Imaging Spectrometer Improves Auroral Understanding

Aaron J. Hand

Researchers at the University of Illinois are taking a fresh look at auroras. They have developed an imaging spectrometer that allows them to study auroral emissions as a function of altitude, offering them more clues about the effects of the solar magnetic storms.
Through experiments done in Greenland in 1994 and 1996, professor of electrical and computer engineering Gary R. Swenson -- along with graduate student Sharath Ananth and researchers at Lockheed Martin Space Sciences Laboratory in Palo Alto, Calif., and the University of Colorado in Boulder -- can better understand how auroras affect the dynamics of the Earth's upper atmosphere.
Auroras form over Earth's magnetic poles when charged particles collide with atoms and molecules in the ionosphere.
"We want to capture spectrum from all altitudes simultaneously," Swenson said. Altitude substantially affects the emissions associated with the aurora. Standard optical systems record data sequentially, making it difficult to get all of the necessary data before an aurora changes.

Altitude and emissions
The Greenland research used two spectrometers to measure the auroral emissions. One was placed at a station in Sondrestrom, where the aurora was directly overhead. This instrument measured no altitude information because it was viewing the aurora directly along the magnetic field line, but it did measure horizontal emission. Another spectrometer -- placed 290 km northwest in Godhavn -- viewed the aurora from a slant path, measuring all altitudes within an auroral arc. To study the aurora, researchers added the data from the two spectrometers with information from radar, an all-sky imager and other optical diagnostics.
The imaging spectrometer has large light-gathering power, Swenson said, focusing the light through fast Nikon lenses to a 1024 X 1024-pixel thermoelectrically cooled charge-coupled device array from Photometrics Ltd. of Tucson, Ariz. The spectrometer includes a diffraction grating on a prism, which sorts the spectral information -- 400 to 900 nm -- horizontally, with altitude displayed along the vertical axis.
One spectrometer remains in Sondrestrom, and the team still measures auroras. The team still hopes to understand the chemistry associated with weak emissions and explain why the oxygen green line has the brightest emission.