Aureole from Mauna Kea
January 04, 2010
The aureole is a bright, white glow around the Sun or Moon. It’s almost always present and is due to forward scattering of sunlight from small aerosol particles. Such particles may be tiny water droplets, haze or dust, smog droplets, pollen, or volcanic dust. Any tiny surface can create them, and I’ve even seen aureoles caused by a cloud of mosquitoes! Air molecules do not produce aureoles.
In the picture at left, an aureole is seen around the Sun, which is hidden behind a light fixture. This was an exceptionally clear day with almost no dust in the air, so the aureole is small. In most cases, the aureole is many times larger. The picture at right was taken the next day when there was no detectable aureole, a rarity. As hard as it may be to believe, the Sun is actually behind the light fixture! The two bright spots are due to diffraction of the hidden solar disk by the edges of the light’s metal housing.Both photos were taken from Mauna Kea, Hawaii. I’ve only seen three days when it was this clear, and each was at Hale Pohaku, the astronomer’s community on Mauna Kea. Such clarity only occurs at high altitudes. From low elevations – specifically in the planetary boundary layer – there are always significant amounts of aerosols, and thus the aureole is always present.