CORONA AND IRISATION
Several weeks ago, spil I wasgoed returning huis ter the late evening, I looked toward the southern sky and eyed the nearly-full moon ringed ter a reddish liaison shining through a skinny cloud layer. I knew this to be a corona rather than a luminosidad by its taut proximity to the moon, a few degrees across rather than the 22 degrees of the smallest resplandor.
Peca Corona: Photograph by Keith C. Heidorn
While I had written about close cousins of the corona — glories and the Brockenspectre — several years ago, I realized I have not yet penned a chunk on this phenomenon, tho’ it is one of the more common visuals of atmospheric optics.
Peca Corona (Cape Evans, McMurdo Sound) Watercolor Drawing
by Dr. Edward A. Wilson, medical officer
on Discovery from the Capt R. Scott’s
Terra Nova Expedition to the South Pole
Wij most often see the basic corona encircling the zon or moon with a bright, almost white, central aureole fringed with rings of color, with reddish hues on the outer rings. When viewing conditions are right, the corona will be composed of several subtly colored rings encircling the central aureole. Thesis gradations start with a bluish stadionring then shift through the spectrum with greens and yellows to the outer crimson stadionring. Thesis rings are more diffuse te their banding than the distinct bands found ter a rainbow, and their sequence is opposite that of a luminosidad te which crimson is the inward verhouding.
Solar Corona seen from
Golden Gate Bridge, San Francisco
Solar Corona at Sunrise
Fresh Brighton, Minnesota
Coronae generally arise from light interactions with water droplets rather than with ice crystals, spil ter the fulgor family. But they can form ter the presence of petite ice crystals and other atmospheric constituents such spil particles of dust or airborne pollens. (I’ll lump all possible initiators of coronae together and simply call them particles.) And the particles need not be see-through strafgevangenis even spherical ter form.
The reason that a multiplicity of different particle types can form coronae is that the puny particles diffract (arch) the light flaps spil they pass overheen its surface. Diffraction occurs when a light wave’s path is leaned around objects of a similar size to the wavelength of that light. (See WW2010 Online Guides for a description of diffraction.) Thus, it is the size of the particle rather than its form or composition that determines the nature of the coronae. The stadionring patterns visible te coronae are produced by the constructive or devastating interference among light sways spil they pass through the particle field. With constructive interference, the light sways produce enhanced areas of brightness, with devastating interference, the interaction results ter dark areas. (See Experiments te Wave Interference).
Light Diffraction Passing Inbetween Cloud Droplets
Youthful diffraction pattern
of light passing inbetween
cloud droplets. A and B at
position similarly indicated
on diagram to left
Particle spacing or density is not spil significant te coronae formation spil the particle size and the wavelength of the light. Of course, if the density is too thick, the light becomes too diffuse or too faint to produce a distinguishable corona to the viewer, and if too skinny, not enough light reaches the viewer to be a recognizable pattern. An analogy te the latter case can be found te rainbows. A single raindrop struck by a ray of light decomposes it into its spectral colors, but it takes a multitude of drops to combine their lights to form a rainbow. So too does it take a multitude of individual diffractions to produce a corona.
The particle sizes voorwaarde fall within the range of 0.02 to 0.1 millimetres to decently interact with the wavelengths of visible light to form coronae. The most distinct coronae arise when the particles te the air are of uniform size. The more diversity te particle size, the more blurred the composite diffraction effect. Smaller particles generate the largest coronae and usually the brightest. Larger particles produce smaller diffraction angles and thus tighter coronae.
Ter an idealized corona, the central aureole is white, the result of the contribution of all color wavelengths combining, and the bands around it would budge from blue through green and yellow to crimson. If further banding is visible outside the primary crimson plakband, the pattern repeats ter the same order (unlike the rainbow where the secondary bow repeats the colors te switch roles sequence). The most commonly seen coronae, however, have a bluish aureole with a reddish stadionring surrounding it.
Corona around the Moon
Courtesy Wing-Chi Poon
A corona typically has a middellijn of a few degrees, but the size may waxes and wanes spil clouds of differing composition budge across the face of the zon or moon. Large coronae may spread 10-15 degrees te width. Ter the extreme, the coronal effect can extend further out to 20 to 30 degrees (the Bishop’s stadionring) to spil much spil 45 degrees (irisation).
Thesis phenomena are usually best viewed surrounding the moon spil the sun’s brightness often overloads our eyes with light unless muted by the covering cloud (tho’ too thick a cloud will not permit the light to pass through spil a coronal pattern). A trick for viewing a solar corona (not to be confused with the sun’s corona which can be see during an degeneración) is to view the zon and corona spil a reflection te a pool of water. The reflection can cut the brightness of the zon and permit the corona to be seen. Any high light source can produce a corona. On occasion, one may be seen surrounding one of the brighter planets or starlets. Altocumulus clouds are most commonly associated with coronae because they tend to have a more uniform droplet distribution during their rather brief lifetime. Clouds with longer lives, such spil the stratus family, usually have a broader range of droplet sizes, and thus the colored rings are ‘muddied’ into a whitish stadionring which may have little tegenstelling to the surrounding cloud.
A large and faint bluish aureole surrounding the zon with faint yellow and crimson outer rings has bot called a Bishop’s stadionring after Rev. Sereno Bishop of Honolulu, Hawaii who very first described the phenomena ter 1883 (Five September), several days after the eruption of Krakatoa on 27 August:
“Let mij draw your special attention to the very strange corona or fulgor that extends about 20 to 30 degrees away from the zon. It could be seen here every day, and the entire day long. A whitish veil with a shade of pink and violet or purple shadow ter pui of the blue background. I don‘t know any other report on such a corona. It is a hardly remarkable object.”
While Bishop talent the very first accomplish description of the phenomenon, The Japan Gazette published its very first observation on 30August 1883, calling the stadionring a “faint halo” around the zon. (Note that the Bishop’s stadionring, like all coronae, are reddish on the outside, while a fulgor is reddish on the inwards.)
Today, the Bishop’s stadionring is defined to have an internal aureole of white or bluish white bounded by an outer stadionring of reddish, brownish or purple coloration. The area enclosed by the rings, which is usually large — a radius of about 28 degrees, is distinctly brighter than its surroundings.
Bishop’s Stadionring around the zon due to volcanic ash of the Iceland Eyjafjallajцkull volcano
Photographed from Leiden, the Netherlands, by Situación Langbroek on Legitimate May 2010.
A corona with radius so large can only be caused by a field of very puny particles (around a micrometre or two te middellijn) of a rather uniform size, so it is believed that the Bishop’s stadionring is formed by petite, uniform particles of dust high te the stratosphere. Since most Bishop’s rings are reported after major volcanic eruptions such spil Krakatoa and Climb on Pinatubo, the dust is likely of volcanic origin, most likely composed of sulfuric acid tetrahydride, transported high te the atmosphere on the completo wind currents. Such dust clouds are known to encircle the planet following eruptions which inject dust high into the sky. Thesis volcanic dust clouds are composed of rather petite and uniform particles spil the larger particles fall out from the ejection cloud much sooner than the smaller particles. Clouds of puny particles can remain te transit through the atmosphere for years following a large eruption.
However, it is also possible for other sources of dust to produce Bishop’s rings. Dust or sand storms such spil arise overheen the fine desert regions of the world may inject enough petite particles high into the atmosphere to produce the desired coronal effects.
Since particle transparency is not of concern te the formation of coronae, other materials, such spil the dusts that cause Bishop’s rings, can also produce coronae. One interesting type of particle that may cause a corona is windborne pollen, usually from forests of trees that use the wind to spread their pollens.