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Polar SubstormLast week, NASA's Polar satellite spotted a geomagnetic storm triggered by a gust of solar wind. |
Above:
This sequence of images
captured by the Ultraviolet
Imager on NASA's Earth-orbiting Polar
satellite shows an auroral substorm over northern Asia on
February 24. Maximum activity, denoted by dynamic yellow blobs
in the aurora oval, occurs around 1400 UT. Because it records
ultraviolet light, Polar's UVI camera can see aurorae from space
on both the day and night sides of Earth.
Our planet's magnetic field usually does a good job protecting
Earth-dwellers from solar wind storms. Magnetic lines of force,
which look a bit like a squashed bar magnet's, deflect charged
particles from the Sun so that they don't hit our atmosphere
head on. Life as we know it depends on our magnetic shield. Our
neighboring planet, Mars, which has little or no magnetic field,
is thought to have lost much of its former oceans and atmosphere
to space. This loss was caused, at least in part, by the direct
impact of the solar wind on Mars' upper atmosphere. Our other
close planetary neighbor, Venus, has no appreciable magnetic
field, either. Venus is also thought to have lost nearly all
of its water to space, in large part owing to solar wind-powered
ablation.
NASA's Polar satellite was busy monitoring Earth's polar auroral ovals last Wednesday when it spotted an intense geomagnetic substorm raging over northern Asia. The substorm was detected just after NASA's ACE spacecraft measured a sharp increase in solar wind speed (from 630 to 750 km/s) and a flip in the sign of the interplanetary magnetic field from predominantly southward to northward. "A geomagnetic substorm is smaller than a full-fledged storm," explained Spann, a co-investigator on Polar's UVI instrument. "A geomagnetic storm is driven by outside forces from the solar wind when a coronal mass ejection hits the magnetosphere. It typically lasts 24 hours or longer. A substorm is shorter, lasting up to a couple of hours and results from the energy stored in the magnetotail being released and accelerated toward the Earth. The substorm trigger is not fully understood but is strongly coupled to a northward turning interplanetary magnetic field."
![]() Above: Click the image for a 3D simulation of the magnetosphere's shape. The Sun is off screen to the left. The animation begins showing the Earth, which recedes as the shape and size of the magnetosphere comes into view. The solar wind deforms the magnetosphere into its characteristic shape. Where the magnetosphere and the solar wind meet is the "bow shock," represented in the animation by a faint, translucent bullet shape. Credit: Digital Radiance "NASA's IMAGE satellite [slated for launch this month] will enhance our understanding of how the energy is transferred to the magnetotail from the solar wind and then to the aurora," continued Spann. "It does this by viewing regions of space in unique and novel ways. It images the ring current and probes the boundaries of the magnetosphere by sensing changes in plasma densities." Where's the best place to watch for aurora if you're stuck
on the surface of the Earth?
Right: What does an aurora look like? This colorful picture taken in January 1998 shows a spectacular aurora borealis above a frozen landscape of snow-covered spruce trees in Alaska. Auroral light results from solar electrons and protons striking molecules in the Earth's atmosphere. Aurorae rarely reach below 60 kilometers, and can range up to 1000 kilometers. Frequently, when viewed from space, a complete aurora will appear as a circle around one of the Earth's magnetic poles. [Picture credits] |
Web Links |
Polar
UVI home page
- from NASA/Marshall. IMAGE home page - from NASA/GSFC. IMAGE home page - from the Southwest Research Institute. |
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