"Warm Plasma Cloak" Discovered Enveloping Earth

Anne Minard
National Geographic News
January 7, 2009
The Earth is dressed in layers that protect it from the sun's fierce winds, and scientists have identified a new one they call a "warm plasma cloak."

The magnetosphere—the shield of ions and electrons that envelops Earth—extends far beyond the atmosphere, defending the planet from the harmful solar wind.

(Related: "Sun's Mysterious Waves Found; May Be Solar Wind Source" [December 6, 2007].)

Charles "Rick" Chappell, a physicist at Vanderbilt University in Nashville, Tennessee, led a research team that assembled information dating back decades to describe the new magnetosphere layer.

Some of the first hints of the cloak first showed up in data from research satellites in the early 1970s. The cloak was finally confirmed by NASA's Polar satellite, which ended a 12-year run in April 2008.

The cloak's discovery creates a theoretical home for particles that didn't fit with any of the other understood parts of the Earth's magnetosphere, Chappell said.

"The cloak particles didn't fit with any of the other regions."

The results appeared in fall 2008 in the Journal of Geophysical Research.

Flying Horseman

Chappell and his colleagues called the layer the "warm plasma cloak" because it conjured an image for them of a person on a horse, wearing a long cloak. Plasma is ionized gas found in space.

The cloak's tails billow in response to the direction of solar winds.

The warm plasma cloak begins thinly on the nightside—or darkside—of the planet and wraps around to the dayside, where it becomes thickest until noon. In the afternoon, convective winds push the cloak out toward the edge of the magnetosphere, where it's peeled off by solar winds.

Depending on where it is relative to Earth, and the energy of the solar wind, the cloak can be found anywhere from 13,000 to 65,000 miles (20,000 to 105,000 kilometers) above the Earth's surface. It is always thickest on the planet's dayside.

The most notable magnetic field elsewhere in the solar system is that of Jupiter's.

"Jupiter's magnetosphere would be the largest object in the solar system if you could see it—larger than the sun," Chappell said.

Earth's magnetosphere is more than a million miles in the tail, which trails off in the downwind direction from the sun. It's so far-reaching that the moon orbits through it every month.

Magnetic Boon and Bane

The formerly mysterious warm plasma cloak is also implicated in one of the menacing effects of the magnetic field—damage to dozens of human-made satellites over the years.

"The warm plasma cloak is part of the environment that communications and weather satellites fly in," Chappell said. "It will play a role in how much the spacecraft charge electrically."

The magnetosphere can induce power surges in the electrical grid on Earth, triggering blackouts, interfering with radio transmissions, and disrupting GPS signals, Chappell pointed out—especially when it's perturbed by changes in the sun's solar wind.

Michelle Thomsen is a space physicist at Los Alamos National Lab in New Mexico who was not involved in the new study, but reviewed it prior to publication. She called the paper "interesting and useful."

"The proposed new name is catchy and could be a worthwhile addition to the magnetospheric taxonomy," she said.

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