How do gusts in solar wind stir the aurora?
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Right: This view of the Aurora Australis (Southern Lights), seen from the Space Shuttle on the STS-39 mission in may 1991, shows a spiked band of red air glow called a "Red Crown" above the Earth's limb. Credit: NASA/Johnson Space Center NASA Photo ID: STS039-23-036
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"This is a well-known phenomenon," explained James Spann, a space plasma physicist at NASA's Marshall Space Flight Center. "However, we're now applying global imaging in the first broad survey to understand what is happening."
Left: A strong auroral storm from August 27, 1998, as seen by the Ultraviolet Imager on board the Polar spacecraft. The view is over the north pole; Greenland is in the lower right quadrant in this image. Click on the image for a.
The magnetosphere, populated with ionized gases and electrons, is like an invisible shield around the Earth. The Earth's magnetic field forces the solar wind to part and slide around it. But at the same time, a gust in the solar wind can squeeze the magnetosphere, forcing some of the magnetosphere's particles earthward along the magnetic field lines. Particles energized enough to burrow as deep as the upper atmosphere produces the dazzling aurora borealis and magnetic storms.
Above: Three images from the Ultraviolet Imager aboard the Polar spacecraft - taken Oct. 23, 1997 - show the effects of a weak puff of solar wind, just 0.1 nanoPascal, sweeping over the magnetosphere. Universal time is used, so local midnight is over western Canada, to the right of each frame. Credit: NASA/Marshall and UVI Imaging Team
Right: The Aurora Australis (Southern Lights), seen from the Space Shuttle on the STS-45 mission in April 1992. The STS-45 crew noted the interesting spiraling or corkscrew appearance of this particular sighting.Credit: NASA/Johnson Space Center NASA Photo ID: STS045-32-014
Spann is a co-investigator on the Ultraviolet Imager (UVI) aboard Polar. UVI's pictures provide a direct measure of activities back in the million-kilometer-long tail of the magnetosphere. In effect, the aurora acts as a mirror that reflects activities in the tail.
"I'm surveying two years' worth of data, covering 1997-98, from Wind, ACE, and Polar," Spann explained. In that period are more than 30 events where Polar was in the right position high above the aurora while wind data were returned by Wind and ACE.
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But when multiplied by the cross section of space carved out by the Earth plus its magnetosphere, it becomes a significant bundle of power that can have global effects. A pulse of 15 nanopascals is rare. Even the powerful Jan. 10, 1999, coronal mass ejection increased
Left: Because the UVI filters out everything but a few narrow bands of light, the Earth is invisible in UVI images (maps have to be added, as in the panel earlier in the story). Geophysicists prefer to adjust the UVI images so local noon - 0 in the image - is down and midnight - 12 - is up. The center of the plot is the magnetic - not geographic - north pole since the magnetic field is the major player. Links to. Credit: NASA/Marshall and UVI Imaging Team. A is also available.
"Under certain conditions, the magnetosphere will contract," Spann said, "and that initiates some brightening on the dayside of the aurora. As the pulse travels down the flank of the magnetosphere, we see the brightening traveling around the auroral arc."
It appears that when the pressure wave hits bow of the magnetosphere, the aurora brightens at local noon. As the pressure wave travels down the magnetosphere, the aurora brightens on each side, from noon to both dusk and dawn when the wave is straddling the poles, and then both close to local midnight as the wave rolls into deep space. The response of the magnetosphere with a magnetic storm may take a few seconds or a few hours, depending on how much energy is stored in the magnetosphere.
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"We're looking at the magnetic field and delays between encounter and the onset of geomagnetic storms to see what conditions must be present to trigger a storm right away or in a few hours."
How often the solar wind gusts depends on the sun. Active regions that increase the solar wind rotate with the sun every 27 days or so. But the sunspot cycle is on the increase, so there will be more active regions over the next few years.
Results from Spann's study could help in determining, before the wind blows, whether conditions are ripe for a storm that could disrupt communications and power supplies.
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