A Coronal Hole Targets Earth
23rd Feb 2000
speed solar wind particles from a large coronal hole are buffeting Earth's
The end of February could mark another good weekend for aurora watchers,
thanks to a large coronal hole currently straddling the visible disk
of the sun. High speed solar wind particles streaming out of the hole
reached the Earth today and began to buffet our planet's magnetosphere.
Space weather forecasters expect moderate to high levels of aurora borealis
with minor impacts on satellite operations and high latitude power grids
for the next few days. Right: This x-ray image of the sun, captured
on Feb 21, 2000, by the Japanese Yohkoh X-ray Observatory
shows the coronal hole that has rotated into a favorable position to
send high-speed solar wind particles toward Earth. The resulting gusts
of solar wind is expected to strike Earth's magnetic field and trigger
moderate geomagnetic disturbances over the next few days. Coronal holes
are easy to spot by looking at the Sun through an x-ray telescope. They
appear as very dark areas that contrast with bright spots overlying
sunspot groups. Hot gas around sunspots is captured by magnetic fields
Fields) that rise up out of one sunspot and bend back to reconnect
at another spot nearby. These glowing 'magnetic bottles' shine brightly
at x-ray wavelengths. The magnetic fields around coronal holes are different.
Instead of looping back to reconnect on the sun's surface, these magnetic
fields are essentially open. They extend far out into the solar system
and no one knows exactly where they reconnect. Rather than trapping
the hot gas, the open field lines of a coronal hole allow high-speed
solar wind particles to escape.
Above: These data from NASA's Advanced Composition Explorer
(ACE) spacecraft show the speed of the solar wind measured 1.5 million
km from Earth at the L1 Sun-Earth libration point. The wind speed began
to increase around 0900 UT on February 23, 2000, as the solar particles
from a coronal hole reached ACE. Solar wind gusts measured by ACE reach
Earth's magnetosphere about an hour later. The solar wind flows away
of the Sun in all directions, not just from coronal holes, but the wind
speed is high (up to 800 km/s) over coronal holes and much lower (300
to 400 km/s) elsewhere. The higher pressure streams from coronal holes
squeeze the Earth's magnetic field and can produce geomagnetic activity.
The coronal hole visible on the sun now is at least 7 months old. It
has been seen by the Yohkoh soft x-ray telescope during each of the
past seven solar rotations. (The sun rotates on its axis once every
27 days.) After this apparition, there is every reason to expect its
return 27 days hence for another bout of solar wind gusts and geomagnetic
activity. It's not yet clear how much activity will result from this
week's coronal hole. The Geophysical Institute
at the University of Alaska predicts: "Auroral activity will be high
today (February 23). Weather permitting, highly active auroral displays
will be visible overhead from Barrow to Anchorage, and visible low on
the horizon from Bethel, Soldotna and southeast Alaska." Observers in
Alaska, Canada, and the upper tier of U.S. states should be on the lookout
for aurora in the coming days. The best time to look is just before
local midnight, before the waning gibbous moon rises. If auroral activity
increases, with displays visible at middle-latitudes, notices will be
posted at SpaceWeather.com.
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