Rare silence of solar wind opens window into sun

Rare silence of solar wind opens window into sun


Sun from SOHO
Sun from SOHO

The constant stream of particles from the sun known as the solar wind all but stopped blowing earlier this year, offering a rare opportunity to study the interaction between Earth and its nearest star.

The unexpected event from late May 10 to early May 12 did not pose any threat to Earth, scientists said Monday, but it did create some unusual effects as the planet’s magnetic field ballooned to more than 100 times its regular size without the confining pressure from the solar wind.

Unlike solar storms, the calm had no apparent impact on orbiting satellites or power grids. But new information can be used to better understand and prepare for periods of high activity – and strong solar wind.

“It was a very exciting event in terms of what it tells us about the physics of what is going on,” said Howard Stringer of the federal Space Environment Center.

Scientists discussed the diminished wind and results from several satellites that monitored the event at the fall meeting of the American Geophysical Union in San Francisco.

The solar wind of hot, charged particles normally blows outward from the sun at more than 1 million mph as the star’s nuclear engine causes churns, boils and explosions on its surface and atmosphere.

The Earth’s magnetic field is constantly buffeted by the stream of particles, causing it to look like a halved apple with a comet-like tail that sweeps away from the sun. Normally, the force extends only 40,000 miles toward the sun.

But on late May 10, satellites recorded that the density of the wind was only about 2 percent of normal and its speed dropped by more than half. With no pressure to hold back the Earth’s magnetism, the force stretched to nearly 235,000 miles by May 11 – about the distance from Earth to the moon.

“This was a very rare occurrence that has been seen only a few times since satellites began taking solar wind measurements 35 years ago,” said Daniel Baker, director of the University of Colorado at Boulder’s Laboratory for Atmospheric and Space Physics.

Normally, the Earth’s magnetic field causes the particles to dilute as they bounce around space. In May, the particles that streamed toward Earth were able to take a more direct path.

Scientists had predicted such an event 14 years ago, theorizing that it would cause electrons from the sun’s corona, or atmosphere, to rain directly over Earth’s northern polar region. On May 11, scientists detected an auroral glow over the region in X-ray images from NASA’s Polar satellite.

“This event provides a window to see the sun’s corona directly,” said Keith Ogilvie, project scientist for NASA’s WIND spacecraft. “The beams from the corona do not get broken up or scattered as they do under normal circumstances.”

Scientists are still studying what caused the wind to diminish as well as the implications of a direct view into the sun. But because the Earth responded as predicted, they are now more confident that their theories are correct.