CORONAS AND HALOS

Water droplets (or ice crystals) interact in a complicated manner with incoming sunlight. When the droplets or crystals are much larger than the wavelength of visible light (as they often are) an approximation can be made by using the theory of diffraction from a knife edge (Fraunhofer diffraction). This relatively simple theory suggests that rings of refracted light, with distinct diameters, could surround an object illuminated in the 'forward direction'.

In nature, usually the water droplets or ice crystals are hetrodisperse, covering a range of droplet diameters: small droplets are mixed in with large droplets. Since each droplet diameter is associated with rings of a particular angular size and since there is a span of particle diameters, the rings interfere with one another, causing a diffuse, whitish glow, or aureole. You can see this by blotting out the sun behind a tree trunk. The sky around the sun will be whitish from the scattered light. Usually the solar aureole is around 20 degrees in diameter.

However, occasionally, droplets or crystals will be almost monodisperse. This often occurs near the edge of evaporating clouds. When it happens colorful rings can surround the sun,

Fraunhofer theory gives the distribution of light around a forward-scattered sphere to be J1(x)/x where x=Pi*Dia/Lamda.

By measuring the distribution of brightness in the aureole, it is possible, though difficult, to estimate the distribution of particle sizes, the so-called particle size distribution function. This is important for building models of climate change. We have investigated this method for deducing aerosol parameters by using an instrument of our own design, constructed to remove forward Fraunhoffer scattering from the blotting-out disk:

The scan above shows the intensity of the sky aureole around the sun caused by aerosol scattering. The different colors are false and represent different intensities of scattered light.

Here is an aureole around the sun caused from Arctic Haze aerosol:

When the atmosphere is nearly devoid of aerosols, as it is over the Antarctic ice sheet, there is practically no detectable aureole, as this picture taken at the South Pole demonstrates: