Watching the Aurora from Above
THE AURORA---Figure A shows an original image of ultraviolet light from Earth's sunlit hemisphere and an active aurora. Figure B shows the original image with light near the outer edge removed, while Figure C depicts a model image for periods when there is very little auroral activity. Figure D shows the percent change in ultraviolet light caused when auroral energy heats gases in the upper atmosphere, thereby altering the thermosphere's composition. It was created by reprocessing the image in Figure B, then subtracting and dividing by the model image in Figure C.
Dynamic auroras do more than paint brilliant displays, they can change the part of Earth's atmosphere where satellites orbit and space shuttles fly.
Auroras change the upper atmosphere mostly by heating it, according to Research Assistant Tom Immel, a graduate student working toward his doctoral degree in physics.
An active aurora can generate heat by freeing electrons that enhance already strong electrical currents flowing in the thermosphere, the atmospheric shell extending from about 50 miles above Earth to outer space.
"These currents heat the thermosphere like electrical currents heat a light bulb," Immel said.
Immel and institute Professor of Physics John Craven recently produced the first two-dimensional images of large-scale changes caused by the aurora in the thermosphere.
Researchers can correlate the colorful 2-D images with theoretical models designed to study the effects of heating in the upper atmosphere. They also can use the images to help determine how such heating might affect the paths of space stations, the flight of the space shuttle, or the orbits of low-altitude satellites in the upper thermosphere.
Craven and Immel created the 2-D images using data from the Dynamics Explorer-1 satellite, which orbits up to 14,500 miles above Earth.
The images from the high-altitude satellite allow researchers to get a broader view of the thermosphere than is available through more traditional measurements taken from the ground or from low-flying satellites.
Craven compared data from the Dynamics Explorer-1 satellite during intense auroral activity to images from a model he constructed of how the thermosphere looks when the aurora is quiet.
By electronically subtracting the model images from the satellite images, the researchers came up with a 2-D format that illustrates where energy from the aurora heats up gases in the thermosphere to change its composition.
The Geophysical Institute's popular weekly predictions of auroral activity over Alaska have had a high accuracy rate of approximately 75 percent since they were initiated in March, according to Professor Emeritus Charles Deehr.
Deehr, a space physicist who began studying the aurora borealis at the Geophysical Institute in 1958, calculated the accuracy of the predictions recently by comparing actual auroral displays with his weekly predictions.
Except for a half-month period in November, Deehr found that the aurora occurred within the range of predictions issued by the Geophysical Institute.
Aurora watchers have accessed the Geophysical Institute's Aurora Prediction Internet page up to 2,000 times per week, and the predictions are distributed weekly to all major media outlets and visitor centers in Alaska and Canada.
To predict the activity level and the viewing range of the auroral displays, Deehr studies data on worldwide disturbances in the earth's magnetic field. The earth's magnetic activity is a direct reflection of auroral activity.
He compares the earth's magnetic profile to active regions on the sun, which face the earth every 27 days as the sun rotates on its axis. These active regions create an electrically charged wind that blows through the solar system and reaches the earth in rings around the north and south geomagnetic poles, creating the aurora.
Aurora predictions, the innovative idea of Director Syun Akasofu, are possible this year because the active regions of the sun are at the low end of an 11-year cycle, when solar activity is relatively stable. Erratic bursts of solar flares, common at the high end of the cycle, make prediction difficult.