Ashen light

While the discovery of the ashen light is often attributed to Italian astronomer Giovanni Battista Riccioli, recent evidence finds that German priest Athanasius Kircher{{cite book |last1=Kircher |first1=Athanasius |title=Iter Exstaticum |language=la |date=1660 |publisher=Herbipoli : sumptibus Joh. Andr. & Wolffg. Jun. Endterorum hæredibus, ; Prostat Norimbergæ : apud eosdem |pages=134–135 |url=https://archive.org/details/rpathanasiikirch00kirc/page/134/mode/2up |access-date=14 July 2023}} might have been the first to observe the ashen light during his one and only trip to Palermo, Sicily in the spring of 1638.{{cite book |last1=Fletcher |first1=John Edward |title=A Study of the Life and Works of Athanasius Kircher |date=2011 |publisher=Brill |pages=36–38 |isbn=9789004216327 |url=https://books.google.com/books?id=QeR5DwAAQBAJ&pg=PA36 |access-date=14 July 2023}} However, the first{{Citation needed|date=March 2025}} distinct and detailed record of the ashen light was produced by Riccioli on 9 January 1643, who ascribed it to the refraction of light within the telescope itself: "The colours arise from the various refraction of light in the glass, as it happens with trigonal glasses."{{Cite book |last=Riccioli |first=Giovanni Battista |url=https://books.google.com/books?id=_mJDAAAAcAAJ&pg=PA484 |title=Almagestum novum astronomiam veterem novamque complectens observationibus aliorum, et propiis novisque theorematibus, problematibus, ac tabulispromotam, in tres tomos distributam quorum argumentum sequens pagina explicabit |date=1651 |publisher=Ex typographia Haeredis Victorii Benatii |language=la}} This is likely a description of a phenomenon now known as chromatic aberration. Subsequent claims have been made by various observers ever since, including Sir William Herschel, Sir Patrick Moore, Dale P. Cruikshank, and William K. Hartmann.{{cite journal |title=The Paradoxical Ashen Light of Venus |author=Gingrich, M. |author2=Myers, E. |url=http://www.eastbayastro.org/2001/0301/r0301-2.htm |journal=Bulletin of the Eastbay Astronomical Society |volume=77 |issue=7 |date=Mar 2001 |location=Oakland, CA |access-date=2007-11-13 |archive-url=https://web.archive.org/web/20080705112912/http://www.eastbayastro.org/2001/0301/r0301-2.htm |archive-date=2008-07-05 |url-status=dead }}{{cite news |last=Inglis-Arkell |first=Esther |url=http://io9.com/the-four-hundred-year-mystery-of-the-ashen-light-of-ven-568548720 |title=The four-hundred-year mystery of the Ashen Light of Venus |work=io9 |date=27 June 2013 |accessdate=2015-09-05 }}

By the late 19th century the light phenomenon was described by those who stated they had observed it as ash-coloured, reminiscent of earthlight, therefore being called Venusian lumière cendrée or in English ashen light.{{cite book | last=Barentine | first=John C. | title=Mystery of the Ashen Light of Venus: Investigating a 400-Year-Old Phenomenon | publisher=Springer International Publishing | publication-place=Cham | date=2021 | isbn=978-3-030-72714-7 | doi=10.1007/978-3-030-72715-4 | doi-access=free | page=87 ff |quote="[...], the ash-colored secondary light of Venus will be found to present bright lines,” the editors of Nature offered in June 1876}}

