Aurora – Northern and Southern Light

Monday, June 11, 2007

Long time ago my professor Nirmal P. Shrestha had written a post about Aurora. I find it so much interesting. I haven't gone through the details in net as of yet but I think by reading what he wrote everyone will get the idea of it pretty much. Here is what he had said and written 10 years back.

Aurora may be described as the luminous display in the sky which lasts for times of few minutes and on occasions even form sunset to sunrise. When it occurs in the northern hemisphere it is known as aurora borealis. Although no two displays are ever exactly alike, a few appear to follow general pattern. The display commonly is in the form of a pale yellow-green color which may be steady or suddenly start moving and fluttering like drapery struck by wind. Many of the forms show enhanced color effects with yellow, green, crimson-red, and violet.

The lower borders of most of the auroras are at about 105 km, and the upper border of the diffuse type are several hundred kilometers above the earth. Regarding the geographic distribution, the region of greatest frequency of auroral occurrence has an average altitude of 67 degrees for the northern hemisphere and perhaps 72 degrees in the southern hemisphere.

A relatively detailed study of the motion of aurora has recently been made possible by the use of “all-sky cameras” (first invented by a German scientist Dr. Willy Stoffregen).

The horizontal thickness of relatively quite aororal arcs as measured at Churchill is 8km. The horizontal length of arc may exceed 2000 km on occasion.

The earth is surrounded by a magnetic field. During storms on the surface of the sun a stream of protons and electrons moves out from the sun at speeds of approximately 1600 to 2400 km/sec. On reaching the earth’s upper atmosphere they collide with the atoms and molecules of oxygen and nitrogen of the atmosphere. Oxygen atoms exited by electrons give out a greenish yellow light while oxygen molecules produce a crimson red glow. Ionized nitrogen and Nitrous oxide molecules also contribute to this phenomenon.

Radio Aurora

An abnormal ionization is associated with aurora (first reported by sir E.V. Appleton and his colleagues). Since 1947 a great variety of techniques have been used to investigate this phenomenon. The term radio aurora is used to the ionization concerned.

Measurements of the height of the aororal ionization carried out by radio techniques have shown that most echoes originate in the height range 100-120 km. Motions of auroral ionization have been deduced from Doppler shift. The velocities involved in the motions are usually in the range 100-600 m/s.

Many attempts have been made to establish a close correspondence between radio aurora, but the results are far from conclusive.

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