Oval aureoles and coronae around stars and planets

Atoptics Highlights - Optics Picture of the Day - OPOD - Atmospheric Optics

A miniature corona the size of the moon surrounds Sirius. Complete with central aureole and an oval ring coloured blue inside and red outside.

Anette Aslan captured it near Vienna, Austria on March 20, �18. Sirius was 25� high and 8� west of the meridian.

It�s a rare sighting. There are many images of elongated aureoles around bright planets and stars (see below). But this is perhaps the first of a complete oval corona.

Anette's camera was on an equatorial telescope mounting and tilted. I (Les Cowley) rotated the images to give the star fields (not the corona!) their actual orientation relative to the local horizon. That the corona then also happens to be elongated vertically is important. It argues against a camera artefact. It indicates how it formed.

Top images ©Anette Aslan Text ©2018 Les Cowley

Aureoles around Sirius (left) and Jupiter (right) imaged within a few minutes of each other by Peter Rosen in 2013.

That around Sirius at altitude 14� is vertically elongated. Jupiter's aureole at 47� high is essentially circular.

This was the first verification of the 2009 prediction. But more are needed.

A tiny circular corona around Jupiter. The Pleiades are at right and Aldebaran at lower left. Pictured in the Mesr Desert of Iran by Mahdi Zamani, November 2012.

Below: Circular coronae around Jupiter (lower) and Venus captured from Colorado by Robert Arn on 14th March 2012.

Doug Zubenel pictured this oval aureole surrounding Venus from Kansas on January 8, '12. Venus was 20-25� high.

Non-spherical airborne objects create oval coronae and aureoles.

Here Venus left) and a crescent moon (right) sport aureoles and a corona made by pollen grains. Imaged by Pierre-Paul Feyte.

Anette's Sirius corona is not from pollen.

Oval aureoles around stars near M42, the Orion Nebula. M42 was 25� high. Why do the fainter stars not have ovals? They do. Stars down to 7th magnitude show them on the original file. Longer exposures would show more. Image by Peter Rosen

About Oval Coronae & Aureoles

Lens artefacts?

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?

Pollen

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.

Suspects

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.

Plate and column ice crystals
Ones of large dimension 100-250 micron (0.1-0.25mm) could form aureoles and corona. As they drift in cloud currents aerodynamic drag orients them as shown.