Explaining Sporadic Layers Of Sodium In The Upper Atmosphere

July 06, 1998

WASHINGTON, D.C. -- Researchers at SRI International in Menlo Park, California, and the University of Alaska's Geophysical Institute in Fairbanks have provided new scientific backing to a theory explaining the presence of thin sporadic layers of sodium in the Earth's upper atmosphere. These sporadic layers are in addition to the better understood layer of neutral sodium atoms found between 80 and 100 kilometers altitude, believed to be a by-product of meteors vaporizing on entering the atmosphere.

Using the National Science Foundation's incoherent scatter radar and sodium lidar near Kangerlussuaq, Greenland, Craig J. Heinselman and Dr. Jeffrey P. Thayer of SRI and Dr. Brenton J. Watkins of the University of Alaska have demonstrated an instance of a thin ion layer being pushed downward to a region where chemical catalysts recombined the ionized sodium in that layer. This left a thin layer of neutral sodium behind.

Contrary to the predictions of competing theories, the formation of that neutral sodium layer was not significantly affected by the presence of intense auroral ionization. The findings will be reported in a forthcoming issue of Geophysical Research Letters, published by the American Geophysical Union.

The existence of sporadic sodium in the upper atmosphere has been observed since the late 1970s, and various theories had been advanced to explain its presence and its apparent correlation with layers of metallic ions, called sporadic E. Astronomers use the background sodium layer to create an artificial guide star for the adaptive correction of telescope optics for atmospheric effects.

The inaccessibility of this high altitude region had made it difficult to test these theories, and it is possible that different mechanisms create the sporadic sodium layers at different times and places, the researchers say. The recent addition of a resonant sodium capability to the Arctic Lidar Technology System at the Sondrestrom facility near Kangerlussuaq made the new measurements possible.

"What we are trying to do," says Heinselman, "is understand the physics and chemistry of this region of the atmosphere. Such an understanding could well lead to significant answers with regard to the coupling and transport of gasses to other regions. For example, the rate at which sodium ions are converted to sodium atoms is thought to be governed by the ratio of carbon dioxide to atomic oxygen. If this is in fact the case and if we can measure that conversion rate, we may be able to infer the concentration of carbon dioxide near 90 kilometers altitude. Such information could then be fed back into global models involving this important greenhouse gas."

The research was supported by a cooperative agreement and grants from NSF.
Editors: A copy of the Heinselman, et. al., paper may be obtained from Harvey Leifert at AGU. It has not yet been scheduled for a specific issue of GRL, but is NOT under embargo.

For further information about this study and its conclusions, you may contact Mr. Heinselman at heinselman@sri.com.

American Geophysical Union

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