1. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, U.S.A.
2. Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
The Ashen Light first reported in 1643 is one of the oldest unsolved mysteries of the solar system. It consists of a faint luminosity on the nightside of the planet, similar in appearance to "earthshine" on the Moon but generally not as bright. Ashen Light is most often reported when Venus is in the evening sky at which time the evening terminator of Venus is toward the Earth. While some of this local time asymmetry may be associated with a tendency of observers to view Venus more in the evening hours than in the morning hours, we believe the asymmetry is too great to be explained by terrestrial effects alone. Thus, it appears that Ashen Light is principally a dusk side phenomenon on Venus. To our knowledge there is only one phenomenon that shows a marked dawn-dusk asymmetry on Venus and that could also explain the occurrence of Ashen Light. That phenomenon is Venus lightning whose local time of occurrence has been inferred from impulsive VLF waves observed by the Pioneer Venus Orbiter in the night ionosphere. The following factors appear to affect the visibility of Ashen Light from Earth: the distance of Venus from the Earth; the length of time Venus is above the horizon and the local time distribution of the Ashen Light source on Venus. Presently an Ashen Light campaign undertaken by amateur observers worldwide is underway. This campaign should help resolve whether the reported local time asymmetry is indeed intrinsic to the planet.
Visible emissions from the nightside of Venus have been reported for over 300 years. Known as the Ashen Light these emissions were first reported by Giovanni Riccioli on January 9, 1643 /1/ and have been observed on many hundreds of occasions since. For example, Levine /2/ reports 129 sightings made between 1954 and 1962. Ashen Light has been observed simultaneously and independently by 2 professional astronomers at least once /3/ and by up to 4 independent amateur observers on many occasions /4/. Ashen Light is thought to be an airglow or auroral phenomenon by some /2,5/, and airglow and aurora are seen from orbiting spacecraft /6,7/. In fact, the visible airglow at Venus is sufficient to saturate the Pioneer Venus star sensor when it looks directly at the planet /8/. However, others have suggested that lightning is responsible for the Ashen Light /9,10,11/. The problem with either explanation is in the energetics. To see the Ashen Light it must have an intensity of about 10-4 of the sunlit disk. Normal airglow on Venus does not appear to be this intense and lightning would have to be more frequent and intense than on Earth to explain the observations.
Searches for the electromagnetic pulses associated with electrical discharges in the Venus atmosphere have been overwhelmingly positive. However, optical searches have been much less positive. Only one optical observation has been reported, that of the spectrometer on Venera 9 which saw irregular optical pulses on October 26, 1975 at 1900 LT and 9o S latitude /1/. To our knowledge this is the only optical observation from orbit of the dusk hemisphere. The other optical searches with the Pioneer Venus star sensor /8/ and the VEGA balloons /12/ were undertaken on the dawn side and did not detect any unambiguous lightning signatures. We note that the VEGA balloons flew right in the clouds and not above or below them where its sensors could detect more distant lightning and the Pioneer Venus search provided only a total observing time equivalent to 10 seconds and involved scattered light not direct observation.
Electromagnetic evidence is not so ambiguous. The Venera 11 and 12 landers included induction coils which could measure electromagnetic impulses in four VLF frequency bands and which found strong and frequent impulsive broadband signals similar to those observed from terrestrial lightning /10/. The Pioneer Venus electric antenna also detected waves at low altitudes in the night ionosphere of Venus that appeared to be generated by lightning /13,14/. These studies examined only the components of the signal below the electron gyro frequency which could propagate to the spacecraft in the whistler mode and were very selective regarding the identification of events. Moreover, they were not designed to measure occurrence rates.
Impulsive VLF signals are also observed on PVO at frequencies above the electron gyro frequency /15,16/. These signals appear to be due to lightning generated waves leaking into the lowest layers of the ionosphere from the atmosphere below. They occur at the very lowest altitudes to which Pioneer Venus travels. Since they appear not to travel far they provide excellent tracers of possible source regions. These signals appear to have weak geographic control /17/ and strong local time control /18/. The local time occurrence pattern is similar to that reported for the Ashen Light. It is the purpose of this paper to examine the Ashen Light local time distribution and compare that variation with that deduced for lightning.
