Corona/Aureole around Venus - imaged February 21, '09 at Veszprém, Hungary by Tamás Ladányi (site, TWAN). ©Tamás Ladányi, shown with permission.

This is the largest aureole/corona around Venus that Tamás has yet photographed, about 5 diameter (other Venus effects 1,2,3). Venus remains dazzling in evening skies and we may see more.

Corona or aureole?

Particles in the atmosphere, water droplets, pollen, dust, smoke, aerosol, the air itself and even small insects - regardless whether the particles are transparent or opaque - scatter the sun's rays. Mostly they do so most strongly with only small deviations from the incoming rays - the forward direction. The result is an intense glow near to the sun called an aureole. They exist around the moon, planets and stars but are usually too faint to be seen.

The size of an aureole in photographs is determined to some extent by the duration of the exposure. Venus is so relatively faint that to see the full extent is difficult. But the other factor is the size of the scattering particles. Small particles give large aureoles, for example, the very small particles of stratospheric dust lofted by volcanic eruptions produce the immense Bishop's Ring.

Aureoles are white or the colour of the source light. They decrease smoothly in intensity away from the source. Their originating particles have a range of sizes.

When the particle size range decreases the glow starts to take on structure and colour. First of all a red edge appears because aureoles from long wavelength red light are larger than those from blue. A further decrease in the particle size range leads to a faint ring beyond the red edge of the central aureole. the ring is blue/violet inside and red outside. Further narrowing of the size distribution produces a second or even more rings. That is corona.

The IRIS Lorentz-Mie simulations at lower left show the effects of changing size distribution for spherical particles. (The variation quoted is the standard deviation of a Gaussian size distribution expressed as a percentage of the mean.)

When exactly can an aureole be called a corona? That is debatable. When we can see a definite red boundary to the central glow is a good start, others might say when the first ring is visible.   But why be pedantic! Admire them all.


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