Ashen light

First sighted by the astronomer Giovanni Battista Riccioli on January 9, 1643, the Ashen
Light is a subtle glow that is seen from the night side of the planet Venus. When
astronomers say "night side" it is meant that the side of the planet in which
the sun is opposite to. (I.e. while it might be light outside in China, it is dark
in North America). This is said to be very similar to Earthshine on our moon,
but not as distinguished in brightness. It has been frequently sighted by
various researchers including Sir William Herschel, Patrick Moore,
Dale P. Cruikshank and William K. Hartmann. Several space missions have been dedicated to
record sightings of the light in hopes of solidifying what Riccioli and others have
claimed. The Pioneer and the Russian Venerall 11 and 12 have been sent out but without
much success; it is believed that the Ashen Light is one that can only be seen through
human eyes. There are a few hypothesizes being tossed around 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 is was carbon dioxide which is known 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.

Amateur astronomers (i.e. students) attempting to view the Ashen Light can try it 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 
our Moon. For example, on July 17, 2001, a crescent Moon temporarily hid the 
illuminated Venus. Unfortunately the location needed to catch this almost perfect 
occultation was somewhere in the middle of the Atlantic Ocean, and it was only going 
to be visible for a 10-20 seconds, not in the favor of anyone trying to catch a 
glimpse.

Another hypothesis for the cause of the Ashen Light is the possibility of lightening.
If there are several strikes in a period of time, the sequence may give off an overall
glow in the skies of Venus, if this happens on the night side. However, in an article
published in the January issue of Nature, a team of astronomers from the University
of Iowa expressed negative criticism and doubt of 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 noise was detected. On Earth we hear "static"-like
fuzz on AM radio stations during thunderstorms. If the light was caused by
lightening, similar fuzz would have been expected. However, this seems to be the most
promising hypothesis, supported by many astronomers, including C. T. Russell]] and
J. L. Phillips, who wrote a paper on their findings, which is examined below.

C. 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 lightening 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.”

In a pictorial representation, Figure 1 shows the occasions of Ashen Light sightings 
versus Earth-Sun- Venus angle both before and after inferior conjunction.

“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.”

[1]

Inferior vs. superior conjunction

Assuming that the location and the ability to observe the Ashen Light is in our favor, 
meaning we don’t 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. 
(http://www-ssc.igpp.ucla.edu/personnel/russell/papers/ashen/) 

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.  

Resources:

The Ashen Light, C. T. Russell and J. L. Phillips. Advances in Space Research, Vol. 10, No. 5, pp. (5)137-(5)141, 1990.

The Paradoxical Ashen Light of Venus, M. Gingrich E. Myers. http://www.eastbayastro.org/2001/0301/r0301-2.htm