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Geophenomena

Storms on Earth stir up space weather
Rare earthquake rattles Gulf of Mexico

Storms on Earth stir up space weather

Space scientists are currently preparing to study a new cycle of solar flares, expected to start around 2011. Such flares, collectively called space weather, are well-known for disrupting satellite-based electronic communications, but a new study now suggests that intense Earth storms can also produce turbulence in the electrically charged upper atmosphere. The study marks the first time that storms on Earth have been shown to affect those “storms” in space.

Two bands of charged gas in Earth’s ionosphere, glowing in ultraviolet light, circle the globe in this picture compiled by NASA’s IMAGE satellite. The charged gas is densest in the bright, blue-white areas, which is shaped by hot air rising from storms near Earth’s equator (marked by dotted white lines). Image is courtesy of NASA/University of California, Berkeley.

At the uppermost reaches of Earth’s atmosphere is a thin layer of plasma, or ionized gas, known as the ionosphere. At heights from about 100 to 600 kilometers above Earth’s surface, the electrically charged ionosphere is important to many different forms of communication, as it helps radio waves travel for great distances around the globe. Within the ionosphere, molecules are constantly breaking apart and recombining, and the excited particles glow when viewed in ultraviolet light.

Researchers were able to see the glowing bands of plasma around Earth for the first time in 2002, when a NASA satellite known as IMAGE took ultraviolet pictures of Earth’s entire surface. Those pictures displayed an intriguing pattern in plasma distribution around the globe, says Thomas Immel, a space scientist at the University of California in Berkeley.

“There are four places on Earth where the ionosphere is generally denser and the [plasma] bands were more separated,” Immel says. Studying the IMAGE pictures, Immel’s team found that those four regions fall roughly along Earth’s magnetic equator, which is tilted and lies south of the geographic equator.

To understand this pattern, the researchers turned to earlier research, which suggested that intense thunderstorms on Earth produce atmospheric “tides” that create peaks of energy in the lower atmosphere. Those energy peaks matched the locations of the four densest parts of the ionosphere, the researchers observed.

Although they suspected a connection, Immel says, “we didn’t know how that could be.” The ionosphere’s glow, he says, was about 400 kilometers above the surface, while the storms’ tides reached only as high as 100 kilometers before their energy dissipated. The researchers discovered, however, that although the thunderstorms’ winds could not directly stir up the ionosphere, the winds do blow charged gas within the bottommost layer of the ionosphere across Earth’s magnetic field. That motion creates an electrical field that then redistributes the plasma in the overlying layer and creates the observed glowing pattern, the researchers reported Aug. 10 in Geophysical Research Letters.

Immel and his team are now working to replicate the observed effects with an ionospheric model. Previous models have not taken into account the effects of storms on Earth, because without observational data, modeling those effects is “hugely complicated,” he says.

“It’s an important finding in terms of our overall approach” to studying space weather, says Rod Heelis, a space scientist at the University of Texas at Dallas. The study shows that “the whole of our geospace environment is coupled from the ground up” to the edges of the atmosphere, he says. “We have to look at our environment in a big-systems way, rather than just isolate a piece of it.”

More data is needed, however — including direct observations of the winds that produce the electric fields — before scientists can accurately predict how Earth storms will affect the ionosphere, Heelis says. “This is a very important first step in being able to quantify the coupling between the ground and the upper atmosphere, but the loop is not closed by any means,” he says.

Carolyn Gramling

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Rare earthquake rattles Gulf of Mexico

A rare magnitude-6.0 earthquake shook the eastern Gulf of Mexico on Sept. 10, producing tremors felt as far away as Louisiana, Florida and even southern Georgia. More than 2,800 people in Gulf Coast states reported shaking from the quake, which was the largest to strike the region in 30 years.

The strength of the temblor, which occurred about 530 kilometers southeast of New Orleans, La., is unusual for that part of the world, says Stuart Sipkin, a seismologist at the U.S. Geological Survey (USGS) in Denver, Colo. That is because the earthquake occurred within the North American tectonic plate, rather than along a fault line. “It did get our attention because it was unusually large for this particular area,” Sipkin says.

Unlike the earthquake that spawned the December 2004 Sumatra tsunami, which occurred along a plate boundary, the source of the mid-plate Gulf earthquake is likely to be seafloor spreading far to the east, in the Atlantic Ocean, according to USGS. As the North American plate moves westward, pulling away from the Eurasian and African plates, it compresses, causing a buildup of stress.

The earthquake knocked items off of shelves and stirred up waves in swimming pools in parts of Florida, according to USGS, and is actually the second earthquake to rattle the eastern Gulf this year. A magnitude-5.2 temblor occurred in the same location in February.

Historically, other earthquakes have occurred in this region, “but up until last February, none were on our radar screen because they were so small,” Sipkin says. Based on historical data, the region is likely a “zone of weakness” within the plate, where the plate releases its accumulated stress. Earthquakes of similar magnitude, therefore, may continue to occur in the region, he says.

“We expect we will continue to see small earthquakes there, but there is no reason to think we’ll see anything larger than this one,” Sipkin says.

Carolyn Gramling

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