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Shedding new light on the aurora

Posted: April 25, 2008

An international team of scientists has detected the unexpected signatures of polarisation in the Earth’s aurora, providing fresh insight into the composition of the Earth’s upper atmosphere, the configuration of its magnetic field and the energies of solar particles.

At the north and south magnetic poles, many charged particles in the solar wind are captured by the Earth’s field and dragged into the atmosphere where they strike atmospheric gases, promoting light emissions. When a beam of light is polarised, its electromagnetic waves share a common orientation, and until now, scientists thought that light from energised atoms and molecules in the upper atmosphere could not be polarised because the collisions between molecules and gas atoms depolarise the emitted light.

Red and green colours predominate in this view of the aurora, as photographed from the Space Shuttle, corresponding to emission of light from oxygen atoms at different wavelengths. Image: NASA.

Claims of polarised auroral light were made around fifty years ago by an Australian researcher Robert Duncan, but based on just one sighting, other scientists were unconvinced. Between 2006 and 2008, Jean Lilensten of the Laboratory of Planetology of Grenoble, France, observed the aurora from Svalbard Island in Norway, which is at a latitude of 79 degrees north. Lilensten and colleagues observed weak polarisation of a red glow that radiates at an altitude of 220 kilometres, indicative of electrons hitting oxygen atoms. The scientists suggest that such light is polarised because Earth's magnetic field at high latitudes funnels the electrons, aligning the angles at which they penetrate the atmosphere.

Fluctuations in the polarisation measurements can reveal the energy of the particles coming from the Sun when they enter Earth's atmosphere and the intensity of the polarisation gives clues to the composition of the upper atmosphere, particularly with regard to atomic oxygen. Because polarisation is strongest when the telescope points perpendicularly to the magnetic field lines, the measurements also provide a way to determine magnetic field configurations, which could prove especially useful as astronomers train their telescopes on other planetary atmospheres. If polarised emissions are observed there as well, the measurements may enable scientists to understand how the Sun's magnetic field is distorted by Venus and Mars, which lack intrinsic magnetic fields, opening up a new field in planetology.








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