Ashen light is a subtle glow that is seen from the night side of the planet Venus. This ashen light is said to be very similar to Earthshine on the Moon, but not as distinguished in brightness. It was first sighted by the astronomer Giovanni Battista Riccioli on January 9, 1643, and has been frequently sighted by various researchers including Sir William Herschel, Patrick Moore, Dale P. Cruikshank, and William K. Hartmann.
Before the development of more powerful telescopes, early astronomer Franz von Gruithuisen believed that ashen light was from the fires from celebration of a new Venusian emperor, and later believed that it was the inhabitants burning vegetation to make room for farmland.
Detection attempts 
Several space missions have been dedicated to record sightings of the light in hopes of solidifying what Riccioli and others have claimed. There are a few hypotheses being considered to try to explain this astronomical phenomenon.
An attempt to view the ashen light was made in Hawaii using the Keck 1 telescope. Researchers claimed to see a faint green glow on the night side of Venus. They suspected that it was carbon dioxide which is known to be of a high concentration in that atmosphere. When the molecules are split by the ultraviolet light from the Sun, they become carbon monoxide and oxygen, which emits a green light. However, this light emitted is very faint, and researchers doubt that it is the explanation surrounding ashen light.
Few other optical observations have been recorded with current instruments. For example, the spectrometer of Venera 9 saw irregular optical pulses.
Amateur astronomers attempting to view the ashen light can try this by using an occulting bar, an opaque mask for one’s eyes. This lens blocks the sunlit portion of Venus which greatly cuts down extraneous light that is scattered in the eye, improving the chances of witnessing the faint Ashen glow. However, Venus light scattered by the Earth's atmosphere and in the telescope's lens still works against the observer, and it is very hard to get the timing just right. Nevertheless, there have been calculated times in which the light scattered by Earth is covered partially by the Moon. For example, on July 17, 2001, the progression of a crescent Moon temporarily hid the illuminated portion of Venus. Unfortunately the location needed to catch this almost perfect occultation was somewhere in the middle of the Atlantic Ocean, and it was only visible in this manner for 10 to 20 seconds.
Other hypotheses 
Another hypothesis for the cause of the ashen light is lightning. 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, in an article published in the January issue of Nature, a team of astronomers from the University of Iowa argued against this idea. After reviewing data taken from the Cassini spacecraft, which flew close to Venus in 1998 and 1999, the team resolved that no high-frequency radio emissions were detected, the presence of which would have indicated lightning. In 2007, whistler waves were detected by Venus Express, confirming the occurrence of lightning on Venus. This hypothesis is supported by many astronomers, including two researchers whose paper on their findings is examined below.
Russell and Phillips 
Christopher T. Russell and J. L. Phillips from the Institute of Geophysics and Planetary Physics at the University of California in Los Angeles and the Los Alamos National Laboratory in Los Alamos, New Mexico have offered a mathematical approach to figuring out how exactly the ashen light is formed. The favored hypothesis, as explained in the paper, is the idea of lightning on Venus. Both believe that the phenomenon has occurred, stating in their paper that:
|“||The fact that many totally independent observers have observed the ashen light simultaneously, that some of these simultaneous observations were made by professional astronomers and that the observations persist today with improved instrumentation suggest the phenomenon is real.
The solid trace in the top panel shows the number of sightings from 1954 to 1962, while the bottom panel shows the same sightings normalized by the total number of observations at that magnitude of Earth-Sun-Venus angle. The dotted trace in each panel represents the probability function of equation, normalized to the same integrated value as the observations...
We interpret this diagram as follows assuming that observers are constantly watching Venus so as to provide a similar amount of observing time at all longitudes. At large Earth-Sun-Venus angles Venus is more difficult to observe because it is further away. Moreover the percentage size of the sunlit hemisphere is larger so that the ashen light will be difficult to see. We would expect that the process causing ashen light should act equally well at all locations of Venus relative to the Earth. The variation in occurrence rate must be due mainly to observability from Earth.
Inferior vs. superior conjunction 
Assuming that the location and the ability to observe the ashen light is in our favor, meaning we do not have to be in the middle of the ocean with technical instruments that allow us to view it without damage done to the eyes, Russell and Phillips found that it is possible to replicate the common characteristics of the sightings measured in the figure above. A more detailed explanation can be found in the paper itself for those interested in the mathematics of it.
Taking into consideration the limited amount of observation and the “simplicity of the model”, according to Russell and Phillips, the relation between observed sightings and the calculation seems to bring more promise in finding out the true cause of ashen light. It supports the hypothesis that the source of ashen light occurs mostly on the night side of Venus. The distance from the Earth to Venus could be a factor that controls the visibility of the light, however it also has to do with the observer’s specific location.
- Russell, C. T.; Phillips, J. L. (1990). "The Ashen Light". Advances in Space Research (Great Britain) 10 (5): (5)137–(5)141. Bibcode:1990AdSpR..10....5S. doi:10.1016/0273-1177(90)90112-D.
- 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". Nature 450 (7170): 661–662. Bibcode:2007Natur.450..661R. doi:10.1038/nature05930. PMID 18046401.
- Gingrich, M.; Myers, E. (Mar 2001). "The Paradoxical Ashen Light of Venus". Bulletin (Oakland, CA: The Eastbay Astronomical Society) 77 (7).