by David Hosansky
In July 2000, an intense solar storm sent billions of tons of plasma
hurtling through space at some 6.4 million kilometers (4 million miles)
per hour. The plasma set off a major geomagnetic storm in Earths
upper atmosphere that raged for nine hours before gradually subsiding.
Satellites and ground-based power stations were affected, and auroral
light shows were visible as far south as El Paso, Texas.
Along with producing auroral displays, geomagnetic storms can
far-reaching effects on communications satellites, radio signals,
and power-plant operations. They may also produce quasicyclonic
vortices in the thermosphere, according to a new NCAR study. (Photo
by Jim Hannigan.)
For NCARs Alan Burns, the event (which solar scientists dubbed
the Bastille Day storm) raised an interesting question:
why did it take more than 24 hours for the thermosphere to recover from
the nine-hour tempest?
Weve always thought the thermosphere is driven by events
from outside, but it doesnt have a long memory, Burns says.
So the High Altitude Observatory physicist, working with colleague
Wenbin Wang and NCAR director Tim Killeen, decided to model the storm.
To his surprise, the modelHAOs Thermosphere Ionosphere Electrodynamic
General Circulation Model (TIEGCM)indicated that the storm spawned
a number of quasicyclonic vortices in the thermosphere, the first time
such systems have appeared in an upper-atmosphere simulation.
Three hours after the plasma struck the atmosphere, the modeled vortices
included updrafts of about 350500 km/hr (217311 mph). After
another three hours, the updrafts were about half as strong. Such vortices
would help explain the prolonged geomagnetic turmoil about 300 km (186
mi) above Earths surface. Typically, the upper atmosphere recovers
from these storms within 12 hours. But the effects can persist for more
than a day in locations where vortices form.
There seems to be an element of self-organization in these events,
explains Burns, who presented his findings at the American Geophysical
Union European Geophysical Society joint meeting in April (see
page 12). Theyre being driven from within.
The duration and size of geomagnetic storms in Earths thermosphere
is an important research area because the storms have far-reaching effects
on technological systems. The storms, associated with coronal mass ejections
that send large amounts of charged matter from the Suns outer
atmosphere, can disrupt communications satellites, radio waves, and
even power plant operations.
Relatively little is known about the thermosphere, partly because
it is difficult to probe with ground-based instruments. Without more
data, Alan can only speculate about the causes of the vortices.
One possibility, he says, is cooling over the poles. Even as geomagnetic
storms heat up most parts of the upper atmosphere, they can cause cooling
above the poles at heights of around 150200 km (90130 mi)
due to a cascade of effects involving nitrous oxide production and resulting
ozone destruction. The downward conduction of heat in these areas could
help form and sustain vortices. Another theory is that the storms create
bulges in the upper atmosphere that are as much as 400°C (720°F)
hotter than surrounding regions. As a result, air would rush from the
heated areas to the cooler ones, fueling a vortex.
Burns notes that in regions where vortices do not form, the thermosphere
recovers relatively rapidly from geomagnetic storms. The thermosphere
is a surprisingly robust system, he says. You hit it with
one of these geomagnetic storm hammers andcertainly in the upper
thermosphereit doesnt take too long to return to its normal