Does the aurora borealis, the grand "northern lights" of the night sky, also shine in daylight when it can't be seen? Researchers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., have a surprising answer to that long-standing scientific question.
"Auroras have been studied for centuries, but always in darkness, when observation was possible," says APL physicist Patrick T. Newell. "It has long been assumed that auroras, like stars, are still present when the sun is up, just hidden."
Not so, according to a report by Newell and APL colleagues Ching-I. Meng and Kevin M. Lyons in the June 27, 1996, issue of the journal Nature. They conclude that the northern lights are present predominantly in darkness, most often between sunset and midnight, and mainly during the spring, winter, and fall months of short days and long nights. This is contrary to the belief that auroral storms usually occur symmetrically in the northern and southern hemispheres.
The APL results were based on scrutiny of more than 150 million measurements made by five U.S. Air Force weather satellites from 1983 to 1992. In order to overcome the problem of observing auroras in daylight, the researchers instead looked at beams of hot electrons from near-Earth space, which can be studied equally well in daylight or darkness. Auroras are thought to originate when those beams of high-speed charged particles from the sun interact with Earth's powerful magnetic field.
The findings also provide strong support for an existing theory that explains the mechanism of aurora formation. One of at least 22 competing theories in the field of auroral research, this explanation contends that auroral intensity correlates to the level of electrical conductivity in the ionosphere.
During sunlit hours, conductivity is high and the electrical circuit between the ionosphere and the energetic particle beams from near-Earth space is stable. But ionospheric conductivity drops at night, particularly on winter nights. This creates instabilities in current flow between the beams and the ionosphere, leading to violent arcing, or electrical discharge -- the aurora -- similar to a lightning flash.
"Our observations strongly support the view that ionospheric conductivity is the key factor controlling the occurrence of discrete auroras," says Newell.
The Applied Physics Laboratory is a not-for-profit laboratory and independent division of The Johns Hopkins University. APL conducts research and development primarily for national security, and for non-defense projects of national and global significance. Located midway between Baltimore and Washington, D.C., in Laurel, Md., APL employs 2,800 permanent staff.
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