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.{{cite news |last=Winder |first=Jenny |url=http://www.universetoday.com/94848/the-mystery-of-venus-ashen-light-2/ |title=The Mystery of Venus' Ashen Light |work=Universe Today |date=April 27, 2012 |accessdate=2015-09-05 }} Observation attempts were made on 17 July 2001, when a 67% illuminated Venus reappeared from behind a 13% illuminated moon. None of the observers of this occurrence (including some using {{cvt|61|cm|||}} 'Super RADOTS'{{Cite web|url=http://www.photosonics.com/super_radot.htm|title=Super Radot Tracking Mount|website=www.photosonics.com|access-date=2019-07-24}} telescopes) reported seeing the ashen light. Video from the event was captured, but the camera was too insensitive to detect even the earthshine.[http://www.smdc.army.mil/KWAJ/Hourglass/issues-archived/01Issues/hourglass7_24_01.pdf 2001 Jul 17 Reappearance of Venus from the Marshall Islands] {{Webarchive|url=https://web.archive.org/web/20161021082240/https://www.smdc.army.mil/KWAJ/Hourglass/issues-archived/01Issues/hourglass7_24_01.pdf |date=2016-10-21 }}, by Peter Rejcek. See page 8. Retrieved 2015-10-25.

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.[https://www.youtube.com/watch?v=r6TsRBQaMzI 2015 October 8 Reappearance of Venus from Australia], by Dunham D.W. & J.B. (2015-Oct-08).{{Better source needed|reason=Relying only on primary source (Youtube video). Also, the source is not peer reviewed, and of unknown reliability.|date=July 2019}}

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 ({{chem|C|O|2}}), known to be common in Venus' atmosphere, into carbon monoxide ({{chem|C|O}}) and oxygen ({{chem|O|2}}). However, the green light emitted as oxygen recombines to form {{chem|O|2}} is thought too faint to explain the effect, and it is too faint to have been observed with amateur telescopes.{{cite news |url=https://www.wired.com/2009/01/jan-9-1643-astronomer-sees-ashen-light-of-venus-2/ |title=Jan. 9, 1643: Astronomer Sees Ashen Light on Venus |work=Wired Science |date=9 January 2009 |accessdate=2015-09-05 }}

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,Dolginov, Nature of the Magnetic Field in the Neighborhood of Venus, Cosmic Research, 1969{{cite book

|author=Kivelson G. M.

|author2= Russell, C. T.

|title=Introduction to Space Physics

|publisher=Cambridge University Press

|date=1995|isbn=978-0-521-45714-9}} 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.{{cite journal

|author=Upadhyay, H. O.

|author2=Singh, R. N.

|title=Cosmic ray Ionization of Lower Venus Atmosphere

|date=April 1995|journal=Advances in Space Research

|volume=15|issue=4|pages=99–108

|doi=10.1016/0273-1177(94)00070-H|bibcode = 1995AdSpR..15d..99U }}

It was hypothesised in 1957 by Urey and Brewer that CO⁺, CO{{su|b=2|p=+}} and O{{su|b=2|p=−}} ions produced by the ultraviolet radiation from the Sun were the cause of the glow.{{Cite journal|bibcode = 1960JRASC..54...97M|title = Some Topics in Molecular Astronomy|last1 = McKellar|first1 = Andrew|journal = Journal of the Royal Astronomical Society of Canada|year = 1960|volume = 54|page = 97}} In 1969, it was hypothesised that the Ashen light is an auroral phenomena due to solar particle bombardment on the dark side of Venus.{{cite journal |title=The Ashen Light: An auroral phenomenon on Venus |journal=Planetary and Space Science |date=June 1969 |last= Levine |first=Joel S. |volume=1 |issue=6 |pages=1081–1087 |doi=10.1016/0032-0633(69)90001-4|bibcode = 1969P&SS...17.1081L }}

