10][11] 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.[12]

It was hypothesized in 1957 by Urey and Brewer that CO+, CO+
2 and O−
2 ions produced by the ultraviolet radiation from the Sun were the cause of the glow.[13] In 1969, it was hypothesized that the Ashen light is an auroral phenomena due to solar particle bombardment on the dark side of Venus.[14]

Throughout the 1980s, it was thought that the cause of the glow was lightning on Venus.[15] The Soviet Venera 9 and 10 orbital probes obtained optical and electromagnetic evidence of lightning on Venus.[16][17] Also, the Pioneer Venus Orbiter recorded visible airglow at Venus in 1978 strong enough to saturate its star sensor.[16] The European Space Agency's Venus Express in 2007 detected whistler waves further confirming the occurrence of lightning on Venus.[18][19] 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.[16]

However, simulations indicate 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.[20] Observers have speculated it may be illusory, resulting from the physiological effect of observing a bright, crescent-shaped object.[21] Spacecraft looking for it have not been able to spot it — leading some astronomers to believe that it is just an enduring myth.[2]

See also

References
  1. Gingrich, M.; Myers, E. (Mar 2001). "The Paradoxical Ashen Light of Venus". Bulletin of the Eastbay Astronomical Society. Oakland, CA. 77 (7). Archived from the original on 2008-07-05. Retrieved 2007-11-13.
  2. Inglis-Arkell, Esther (27 June 2013). "The four-hundred-year mystery of the Ashen Light of Venus". io9. Retrieved 2015-09-05.
  3. Winder, Jenny (April 27, 2012). "The Mystery of Venus' Ashen Light". Universe Today. Retrieved 2015-09-05.
  4. "Super Radot Tracking Mount". www.photosonics.com. Retrieved 2019-07-24.
  5. 2001 Jul 17 Reappearance of Venus from the Marshall Islands Archived 2016-10-21 at the Wayback Machine, by Peter Rejcek. See page 8. Retrieved 2015-10-25.
  6. 2015 October 8 Reappearance of Venus from Australia, by Dunham D.W. & J.B. (2015-Oct-08).
  7. Hunt for optical lightning flash in Venus using LAC onboard Akatsuki spacecraft. Takahashi, Yukihiro; Sato, Mitsuteru; Imai, Masataka. 19th EGU General Assembly, EGU2017, proceedings from the conference held 23–28 April 2017 in Vienna, Austria., p.11381.
  8. Lorenz, Ralph D.; Imai, Masataka; Takahashi, Yukihiro; Sato, Mitsuteru; Yamazaki, Atsushi; Sato, Takao M.; Imamura, Takeshi; Satoh, Takehiko; Nakamura, Masato (2019). "Constraints on Venus Lightning From Akatsuki's First 3 Years in Orbit". Geophysical Research Letters. 46 (14): 7955–7961. Bibcode:2019GeoRL..46.7955L. doi:10.1029/2019GL083311. ISSN 1944-8007.
  9. "Jan. 9, 1643: Astronomer Sees Ashen Light on Venus". Wired Science. 9 January 2009. Retrieved 2015-09-05.
  10. Dolginov, Nature of the Magnetic Field in the Neighborhood of Venus, COsmic Research, 1969
  11. Kivelson G. M.; Russell, C. T. (1995). Introduction to Space Physics. Cambridge University Press. ISBN 978-0-521-45714-9.
  12. Upadhyay, H. O.; Singh, R. N. (April 1995). "Cosmic ray Ionization of Lower Venus Atmosphere". Advances in Space Research. 15 (4): 99–108. Bibcode:1995AdSpR..15...99U. doi:10.1016/0273-1177(94)00070-H.
  13. Some Topics in Molecular Astronomy. McKellar, A. Journal of the Royal Astronomical Society of Canada, Vol. 54, p.97. Bibliographic Code: 1960JRASC..54...97M
  14. Levine, Joel S. (June 1969). "The Ashen Light: An auroral phenomenon on Venus". Planetary and Space Science. 1 (6): 1081–1087. Bibcode:1969P&SS...17.1081L. doi:10.1016/0032-0633(69)90001-4.
  15. Ksanfomaliti, L. V. (20 March 1980). "Discovery of frequent lightning discharges in clouds on Venus". Nature. 284 (5753): 244–246. Bibcode:1980Natur.284..244K. doi:10.1038/284244a0. S2CID 11234166.
  16. Russell, C. T.; Phillips, J. L. (1990). "The Ashen Light". Advances in Space Research. 10 (5): 137–141. Bibcode:1990AdSpR..10..137R. doi:10.1016/0273-1177(90)90174-X.
  17. V. A. Krasnopol'skii, Lightning on Venus according to information obtained by the satellites Venera 9 and 10. Kosmich. Issled. 18, 429-434 (1980).
  18. Russell, C. T.; Zhang, T. L.; Delva, M.; Magnes, W.; Strangeway, R. J.; Wei, H. Y. (29 November 2007). "Lightning on Venus inferred from whistler-mode waves in the ionosphere" (PDF). Nature. 450 (7170): 661–662. Bibcode:2007Natur.450..661R. doi:10.1038/nature05930. PMID 18046401. S2CID 4418778. Archived from the original (PDF) on 4 March 2016. Retrieved 29 January 2012.
  19. "Venus also zapped by lightning". CNN. 29 November 2007. Archived from the original on 30 November 2007. Retrieved 2007-11-29.
  20. Williams, Mark A.; Thomason, Larry W.; Hunten, Donald M. (October 1982). "The transmission to space of the light produced by lightning in the clouds of Venus". Icarus. 52 (1): 166–170. Bibcode:1982Icar...52..166W. doi:10.1016/0019-1035(82)90176-2.
  21. Baum, R. M. (2000). "The enigmatic ashen light of Venus: an overview". Journal of the British Astronomical Association. 110: 325. Bibcode:2000JBAA..110..325B.
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