Circumsolar radiation refers to light that, to an observer on the ground, appears to originate from the region around the sun. The term solar aureole is often used to describe easily observable or characteristic occurrences of circumsolar radiation. The phenomenon can easily be observed by using a finger or nearby object to block the direct sunlight from entering the eye and examining the light that streams around the occulting object.
Circumsolar radiation is caused by forward scattering of light through small angles by particles (aerosols) in the earth's atmosphere with dimensions on the order of or greater than the wavelength of light. The aerosol particles may be composed of ice crystals or water droplets in thin clouds. They may be dust or sea salt particles, smoke or fumes, photochemical pollutants, sulfuric acid droplets, solid particles with a water mantle, flocks formed of a loose aggregate of smaller particles, or any of a large variety of solid, liquid or heterogeneous materials that are small enough to be airborne. The amount and character of circumsolar radiation vary widely with geographic location, climate, season, time of day, and observing wavelength. Some of the more striking cases can be observed in the presence of high, thin cirrus clouds.
Under some circumstances these aerosols can cause a significant fraction of the solar flux to be deviated to angles of several degrees or more. Solar energy conversion techniques using high concentration ratios, such as solar central receivers and parabolic dishes, only collect light from the solar disk and a small portion of the circumsolar region. Pyrheliometers, the instruments normally used to measure the direct solar radiation, typically have a field of view of 5 degrees to 6 degrees. The pyrheliometer measurement includes a large portion of the circumsolar radiation and thus overestimates the amount of direct sunlight that would be collected by a concentrating system. The detailed angular distribution of the circumsolar radiation is important, as it affects the radiant energy distribution on the surface of the receiver in concentrating solar energy conversion systems.
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