Many dismiss Ashen Light as an artifact and do not believe that Ashen Light arises on Venus. 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 /2,3,4/. Another piece of evidence for the reality of the Ashen Light phenomenon is the fact that its occurrence correlates with a phenomenon that should not affect seeing conditions and of which the observers should have been completely unaware. Ashen Light at Venus' inferior conjunction is more prevalent when geomagnetic activity is high /2/. Levine interpreted this fact to imply that energetic particles caused nighttime aurora on Venus. However, increased energetic particle flux could also lead to increased electrification of the atmosphere and hence greater lightning activity.
Levine /2/ also showed the occurrence of Ashen Light versus a Venus reference angle that was zero at inferior conjunction and 90o at Venus quadrature. These data showed a significant drop near inferior conjunction. He gave no explanation for this observation. As discussed below there may be a simple explanation of this phenomenon.
The occurrence of Ashen Light has been studied very carefully many times by amateur groups. A particularly useful summary has been provided by the British Astronomical Association /4/. In these lists it is obvious that there is a strong dawn-dusk asymmetry. Most reports of Ashen Light were made during the evening hours. It is possible but unlikely that this is due to a paucity of morning observations of Venus. We feel it is unlikely because of the seriousness with which these campaigns were conducted and the occurrence of reports of other phenomena during the morning hours. Thus we will proceed under the assumption that the Ashen Light phenomenon was adequately sampled both morning and evening during these early campaigns but we will keep in mind the possibility that there may have been fewer morning time observations.
It is surprising to us that this dawn-dusk asymmetry has received little notice. In fact, Levine did not plot data from east and west longitudes separately. Had he done so he would have obtained the plot shown in the upper panel of Figure 1. There is a rapid fall off in occurrence rate across inferior conjunction. While this variation may be affected by variations in seeing conditions as Venus approaches the Sun at inferior conjunction, the qualitative behavior is correct.
|Fig. 1. The occurrence of Ashen Light sightings versus Earth-Sun- Venus angle both before and after inferior conjunction /2/. 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 (1), 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.
Since observability due to distance and geometry does not depend on whether Venus is pre- or post-inferior conjunction, we have normalized the occurrence rate of Ashen Light as follows in the bottom panel of Figure 1. We added up the occurrence in each 10o bin at equal distances from inferior conjunction. Then we calculated the percent of the Ashen Light occurrences that were observed in each of the two contributing bins. In the bins from 70o to 80o, for example, all observations occurred when Venus was in the evening sky. In fact, morning occurrences were reported only within 30o of inferior conjunction.
If our assumption about the equality of the observing times on each side of inferior conjunction is correct or even approximately so this behavior has a simple explanation. There is a region on Venus which is the source of the Ashen Light which is always visible in the evening sky and which disappears when Venus is in the morning sky. Note that the motions of Venus are such that a viewer from Earth sees the dusk or leading hemisphere during terrestrial evening, and the morning or trailing hemisphere during terrestrial morning. Thus a source near the Venus evening terminator would have this property. If this is true then occurrence of Ashen Light seen prior to inferior conjunction should be strongest near the day-night terminator. Occurrences after inferior conjunction should have strongest emissions away from the day-night terminator.
|Fig. 2. The local time distribution of 100 Hz impulsive bursts when the magnetic field strength exceeds 15 nT, the distribution of bursts at the higher three frequencies (combined) and the occurrence rate of magnetic fields greater than 15 nT.|
The impulsive VLF signals seen at low altitudes in the night ionosphere of Venus are almost certainly due to lightning. Their occurrence rate drops rapidly with increasing altitudes except when the wave frequency is less than about one-quarter of the electron gyro frequency /18/. At these latter times the wave occurrence is almost constant with altitude. This behavior is what one would expect for an electromagnetic source of these waves at low altitudes. The low frequency waves are also observed to be electromagnetically polarized /19/. As mentioned above though, if we wish to determine the local time of the source region the higher frequencies are better for mapping than the lower because the signals attenuate more rapidly. Figure 2 illustrates this by comparing the local time occurrence rate at 100 Hz with that of waves at frequencies above the local electron gyro frequency /18/. The higher frequency waves maximize at about 2100 LT and fall off in occurrence sharply to earlier and later local times. We attribute the fall off toward dusk as due to increasing ionospheric density and hence absorption. Thus the source of these waves may extend well into the dayside of Venus. The fall off toward dawn must be due to source strength variations. We note that the distribution of strong magnetic fields is different from that of the VLF waves showing that we are not observing local time distribution imposed by the available propagation paths. In short, it appears that the local time sector in which lightning is generated is appropriate to also cause the Ashen Light.
