Entire sequence of auroral storm studied at poles simultaneously

Entire
sequence of auroral storm studied at poles simultaneously
…………………………………………………………………

BY B.J. ALMOND
Rice News Staff

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 Patricia Reiff presented Dec. 13
at the American Geophysical Union meeting in San Francisco.
Reiff is director of the Rice Space Institute and a professor
of astronomy and physics.

As part of her
presentation, she interpreted images of aurora borealis
(the Northern Lights), the aurora around the North Pole,
and aurora australis (the 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,”
Reiff said.

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 are available on the Web at <www.spaceupdate.com/IMAGE/News121201.html>.