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The corona is observed around the sun as it is about to rise over the mountain ridge approximately
1.5 hours after sunrise. An extremely thin overcast of Altostratus or Cirrus cloud is present, but is
not thick enough to whiten the sun. The inside ring is green to violet, while the outer ring is red.
A faint secondary blue ring is visible outside the red ring (Medicine Bow Mountains, Colorado, USA,
photo by D.O. Blanchard)
Diffraction corona around the sun. Only a part of this phenomenon is visible. The star-shaped
appearance of the sun is caused by over-exposure in the camera. (photo by G.P. Können)
The un Polarized diffraction corona appears as a bright coloured aureole around a source of light. It is much smaller than the halo (some degrees in diameter, just like the glory) and is very bright. It is best seen near the moon or near a terrestrial source of light; near the sun it is difficult to observe with the naked eye because of its brightness. Yet this is when it is at its most beautiful. It is usually noticed near the moon and it is almost the only optical phenomenon that maintains a wealth of colours by moonlight. One sees it appear when there are thin clouds in front of the moon. It is formed by diffraction by the cloud particles; its centre is vividly white, with a red edge round it. Round it appear coloured rings, which in favourable circumstances can sometimes recur but then do so with a continually decreasing intensity of light. The size of the rings is inversely proportional to the diameter of the particles, as in the case of the glory. When a second cloud passes in front of the moon, a change in the diameters of the rings ca be noted; near the edge of a cloud the corona is often slightly egg- shape (this also occurs with the glory). During a foggy night one often sees a corona appear around a lamp-post or around the head-lights of a motorcar. Coronas can also frequently be observed through blurred window panes; then, however, the centre is dark. In this case they are not formed by diffraction but rather by interference of light that has passed through several little 'holes' between the drops. Atmospheric coronas can be formed in both water-clouds and ice-clouds, but in the latter case the coloured rings are less well developed. Even around bright planets like Venus or Jupiter the corona can be seen; here, however, it is only the uncoloured centre that appears since the intensity of light in the coloured parts is too low for any observation. On the other hand, a complete diffraction corona has sometimes been seen around the totally eclipsed sun: then the solar corona was the source of light.
source: Polarized light in Nature