Dynamics of auroras at north and south poles studied simultaneously for first time

CONTACT: B.J. Almond
PHONE:
(713) 348-6770
EMAIL: balmond@rice.edu

DYNAMICS OF
AURORAS AT NORTH AND SOUTH POLES
STUDIED SIMULTANEOUSLY FOR FIRST TIME

Rice University professor to present findings at American Geophysical
Union meeting Dec. 13

For the first time, the
entire sequence of an auroral storm has been studied simultaneously over both
hemispheres, yielding information that might help scientists issue warnings
about potentially harmful storms.

The study is reported in
a paper that will be presented Thursday at the American Geophysical Union
meeting in San Francisco by Patricia Reiff, director of the Rice Space Institute
at Rice University in Houston. She will interpret images of aurora borealis
(northern lights), the aurora around the north pole, and aurora australis
(southern lights), the corresponding aurora around the south pole, taken Aug. 17
this year by the IMAGE and Polar NASA satellites, respectively.

“The question we’ve been
wanting to answer is whether the southern and northern auroras are conjugate or
anticonjugate,” said Reiff, a professor of physics and astronomy at Rice.

Previous studies have
indicated that auroras inside the polar caps are anticonjugate, or opposites; a
high-latitude aurora observed on the dawn side of the northern hemisphere would
be accompanied by a “mirror image” on the dusk side of the southern hemisphere.
Lower- latitude auroras in the main auroral oval have been known to be nearly
conjugate, or appearing in nearly the same local time and latitude in both
hemispheres.

But images of the
auroras taken during a geomagnetic substorm Aug. 17 were nearly conjugate; the
highest-latitude auroras appeared on the dusk side of both polar
caps.

Auroras occur when
electrons and protons that are trapped in Earth’s magnetic field hit the gases
of Earth’s upper atmosphere. Those electrical particles can move only along the
invisible magnetic field lines that are connected to Earth near the north and
south poles. When the magnetic field is energized by a storm, the particles
travel to both ends of the field lines, creating bright auroral displays of
streamers or arches of light in rings measuring 2,500 miles in diameter around
each pole.

Reiff said the auroras
observed Aug. 17 were actually anticonjugate early in the storm when the polar
cap was connected to the solar wind. The very large dawn-dusk component of the
interplanetary magnetic field pulled the open field lines to the dusk in the
southern hemisphere and toward the dawn in the northern hemisphere. But once the
storm got started and the electrical particles were blasted by Earth’s magnetic
tail, the auroras were conjugate, as evidenced by both the northern and southern
auroral displays being brightest at the same magnetic locations at the same
time. The aurora is nearly symmetric when mapped in magnetic coordinates, not in
geographic coordinates. Reiff was able to observe these changes as they occurred
because the images taken by both satellites were produced in a real-time mode
and available to the public through the Space Environment Center.

This finding could be
particularly useful when scientists need to issue warnings in real time about
potentially dangerous auroras, Reiff said. If the main auroral oval is found
over populated areas, it has the potential to trip electrical circuits and
destroy transformers. The poleward edge of the aurora also marks the “danger
zone.” Astronauts or high-altitude pilots who fly poleward of the aurora can be
in danger if there is a solar energetic-particle event in progress.

“If we can trust that
the aurora in the northern hemisphere is the same as in the southern hemisphere,
we can use imagers over either pole as our monitors,” Reiff said. “This will
enable us to predict where the northern aurora will be located based on its
appearance in the south, or vice versa.”
She added, however, that although
the large-scale aurora is nearly conjugate, including undulations at the
poleward edge, the hemispheres have significant differences. “There’s still a
lot more to be learned,” she said. “Some of the details may be caused by
near-Earth instabilities that can be different in the two hemispheres. This
study, like others previously, was serendipitous, based on a happy accident of
orbit and timing. A pair of spacecraft designed to monitor both poles
simultaneously would be ideal to discover how much of the structure is locally
determined.”

Reiff is lead author of
the paper, which she co-wrote with Harald Frey and Steve Mende at the University
of California, Berkeley, and John Sigwarth and Louis Frank at the University of
Iowa, Iowa City. Others who participated in the study were Terry Onsager of the
Space Environment Center at National Oceanic and Atmospheric Administration in
Boulder, Colo., and Jerry Goldstein at Rice. The principal investigator of the
IMAGE mission is J.L. Burch of Southwest Research Institute, and the Polar
spacecraft is managed by the Goddard Space Flight Center in Greenbelt,
Md.

Images and movies from
Reiff’s talk will be available on the Web at <www.spaceupdate.com/IMAGE/News121201.html>.
A CD-ROM, “Space Weather,” which describes the science of space weather and
highlights of the IMAGE mission, will be available.