Throughout the 1980s, it was thought that the cause of the glow was lightning on Venus.{{cite journal |title=Discovery of frequent lightning discharges in clouds on Venus |journal=Nature |date=20 March 1980 |last=Ksanfomaliti |first=L. V. |volume=284 |issue=5753 |pages=244–246 |doi=10.1038/284244a0 |bibcode = 1980Natur.284..244K |s2cid=11234166 }} The Soviet Venera 9 and 10 orbital probes obtained optical and electromagnetic evidence of lightning on Venus.V. A. Krasnopol'skii, Lightning on Venus according to information obtained by the satellites Venera 9 and 10. Kosmich. Issled. 18, 429-434 (1980). Also, the Pioneer Venus Orbiter recorded visible airglow at Venus in 1978 strong enough to saturate its star sensor. 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. The European Space Agency's Venus Express in 2007 detected whistler waves, providing further evidence for lightning on Venus.{{cite journal|author=Russell, C. T.|author2=Zhang, T. L.|author3=Delva, M.|author4=Magnes, W.|author5=Strangeway, R. J.|author6=Wei, H. Y.|date=29 November 2007|title=Lightning on Venus inferred from whistler-mode waves in the ionosphere|url=http://aten.igpp.ucla.edu/personnel/russell/papers/lightning_venus_whistler-mode_waves.pdf|url-status=dead|journal=Nature|volume=450|issue=7170|pages=661–662|bibcode=2007Natur.450..661R|doi=10.1038/nature05930|pmid=18046401|archive-url=https://web.archive.org/web/20160304120534/http://aten.igpp.ucla.edu/personnel/russell/papers/lightning_venus_whistler-mode_waves.pdf|archive-date=4 March 2016|access-date=29 January 2012|s2cid=4418778}}{{cite news|date=29 November 2007|title=Venus also zapped by lightning|publisher=CNN|url=http://www.cnn.com/2007/TECH/space/11/28/venus.lightning.ap/index.html|accessdate=2007-11-29|archiveurl=https://web.archive.org/web/20071130201237/http://www.cnn.com/2007/TECH/space/11/28/venus.lightning.ap/index.html|archivedate=30 November 2007}}

The Akatsuki spacecraft, by Japan's space agency JAXA, entered orbit around Venus 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.[http://adsabs.harvard.edu/abs/2017EGUGA..1911381T 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. No lightning has been detected in 16.8 hours of night-side observation (July 2019).{{Cite journal|last1=Lorenz|first1=Ralph D.|last2=Imai|first2=Masataka|last3=Takahashi|first3=Yukihiro|last4=Sato|first4=Mitsuteru|last5=Yamazaki|first5=Atsushi|last6=Sato|first6=Takao M.|last7=Imamura|first7=Takeshi|last8=Satoh|first8=Takehiko|last9=Nakamura|first9=Masato|date=2019|title=Constraints on Venus Lightning From Akatsuki's First 3 Years in Orbit|journal=Geophysical Research Letters|language=en|volume=46|issue=14|pages=7955–7961|bibcode=2019GeoRL..46.7955L|doi=10.1029/2019GL083311|issn=1944-8007|doi-access=}}

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.{{cite journal |title=The transmission to space of the light produced by lightning in the clouds of Venus |journal=Icarus |date=October 1982 |last1=Williams |first1=Mark A. |last2=Thomason |first2= Larry W. |last3= Hunten |first3=Donald M. |volume=52 |issue=1 |pages=166–170 |doi=10.1016/0019-1035(82)90176-2 |bibcode = 1982Icar...52..166W }} Observers have speculated it may be illusory, resulting from the physiological effect of observing a bright, crescent-shaped object.{{cite journal|last=Baum|first=R. M.|date=2000

|title=The enigmatic ashen light of Venus: an overview

|journal=Journal of the British Astronomical Association|volume=110|page=325|bibcode=2000JBAA..110..325B

}} Spacecraft looking for it have not been able to spot it — leading some astronomers to believe that it is just an enduring myth.

A more recent hypothesis is that unusually high solar activity could induce auroral or airglow-like effects on the dark side of Venus. It has been observed that after major solar storms, an emission of light with a wavelength of 557.7 nm (the oxygen green line) occurs across the entire upper atmosphere of Venus. This is the same phenomenon which gives some aurorae on Earth their greenish appearance. Generally, this emission does not occur except for during major solar events such as coronal mass ejections (CMEs) or solar flares. However, dim emissions have been detected twice outside of solar storms, on December 27, 2010, and December 12, 2013 respectively. Both of these detections coincided with the passage of a “Stream Interaction Region”, a denser than average solar wind. In July 2012, a CME struck Venus producing a very bright green line emission. It is notable that after every CME impact on Venus, this emission is detected, but not after every flare. This is taken to indicate charged particles are what is responsible for the green line emission, similar to Earth's aurora.

