Spectrometer Measures Auroras' Impact On Ionosphere

November 03, 1998

CHAMPAIGN, Ill. -- A new imaging spectrometer developed at the University of Illinois will assist scientists who are studying auroras and their effects upon Earth's atmosphere. By discerning spectral characteristics of auroral emissions as a function of altitude, the instrument will aid in the verification and refinement of atmospheric chemistry models.

Commonly referred to as "northern lights" or "southern lights," auroras form over Earth's magnetic poles when charged particles in the solar wind bombard atoms in the ionosphere. The excited atoms release their energy in the form of light ? often as shimmering curtains or pulsing filaments. The solar magnetic storms that create these ribbons of light can disrupt power grids in northern Canada and wreak havoc with communications systems that propagate signals through the ionosphere.

"Analysis of the spatial distribution of auroral emissions is important to the study of chemical and dynamical processes occurring in the Earth's ionosphere," said Gary Swenson, the U. of I. professor of electrical and computer engineering who directed the spectrometer project. "This spectrometer tells us which emissions are present, how strong they are, and at what altitude they occur. From this information we can determine the energy of the particles that are penetrating the ionosphere, and compare their observed effects with what our models predict."

The spectrometer works by focusing auroral features onto a grism (a diffraction grating on a prism) and projecting the resulting image onto a CCD (charge-coupled device) array detector. "The spectrometer disperses spectrally in one direction while preserving spatial information in the orthogonal direction," Swenson said.

Unlike standard optical systems that must take data sequentially -- by stepping through filters or by tilting the prism -- the new instrument can simultaneously record all emissions at all observed altitudes. "This is a key advantage when observing auroras," Swenson said, "which are temporally active and can change extremely rapidly."

In recent measurements conducted at Sondrestrom, Greenland, the spectrometer successfully characterized the spatial distribution of auroral features between a height of 80 and 320 kilometers.

"By comparing these detailed altitude distributions of respective emissions with our atmospheric models, we can improve our understanding of the effect that solar magnetic storms have on geophysical processes in the Earth's upper atmosphere," Swenson said.

In addition to Swenson, Richard Rairden of the Lockheed Martin Space Sciences Laboratory, Stanley Solomon of the University of Colorado, and U. of I. graduate student Sharath Ananth assisted in developing the spectrometer and taking the auroral measurements. The researchers described the instrument in the Aug. 20 issue of Applied Optics.

University of Illinois at Urbana-Champaign

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