By making simple assumptions about the source location and the observability of the Ashen Light, it is possible to reproduce the general characteristics of the sighting distributions of Figure 1. For example, if we assume that the probability of viewing the Ashen Light at a particular Earth-Sun-Venus angle varies with viewing time and with the apparent size of a source region bounded in Venus local time, the probability can be modeled as follows:
where = Earth-Sun-Venus Angle
T = Time between terrestrial sunset and sunrise during which source zone is in view
= Arc subtended by Venus viewed from Earth
= Venus local time
1, 2 = Limits of Venus local time observable from Earth
= Earth zenith angle at Venus equator
f = Ashen Light source function
If we further assume, based on the high-frequency burst distribution of Figure 2, that the source function is a step function which is non- zero only between 1800 and 2230 local time, the result is the predicted sighting distribution shown as a dotted trace in Figure 1. Considering the limited observational statistics and the simplicity of the model, the agreement between calculated and observed sightings is encouraging, and supports the hypothesis of an Ashen Light source which occurs primarily at local Venus evening. It should be noted that there is an alternative mechanism to lightning that has been proposed to cause a dusk enhancement of emissions. That mechanism is the impact of energetic solar particles primarily on the evening side of Venus as a result of the prevailing interplanetary magnetic field direction /7/.
Williams at al. (20) have dismissed the notion that lightning can be the source of Ashen Light because the Ashen Light would have to have an intensity of about 10-1 W m-2 to be seen. If an individual flash had a power of 109 W as Williams et al. assume then to maintain Ashen Light over about 107 km2 on the nightside of Venus requires about 1012 W which in turn requires 103 intra-cloud flashes per second. This power in lightning is similar to that observed on Earth but the flash rate is 10 times as high because a lower energy per flash has been assumed than is typical for Earth (ground to cloud) discharges. on the other hand, intra-cloud discharges on Earth, while weaker, occur more frequently /21/. We might expect such a compensation on Venus also.
The above analysis says that if lightning occurs as frequently on Venus as it does on the Earth we should see it as Ashen Light. This same conclusion was reached by both Ksanfomaliti /10/ and Krasnopol'skii /11/ arguing from different models. The question then becomes how frequent is Venus lightning. Could there be 1000 weak flashes per second or 100 more powerful flashes per second? It is difficult to answer this question from the Pioneer Venus Orbiter data because the one-second temporal resolution of the data does not allow all the submillisecond pulses associated with each flash to be measured. All we can tell with Pioneer Venus is that on nearly every orbit when conditions are right for propagation to the satellite there seems to be several bursts at least in each 30-second interval studied. If the satellite is sensitive to a region of about 1010 m2 then a flash rate of 102 per second or more in an area of about 1013 m2 is consistent with our data.
The Ashen Light has many properties that one would expect for a phenomenon intrinsic to Venus rather than an artifact introduced by the observers, In particular, it appears that it has a local time of occurrence that is very similar to that of Venus lightning. Consideration of the frequency with which lightning flashes are observed to occur according to the proxy electromagnetic wave data suggests that the Ashen Light could be powered by lightning. Factors that appear to control the visibility of Ashen Light from the Earth include the distance of Venus from the Earth, the length of time Venus is above the horizon, and the local time distribution of the source of Ashen Light on Venus. The ongoing Ashen Light campaign should help answer the question concerning the reality of the Ashen Light and further refine the local time of occurrence of this phenomenon,
This work was supported by the National Aeronautics and Space Administration under research grant NAS2-501 and by the U. S. Department of Energy.
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