File:Wispr 4thflyby.gif of the Parker Solar Probe took this visible light footage of the nightside in 2021, showing the hot faintly glowing surface, and its Ovda Regio plateau of Aphrodite Terra as large dark patch, through the clouds, which prohibit such observations on the dayside when they are illuminated.{{cite web |last1=Hatfield |first1=Miles |title=Parker Solar Probe Captures Visible Light Images of Venus' Surface |url=https://www.nasa.gov/feature/goddard/2022/sun/parker-solar-probe-captures-its-first-images-of-venus-surface-in-visible-light-confirmed |website=NASA |access-date=29 April 2022 |date=9 February 2022 |archive-date=14 April 2022 |archive-url=https://web.archive.org/web/20220414155959/https://www.nasa.gov/feature/goddard/2022/sun/parker-solar-probe-captures-its-first-images-of-venus-surface-in-visible-light-confirmed/ |url-status=live }}{{cite journal | journal=Geophysical Research Letters | last1=Wood | first1=B. E. | last2=Hess | first2=P. | last3=Lustig-Yaeger | first3=J. | last4=Gallagher | first4=B. | last5=Korwan | first5=D. | last6=Rich | first6=N. | last7=Stenborg | first7=G. | last8=Thernisien | first8=A. | last9=Qadri | first9=S. N. | last10=Santiago | first10=F. | last11=Peralta | first11=J. | last12=Arney | first12=G. N. | last13=Izenberg | first13=N. R. | last14=Vourlidas | first14=A. | last15=Linton | first15=M. G. | last16=Howard | first16=R. A. | last17= Raouafi | first17=N. E. | doi=10.1029/2021GL096302 | date=9 February 2022 | title=Parker Solar Probe Imaging of the Night Side of Venus | volume=49 | issue=3| pages=e2021GL096302 | pmid=35864851 | pmc=9286398 | bibcode=2022GeoRL..4996302W }}]]

Images of the night side of Venus in visible light, by the WISPR instrument of the Parker Solar Probe, have shown in 2022 that the surface is visible through the clouds at the night side. The study of the data though points at nightglow, also observed in visible light, as the more likely candidate for the ashen light.{{cite web | last=Dobbins | first=Thomas A. | title=The Parker Solar Probe Captures Surprising Images of Venus Nightside | website=Sky & Telescope | date=2022-02-22 | url=https://skyandtelescope.org/astronomy-news/the-parker-solar-probe-captures-surprising-images-of-venus-nightside/ | access-date=2025-03-14}} Human eyes are less sensitive to the visible light near-infrared red glow from the surface, which also mostly obscured by the clouds, but if this light is an ashen light, then it might have been reported by people with eyesight more sensitive to the red spectrum.{{cite web | title=Nightside observations by the Parker Solar Probe: implications for the reality of the Ashen Light – British Astronomical Association | website=British Astronomical Association – Supporting amateur astronomers since 1890 | date=2024-05-21 | url=https://britastro.org/journal_contents_ite/nightside-observations-by-the-parker-solar-probe-implications-for-the-reality-of-the-ashen-light | access-date=2025-03-14}}

• Atmosphere of Venus

{{reflist |colwidth=25em |refs=

{{cite journal |url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/ashen/ |title=The Ashen Light |author=Russell, C. T. |author2=Phillips, J. L. |journal=Advances in Space Research |year=1990 |pages=137–141 |volume=10 |issue=5 |bibcode=1990AdSpR..10e.137R |doi=10.1016/0273-1177(90)90174-X |access-date=2006-05-30 |archive-date=2015-12-08 |archive-url=https://web.archive.org/web/20151208005538/http://www-ssc.igpp.ucla.edu/personnel/russell/papers/ashen/ |url-status=dead }}

}}

Category:Planetary science

Category:Venus

Category:Light sources

Category:Unexplained phenomena