About Oval Coronae & Aureoles
Aureoles and coronae around stars are easily ascribed to lens dew. Many are! Inspect your lenses. Suspect images, especially when nothing was noticed by eye.
Yet some are real
Aureoles around bright planets were reported in Finland by Matti Lamminen and others as early as 2001. There is now a body of reported sightings.
Oval observations are perhaps more recent. OPOD's first reports were in 2009. The ovals were all vertically elongated and in 2009 OPOD hypothesised a mechanism for their formation. Later observations support it.
OPOD has several images of oval and non-oval aureoles and one or two coronae around Venus, Jupiter and stars. Thumbnail links below.
Their formation by diffraction
Diffraction forms aureoles and coronae. Cloud water droplets, ice crystals and airborne pollen grains scatter light waves. The scattered waves interact (interfere) to give a pattern of coloured rings around a central bright aureole.
That's the easy bit. But how do the oval ones form?
Spherical particles make circular coronae. Some pollen grains are famously non spherical and – when aerodynamically aligned – produce oval coronae. At right are elongated pollen coronae/aureoles around Venus and the Moon. So far so good but other ovals were obviously formed in high cirrus out of the pollen season.
Oval coronae need:
Oval coronae and aureoles need elongated objects. Non-spherical like pollen grains. Vertical ovals need horizontally elongated objects. Size matters. I calculate from measurements of the star field that Anette’s corona came from particles around 200 micron across. Other oval aureoles yield similar particle sizes.
There is only one suspect in that size range. Ice crystals. They can be up to 1000 micron (1mm) across. They are variously oriented. Plates with their large hexagonal faces horizontal are possible culprits. So are hexagonal columns with their long axes horizontal.
Suspect 1 - Plate crystals
Look directly upwards at a cirrus cloud full of plate crystals. With ultra telescopic vision you see a multitude of tiny hexagons. No good for oval coronae, they would produce circular ones.
But look at the same cloud closer to the horizon, say 10 to 30 degrees up. Now they are more edge on – horizontally elongated as required for vertical oval coronae.
Suspect 2 – Horizontal column crystals
Look up at a cloud of columns and you see sticks aimed in all directions. Their corona is the sum of those from individuals – circular. Closer to the horizon some crystals appear end on, dots. Others are shorter and longer horizontal sticks. Their average is horizontally extended. Their corona is a vertical oval.
Predictions and observations
OPOD predicted vertically elongated coronae/aureoles near the horizon and circular ones high up. If made from large(ish) plate crystals or columns of course. A later simultaneous sighting of Venus and Jupiter at right confirmed this. Vertical ovals would only be expected from lowish altitude planets and stars. This also is as observed.
So it all fits the sightings, qualitatively at least. Now it needs some quantitative predictions using Mie-Lorentz or equivalent scattering theories.