March 2, 2000 -- Last week an
interplanetary wind storm hit our planet. For over two days,
a gale of energetic particles from the Sun blew past the Earth
at speeds exceeding 500 times that of a speeding bullet. The
source of all this activity was a large coronal hole stretched
across the face of the Sun. The hole has since departed and the
powerful interplanetary breeze of magnetized gas has subsided.
Although the storm has subsided, scientific data about the event are still pouring in.
"While the solar wind velocity was high last week, a strong gust triggered some interesting geomagnetic activity," said Dr. Jim Spann of the NASA/Marshall Space Flight Center. "I found it -- a geomagnetic substorm over Asia -- in the ultraviolet imaging data from the Polar satellite, which monitors aurorae from space. Without Polar we might not have noticed."
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
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."
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The magnetotail is a region of space behind the night side of Earth where the solar wind stretches our planet's magnetic field out into a long tail. Although it's over 60,000 km away, what happens there is crucial to auroral substorm activity. The magnetotail contains a region called the plasma sheet, which is filled with dense, charged gas. When an energetic burst of solar particles hits the magnetosphere on the dayside, it compresses the plasma sheet on the nightside. This forces neighboring magnetic field lines with opposite polarities to connect inside the plasma sheet. Intense electric fields are suddenly created by the magnetic reconnection and highly energized plasma shoots toward the aurora ovals over the Earth's north and south poles.
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?
"I must admit that I have never seen the aurora with my own eyes," says Spann, who spends lots of time poring over images of aurorae from space. "However, I understand that Alaska and across Canada above the lower extent of the Hudson Bay are good places to watch. The northern Scandinavian countries provide good views also, along with a lot of good coffee!"
Aurora-watching could become a popular past time for residents of the North in the coming months. The solar maximum in mid-2000 will bring with it lots of coronal holes, solar flares, and other events to trigger magnetic disturbances. While these geomagnetic storms are a headache for satellite operators and power technicians, they will frequently stage colorful displays of Northern lights for nature lovers.
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
IMAGE home page - from NASA/GSFC.
IMAGE home page - from the Southwest Research Institute.