A small gap in the aurora borealis has scientists wondering what's really happening deeper in space.
"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.
"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.
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.
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."
Or, the answer may lie in a region just 5,000 km high where particles get their strongest acceleration towards the Earth.
"Perhaps that acceleration region is perturbed in some fashion," Spann said. "Perhaps it's depleted in particles or energy.