Aureoles are glows around the sun, moon and very bright planets. Scattering by water droplets, dust or aerosol produces them. Sometimes they can be regarded as the inner part of a corona.
This aureole around bright Venus is enigmatic because it is oval rather than the usual circular shape. Doug Zubenel (TWAN) took the image from Kansas on 8th January when Venus was 20-25 degrees high. The two stars are delta and gamma Capricorni. If they too had aureoles they would have been too faint to show in the 3.2s exposure at f/5.6 and ISO 100.
Oval aureoles around Venus and Jupiter were reported by Finnish observers including Matti Lamminen and others as early as 2001 but although they may be common they are relatively little known. Recent images are by Doug Zubenel (1 ) by Tamás Ladanyi and Monika Landy-Gyebnar (2) and others (3 4).
©Doug Zubenel, shown with permission
|Oval aureoles could be produced by light scattering from horizontally elongated objects, horizontal elongation gives a vertically elongated diffraction pattern.
Hexagonal ice plate crystals fit the bill because they are horizontally aligned in clouds.. Horizontal column crystals are another possibility. The stars in Doug’s picture allow us to estimate the aureole size as ~ 0.21 x 0.35°. An IRIS simulation then shows that scatterers of about 0.25mm across created the aureole. Ice crystals of that dimension can be well oriented.
If oriented ice crystals are responsible then the aureole elongation should depend on Venus's height above the horizon. For example, if Venus were overhead then plate crystals would have an almost circular profile and give a circular aureole. Column crystals would also give an averaged circular aureole because the columns take up all azimuthal orientations. A high Venus should give a less elongated aureole than a low one.
To help prove the above we need simultaneous halo observations or better, near simultaneous images of aureoles around low and high bright stars. None of this is easy and is complicated by minute to minute changes in atmospheric conditions. Doug’s lower image taken 8 minutes before the upper one shows much less elongation. Nor do Venus or Jupiter always show elongated aureoles – when they are created by water droplets they are circular.
A challenge for photographers!
Why not ovals around the moon? The aureole would have to be large to be visible. That implies smaller crystals. Small crystals are less oriented.