The northern lights, or Aurora Borealis, occur when high-energy particles emitted by the sun get captured by the Earth's magnetic field, funneled downwards, and hit atoms high in the Earth's atmosphere. Up until recently, it was believed that the light emitted by the excited atoms was unpolarized. Even high up, any polarization would be removed as the light collides with the sparse gasses present high above the 60 mile altitude mark.
However, new work has shown that this is not the case. The recent work, published in the April 19th edition of Geophysical Research Letters, is not the first to suggest that the Northern lights are polarized. Fifty years ago, Australian researcher Robert Duncan reported observations he made that suggested that polarized auroral light existed; other scientists of the time were unconvinced by this single data point.
The new observations were carried out by Jean Lilensten of the Laboratory of Planetology of Grenoble, France, and his colleagues. Their observations were carried out using a custom telescope on Svalbard Island, Norway at a co-latitude of 11o (79 oN). At this location, they observed a weak polarization of a red glow 140 miles above them. The red is the result of electrons hitting oxygen atoms high in the atmosphere, and the polarization is hypothesized to be caused by the angle at which the electrons are funneled into the atmosphere by the magnetic field.
Beside the observation of polarized light, this discovery creates a new tool for researchers to study the dynamics of the upper atmosphere. The degree of polarization should be related to the concentration of molecular oxygen in the upper atmosphere. Measurements of this type from the atmospheres of other planets may also reveal details about the solar magnetic field. If polarized light is observed at Venus or Mercury, then this will tell planetary and solar researchers some details about the Sun's magnetic field at those points in space.
Geophysical Research Letters, 2008. DOI: 10.1029/2007GL033006