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Lopsided lights

Auroras in Northern Hemisphere not always a mirror image of those over Antarctica

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2:56pm, July 22, 2009
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The serendipitous observations of two Earth-orbiting satellites, one passing high over the North Pole while the other whizzed over Antarctica, have revealed that Earth’s auroras aren’t symmetrical.

Auroras, commonly called the northern and southern lights, are caused by charged particles from space slamming into gas molecules in the upper atmosphere. These ghostly, flickering phenomena are most commonly seen at high latitudes because the charged particles follow the lines of Earth’s magnetic field, which pierces the atmosphere in polar regions.

Scientists have presumed that the aurora encircling Earth’s northern magnetic pole, the aurora borealis, mirrors that seen in the Southern Hemisphere, the aurora australis, because the charged particles follow magnetic field lines that connect the two hemispheres, says Nikolai Østgaard, a space physicist at the University of Bergen in Norway. But new observations, reported by Østgaard and his colleagues in the July 23 Nature, reveal that the intensity and pattern of the northern and southern auroras can differ substantially on some occasions.

To date, no space mission has simultaneously observed the northern and southern lights. So scientists depend on data from different probes that orbit Earth at various heights and therefore at different speeds, says Østgaard. For the new research, Østgaard and University of Bergen colleague Karl Laundal took data from satellites that are normally out of sync but occasionally are positioned so that both poles can be observed simultaneously. The satellites lined up this way only a dozen or so times between 2000 and 2005, the time period during which both were in orbit.

One of those occasions fell on May 12, 2001, when many northern polar regions were bathed with sunlight for much of the day and Antarctica spent much of the day in darkness. Images show that while areas of Antarctica had a bright, persistent aurora after sunset, the brightest portions of the Northern Hemisphere’s aurora were transient and located over pre-dawn areas. Although Earth’s magnetic field strength is slightly asymmetrical at heights where auroras occur, those differences don’t explain the new observations, says Østgaard.

Auroral asymmetries were seen on other occasions as well, but those from the date described in the Nature paper were the most dramatic.

The auroral asymmetries may be driven by from large numbers of charged particles flowing between Earth’s northern and southern hemispheres along magnetic field lines — a type of electrical current that has been previously proposed but never observed. According to theory, says Østgaard, those currents arise because sunlight-driven reactions in the atmosphere over a fully sunlit hemisphere cause the electrical conductivity there to differ from that in the atmosphere over the darkened hemisphere half a world away.

The new findings are interesting and unusual, says Arthur Richmond, a space physicist at the National Center for Atmospheric Research in Boulder, Colo. Although he notes that the instruments viewing the northern aurora were of a different type from those observing the southern lights, the asymmetries seem to be real, he says. An aurora-observing mission that included several craft with common instruments would help scientists better understand the newly observed asymmetries, he adds.

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