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SUBSIDING
STORM: The intense geomagnetic storm
of Oct. 24-25 (described below) has subsided and
US skies are returning to normal. If you missed
the show, don't worry. The Northern Lights will
be back. For much of the past few years, the sun
has been in a quiet state; but solar activity is
cyclical and the sun appears to be waking up again.
Forecasters expect new Solar Cycle 24 to peak in
2012-2013 with many more chances to see auroras
in unfamiliar places. Aurora
alerts: text,
voice.
AURORAS
IN THE USA: A coronal mass ejection
(CME) hit Earth on Oct. 24th at approximately 1800
UT (2:00 pm EDT). The impact strongly compressed
Earth's magnetic field, directly exposing geosynchronous
satellites to solar wind plasma, and sparked an
intense geomagnetic storm. As night fell over North
America, auroras spilled across the Canadian border
into the contiguous United States. A US Department
of Defense satellite photographed the crossing:
"This shows the auroras on Oct.
25th at 0140 GMT," says Paul McCrone of the
Fleet Numerical Meteorology and Oceanography Center
in Monterey, California. He created the image using
visual and infrared data from the Defense Meteorological
Satellite Program's F18 polar orbiter. DMSP
satellites carry low light cameras for nightime
monitoring of moonlit clouds, city lights and auroras.
Some of the auroras recorded by the F18 on Oct.
25th were as bright as the city lights underneath.
This "big picture" from
orbit makes sense of what happened next. The bright
band swept south and, before the night was over,
auroras were sighted in more than thirty US states:
Alabama,
Wisconsin,
New
Mexico, Tennessee,
Missouri,
Illinois,
Nebraska,
Kentucky,
North
Carolina, Indiana,
Oklahoma,
Kansas,
Iowa,
Maryland,
New
York, Montana,
Ohio,
Colorado,
Pennsylvania,
Washington,
Virginia,
Texas,
Arizona,
Minnesota,
Maine,
Michigan,
Montana,
Oregon,
Arkansas
and California.
Many observers, especially in the
deep south, commented on the pure red color of the
lights they saw. These rare all-red
auroras sometimes appear at low latitudes during
intense geomagnetic storms. They occur some 300
to 500 km above Earth's surface and are not yet
fully understood.
UPDATED:
October
2011 Aurora Gallery
[previous Octobers: 2010,
2009,
2008,
2007,
2006,
2004,
2003,
2002]
THE
INSTIGATING EXPLOSION: The CME
that hit Earth's magnetic field on Oct. 24th left
the sun almost two days earlier. It was propelled
in our direction by an unstable magnetic filament,
which erupted around 0100 UT on Oct. 22nd. This
movie from the Solar and Heliospheric Observatory
shows the cloud expanding toward Earth in the first
hours after the explosion:
Traveling faster than two million
mph, the cloud took about 41 hours to cross the
sun-Earth divide. The CME was so geoeffective because
it contained a knot of south-pointing magnetic fields.
These fields partially cancelled Earth's north-pointing
magnetic field at the equator, allowing solar wind
plasma to penetrate deeply into Earth's magnetosphere.
The rest, as they say in
Alabama, is history.
