Geomagnetic storms may spur thermospheric vortices

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 Earth’s 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 have 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 NCAR’s 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?

“We’ve always thought the thermosphere is driven by events from outside, but it doesn’t 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 model—HAO’s 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 350–500 km/hr (217–311 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 Earth’s 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). “They’re being driven from within.”

The duration and size of geomagnetic storms in Earth’s 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 Sun’s 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 150–200 km (90–130 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 and—certainly in the upper thermosphere—it doesn’t take too long to return to its normal state.”

 


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