Northern lights glimmer with unexpected trait

Northern lights glimmer with unexpected trait

Top image: Aurora lights up the polar sky above the Kjell Henriksen Observatory, where the study of the polarization of the aurora was carried out last winter. Photo: Fred Sigernes/UNIS.

UNIS aurora scientists Dag A. Lorentzen and Fred Sigernes have, together with an international team of scientists, detected that some of the glow of Earth’s aurora is polarized, an unexpected state for such emissions.

25 April 2008
Press release from AGU/UNIS

Measurements of this newfound polarization in the Northern Lights may provide scientists with fresh insights into the composition of Earth’s upper atmosphere, the configuration of its magnetic field, and the energies of particles from the Sun, the researchers say.

If observed on other planets, the phenomenon might also give clues to the shape of the Sun’s magnetic field as it curls around other bodies in the solar system.

Polarized light is distinguished by the regular orientation of its electric and magnetic fields. Until now, scientists thought that light from energized atoms and molecules in planetary upper atmospheres could not be polarized. The reason is simple: in spite of the low number of particles at the altitudes concerned (above 100 kilometers), there are still numerous collisions between molecules and gas atoms. Those collisions depolarize the emitted light.

Discarded observation now confirmed
Fifty years ago, an Australian researcher, Robert Duncan, claimed to observe what looked like polarization of auroral light, but other scientists found that single observation unconvincing.

To revisit the question, Jean Lilensten, of the Laboratory of Planetology of Grenoble, France and his colleagues from France, the University of Oslo and UNIS, used a custom-made telescope to study auroral light from the UNIS aurora station and the newly opened Kjell Henriksen Observatory during the winters of 2006-2007 and 2007-2008.

At the north and south magnetic poles, charged particles in the solar wind are captured by the planet’s field and forced to plunge into the atmosphere. The particles strike atmospheric gases, causing light emissions. The research group observed weak polarization of a red glow that radiates at an altitude of 220 kilometers—the result of electrons hitting oxygen atoms. The scientists had suspected that such light might be polarized because Earth’s magnetic field at high latitudes funnels the electrons, aligning the angles at which they penetrate the atmosphere.

The finding of auroral polarization “opens a new field in planetology,” says Lilensten, who is the lead author of the study. He and his colleagues reported their results on 19 April in Geophysical Research Letters, a publication of the American Geophysical Union, or AGU.

Fluctuations of the polarization measurements can reveal the energy of the particles coming from the Sun when they enter Earth’s atmosphere, Lilensten notes. The intensity of the polarization gives clues to the composition of the upper atmosphere, particularly with regard to atomic oxygen.

Opens up a new field in aurora research
– This result has great significance, because the polarization has never before been regarded as important when measuring the aurora, says co-author and UNIS associate professor in upper polar atmosphere Dag A. Lorentzen.

– This finding gives us an extra parameter to measure the aurora; in addition to study the wave length and intensity, we can now also measure the polarization. This will be an important aspect in the future when we measure the aurora at other planets in the solar system as it will provide us with more information about the atmosphere than we previously could obtain, Lorentzen says.

Because polarization is strongest when the telescope points perpendicularly to the magnetic field lines, the measurements also provide a way to determine magnetic field configurations, Lilensten adds. That could prove especially useful as astronomers train their telescopes on other planetary atmospheres.

If polarized emissions are observed there as well, the measurements may enable scientists to understand how the Sun’s magnetic field is distorted by obstacles such as the planets Venus and Mars, which lack intrinsic magnetic fields.

The research group will continue its studies of the aurora from the Kjell Henriksen Observatory just outside Longyearbyen. – This result is a great achievement for the Kjell Henriksen Observatory and we are looking forward to continue studying the polarized emissions in the aurora next season, UNIS professor Fred Sigernes says.

More information:
Lilensten, J., J. Moen, M. Barthélemy, R. Thissen, C. Simon, D. A. Lorentzen, O. Dutuit, P. O. Amblard, and F. Sigernes (2008): Polarization in aurorae: A new dimension for space environments studies, Geophys. Res. Lett., 35, L08804, doi:10.1029/2007GL033006.

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