Oval Coronae around Venus (Esthajnalcsillag)
1) Kansas Jan 22, '09 Doug Zubenel (TWAN)
Veszprem, Hungary Feb 12, '09 Tamás Ladanyi (site, TWAN)
3-4) Monika Landy-Gyebnar also at
Veszprem, Feb 12.
And Hungarian Halos (5-6) imaged at Ludanyhalaszi Feb 12 by Erno Berkó
Coronae around the moon and sun are fairly commonplace. More rarely,
Venus can be a bright enough luminary to make them but usually only the smudge of the central aureole is seen.
Doug Zubenel imaged one in January and noticed that it was vertically elongated. Yesterday (Feb 12) more oval Venus coronas were seen over several places in Hungary.
Oval aureoles around Venus and even Jupiter are not new. They were reported by Finnish observers including Matti Lamminen and others as early as 2001 but although they may be relatively common they are little known.
What is going on? We can discount effects from the phase of Venus - That does not produce them.
Most coronae are made when tiny droplets in clouds diffract the light. Small droplets are always tightly spherical and give circular coronae. To produce non-circular coronae we need oriented non-spherical objects. Non circular solar and lunar coronae are known - pollen grains make them as they drift, oriented by their air sac, in the wind. But mid-winter Kansas and Hungary do not have dense pollen clouds!
The clue was provided by the ice halo images sent by Erno Berko showing an intense upper tangent arc and above it a supralateral arc over Hungary. These halos were seen elsewhere in Hungary and were produced by column shaped ice crystals oriented in the air with their long axes nearly horizontal.
Could the column crystals have also made the Venus coronae?
Monika Landy-Gyebnar confirmed that the clouds at the time of her corona image were "cirrostratus with maybe maybe a small amount of very thin altostratus too". Ice crystal clouds. Some coronae are made by ice crystal cirrus but the the crystals are generally too small (10- 20 micron particles) and near spherical in shape to orient themselves by arodynamic drag forces.
What size objects made the coronae here? Tamás Ladanyi's image showed stars and he measured their angular separation. This gave the dimensions of his corona as 0.4°x0.3°. Measurements on the other uncropped images combined with lens data in their EXIF files gave (not quite as reliably) Doug's corona as 0.44° vertically and Monika's also 0.44°.
Calculations with IRIS indicate that coronae of this (small) angular size need particles with a dimension of about 0.175 to 0.350 mm depending on whether the outermost extent of the oval is the edge of the aureole or the first ring. That's large enough to be oriented by aerodynamic drag forces and large enough to make halos. Horizontally oriented crystals will make a vertically elongated corona.
It's therefore likely that these Venus coronae were made by horizontal columns. Plate crystals could also give rise to vertically elongated coronae. The elongation of both would depend on the altitude of Venus.
Venus shows these effects because it is effectively a point light source compared with the large disks of the sun and moon.
Images © the individual photographers.