Polar
Substorm
Last week, NASA's Polar satellite
spotted a geomagnetic storm triggered by a gust of
solar wind.
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."
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.
Magnetic field, magnetic shield...
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 magnetic, 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.
Fortunately for aurora lovers, the
magnetosphere (an area of space controlled by Earth's
magnetic field) is not invincible. If the solar wind
is strong enough or if the magnetic field inside the
wind partially cancels the magnetic field of the
Earth, some plasma (ionized gas) can get through.
Strong bursts of solar wind can also squeeze our
magnetosphere, compressing it until it bounces back
like a vibrating rubber ball. These events can trigger
beautiful aurorae that are most often visible at high
latitudes (above 60 degrees magnetic latitude).
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."
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.
For
more information about space weather and current solar
activity please visit NOAA's Space
Environment Center and SpaceWeather.com.
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]
Related Links:
Polar
UVI home page - from NASA/Marshall.
Advanced
Composition Explorer - a NASA spacecraft that
monitors the solar wind.
IMAGE
home page - from NASA/GSFC.
IMAGE
home page - from the Southwest Research
Institute.
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