Atmospheric
Optics
Corona ring brightness & colour
A magnificent corona imaged by Cortney Litwin at Spokane, Washington, USA. A rare third ring is faintly visible.
Coronae are the result of diffraction by individual tiny water droplets. Each droplet scatters light waves which then overlap and interfere to produce the ringed pattern.
The figure at right is from "
Rings around the sun and moon: coronae and diffraction" by Les Cowley, Philip Laven and Michael Vollmer, J. Physics Education, 40 (1) 2005.
Read it here.
More about corona intensity.
The graph shows corona intensities versus angle for three colours and for their integrated light. The integrated light curve is coloured to represent hues visible by eye.
Monosized 25 micron diameter water droplets were used in the accurate Mie scattering calculations.
The vertical intensity scale is logarithmic.
Look at the green light curve. Its intensity falls by 100X between the central aureole and the first ring. Subsequent rings are less intense but not by much. There is a 10X difference between ring peaks and the troughs between them. A corona in filtered green light (try it someday!) has many well defined rings.
The same is true for other wavelengths but their rings are in different positions.
The corona in integrated white light is far less well defined. The first and second rings are no more intense than the spaces between them. They are only visible because of colour differences.
Paradoxically, at larger angles there are greater intensity variations. However, these would be rarely seen because differences in droplet size usually blur them away.
The graph is based on output from Philip Laven's excellent MiePlot program.
Colours not brightness
We see coronal rings more for their colour variation. The rings are only barely brighter - if at all - than the 'gaps' between them.
At left, half of Cortney's image has been converted to black and white tones. The rings almost disappear.
