Surprising gap in auroral oval puzzles scientists
Updated: June 18th, 2018
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"Right here, at local midnight, you have this gap where things should be happening," explained Dr. James Spann of NASA's Marshall Space Flight Center. "Something special may be happening back in the magnetosphere. The truth is, we don't know."
The gap is described this week in "A new auroral feature: The nightside gap," the cover story of the Oct. 15 issue of Geophysical Research Letters. The lead author is Damien Chua, a graduate student at the University of Washington, who discovered the gap using the Ultraviolet Imager aboard the Polar spacecraft.
Right: A composite of UVI views of the aurora borealis - seen from space - is on the cover of the Oct. 15 Geophysical Research Letters. It shows a small notch in the auroral oval above the tip of Greenland, which then was at local midnight.Credit: Damien Chua, University of Washington, and the American Geophysical Union.
"This gap has been alluded to in past articles, but it's not been fully described," explained Spann, a UVI co-investigator and a co-author on the paper.
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The UVI is a camera equipped with a unique series of filters that allow it to view the Earth's aurora borealis - the Northern Lights - even during daylight. It is aboard the Polar spacecraft which, as its name implies, orbits over the North Pole so scientists can investigate conditions where the Earth's magnetic field lines are vertical and expose the upper atmosphere directly to space.
The aurora itself is one of the effects caused by that exposure as electrons zip back and forth along the magnetic field lines and - if energized high enough - slam into the atmosphere to create a light show.
Spann says that scientists believe the aurora reflects what happens in the magnetosphere, an immense region of charged particles trapped by Earth's magnetic field. The solar wind squeezes the magnetosphere close to the Earth on the dayside, and drags it out to a million or more kilometers on the night side. The wind also helps energize the magnetotail so it sends stored particles zipping back to Earth.
Usually the auroral arc is strongest around the night side during geomagnetic substorms. The surprise that Chua found is that about 7 percent of the time a small gap appears between 10 p.m. and midnight, local time. A total of 22 gaps were found in auroral ovals between December 1966 and February 1997.
"Typically they occur 10 to 20 minutes before the onset of the storm, but often they occur right after onset," Spann explained. The gap may appear as a full break in the auroral arc, or it may be just a notch as if whatever effect causes it is not quite strong enough to complete the break.
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Click on the image to view a larger picture.
The image, above, was made with NASA's Ultraviolet Imager, a polar spacecraft whose main job is to take ultraviolet pictures of the aurora. By taking the pictures from space, rather than from Earth, glare from the bright Sun is not a problem and the aurora can be seen both on the night and day side of the Earth. For more information on aurora and the interpretation of images like the one above, please visit UVI Aurora Headquarters at the NASA Marshall Space Sciences Lab.
For now, the cause is a mystery.
"There does not appear to be much correlation with solar wind pressure," Spann continued. "It does appear to have some correlation with the plane of the interplanetary magnetic field [IMF] along the plane of the Earth's orbit, but we're not ready to conclude that."
"Changes in the IMF orientation may be giving rise to perturbations in the currents aligned with the earth's magnetic field that are directly tied to auroral activity," Chua commented. "This effect has the most observational evidence so far.
"We think that this feature may shed some light on how the auroral ionosphere is coupled to the magnetosphere, but we're still trying to make heads or tails out of what this all means."
Abstract: The nightside gap is an unusual feature of the auroral oval marked by roughly a factor of four decrease in intensity near local midnight during some substorms. This feature has not been previously described in detail. The nightside gap appears in roughly 7% of the substorms observed by UVI during our study period from December, 1996 to February, 1997. The cause of this feature is speculative at this time. However, it is postulated that it is due to a significant reduction in field-aligned currents along with an insufficient ionospheric potential to accelerate precipitating electrons within the gap region. The full paper is on line at the University of Washington.
"Perhaps that acceleration region is perturbed in some fashion," Spann said. "Perhaps it's depleted in particles or energy.Or, the answer may lie in a region just 5,000 km high where particles get their strongest acceleration towards the Earth.
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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack