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Optical Effects
by C. A. Linder

Aurora Borealis
The aurora borealis, or northern lights, is a shimmering curtain of light which can be seen in the clear night sky in the Arctic when solar energy is strong. Auroras are produced by the interaction of the solar wind (a stream of protons and electrons from the sun) with oxygen and nitrogen atoms in the earth's atmosphere. Auroral displays are centered in a ring around the geomagnetic pole, but can be seen many thousands of miles away. The fantastic light shows range in altitude from 40 to 120 miles above the earth's surface. The color ranges from high-altitude red displays to the more common, low-altitude yellow-green displays.

Halos
Halos are bright arcs or spots of light which form due to the reflection and refraction of light passing through ice crystals. Ice crystals are commonly found in cirrus (high, thin, wispy) clouds and in ice fog. Halos can be seen anywhere in the world, but are extremely common in the polar regions due to the cold air (ice crystals are much more common). The type of halo encountered is mainly dependent on the orientation of the ice crystals in the atmosphere.

The 22 Halo
This is the most common type of halo. It appears as a thin whitish circle centered around the sun or moon. If you draw a line from your eye to the edge of the arc, the angle between this line and a line drawn straight into the sun would be 22. The ice crystals for this kind of halo are oriented randomly in the air.

Sun Dogs
Sun dog is another name for a halo called parhelia. Parhelia are bright spots seen in cirrus clouds at the same altitude as the sun. Although similar to the 22° halo, the spots are considerably brighter. The ice crystals that cause parhelia are oriented horizontally.

aqua berg
Arctic Ocean fogbow.
photo © C. A. Linder, WHOI
Click to enlarge.
Fogbow
A fogbow is a rare form of rainbow. Rainbows are formed by the refraction of light through water droplets. The larger the water droplets, the more the light is "bent" and the brighter the colors of the rainbow. If the droplets are very small, such as those in fog, interference causes the colors to blend into a pure white -- the fogbow.

Anticorona (Glory)
The anticorona, also known as a "glory" occurs at the point opposite a source of light. It is made up of one or more colored rings that appear around the shadow cast by an observer on a cloud or in fog. An image from the 2002 cruise here is an example of a glory. Glories can also be observed from airplanes. Researcher Dan Torres observed glory rings around the shadow of the Coast Guard helicopter he was flying in as they passed over a fog bank on the way to the ship in 2002.

Mirages
A mirage is a false image formed by the refraction, or bending, of light. There are several types of mirages. The most common type of mirage is an inferior mirage. These can be seen on any warm surface, such as a highway. The shimmering pools of "water" are actually blue skylight whose rays have been refracted upward by the layer of heated air above the asphalt. It is called an inferior mirage because the flase image occurs below the real image. Another type of mirage is a superior mirage. This image occurs above the real object. In contrast to inferior mirages, superior mirages occur when warm air overlies cold air.

Fata morgana mirage observed on the 2002 cruise.
photo © C. A. Linder, WHOI

Fata Morgana
The fata morgana is a superior mirage formed when a layer of warm air overlies a cold surface (known as an inversion). Inversions form frequently in the Arctic in summer when warm air travels (is advected) over the cold water and ice. The light is bent in such a way that distant ice appears to rise vertically out of the horizon. The term "fata morgana" comes from the Italian for "fairy Morgan," the half-sister of King Arthur. Legend has it that Morgan lived in a crystal palace underwater, and could use magical powers to build castles out of thin air.


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