A bright "polar rain" of x-rays, triggered by particle collisions high in the planet's atmosphere, shone over Earth's north magnetic pole on 11 May when the sun's solar wind dwindled to a near calm. This radiation, and an accompanying auroral display in visible light, came from electrons that smashed into Earth's atmosphere on a direct line from the sun.

Solar Wind Vanishes for a Day

SAN FRANCISCO--The solar wind, a steady gale of high-energy particles from the sun, died down to a mere zephyr for more than a day earlier this year. This sudden--and unexpected--pause allowed Earth's magnetic shield in space to balloon to more than 100 times its usual volume and offered a clear glimpse of the wind's birthplace, researchers announced today.

Electrons, protons, and other charged particles in the solar wind stream from the sun's outer atmosphere, or corona, at speeds of about 1.5 million kilometers per hour. These particles bombard Earth's magnetosphere, a vast bubble of space within which our planet's magnetic field is strong enough to deflect most of the solar wind. The magnetosphere absorbs the rest of the particles, charging up intense radiation belts around Earth and creating shimmering curtains of light, the auroras seen near the poles.

The steady flow of particle traffic tapered to an unprecedented lull from late on 10 May until early on 12 May, according to measurements by a fleet of sun-observing satellites. Particle densities were 50 times lower than normal during that time. This allowed the leading edge of Earth's magnetosphere, normally squeezed by the pressure of the incoming particles to just 65,000 kilometers from the planet, to swell 375,000 kilometers toward the sun--nearly reaching the orbit of the moon, space physicist Keith Ogilvie of NASA's Goddard Space Flight Center in Greenbelt, Maryland, reported here at a meeting of the American Geophysical Union.

Although the sun churned out far fewer particles, some penetrated this extended magnetic shield and slammed into Earth's atmosphere above the planet's north magnetic pole, says space physicist Jack Scudder of the University of Iowa in Iowa City. This stream sparked a rarely seen "polar rain" of energetic particles that lit up a bright patch of aurorae above the north pole. "When we see these particles, we are looking directly into the heart of the solar acceleration mechanism," Scudder says. That's because the low-density particle beams didn't interact with each other or scatter off the magnetosphere on their way to Earth, he notes.

Researchers hope this brief window on the corona will help them unravel the mysterious mechanisms that control how fiercely the solar wind blows. Solar physicist Nancy Crooker of Boston University in Massachusetts analyzed 16 similar lulls in the last 6 years, all shorter-lived than the May event. She found that 10 of them happened near the time of coronal mass ejections--bursts of charged gas that billow outward from the sun in vast bubbles. Some of the solar wind interludes are "dead center within these magnetic clouds," she says, like the calm at the eye of a hurricane. However, researchers have found no obvious link between the 11 May event and a coronal mass ejection.

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