A Star With Two North Poles

Sometimes the Sun's magnetic field goes haywire, and the effects are felt throughout the solar system.

Three years ago, something weird happened to the Sun.

Normally, our star, like Earth itself, has a north and a south magnetic pole. But for nearly a month beginning in March 2000, the Sun's south magnetic pole faded, and a north pole emerged to take its place. The Sun had two north poles.

"It sounds impossible, but it's true," says space physicist Pete Riley of Science Applications International Corporation (SAIC) in San Diego. "In fact, it's a fairly normal side-effect of the solar cycle." Every 11 years around solar maximum, the Sun's magnetic field goes haywire as the Sun's underlying magnetic dynamo reorganizes itself. The March 2000 event was simply a part of that upheaval.

"The south pole never really vanished," notes Riley. It migrated north and, for a while, became a band of south magnetic flux smeared around the Sun's equator. By May 2000 the south pole had returned to its usual spot near the Sun's southern spin axis - but not for long. In 2001 the solar magnetic field completely flipped; the south and north poles swapped positions, which is how they remain now.

Using a supercomputer named Blue Horizon and data from spacecraft (especially NASA's ACE and ESA-NASA's Ulysses) Riley and colleagues are studying how these complex changes can affect our planet. "The Sun's magnetic field permeates the entire solar system," explains Riley. "It interacts with Earth and is the primary driver of space weather."

The vast region of space filled by the Sun's magnetic field is called the heliosphere. All nine planets orbit inside it. But the biggest thing in the heliosphere is not a planet, or even the Sun. It's the current sheet - a sprawling surface where the polarity of the Sun's magnetic field changes from plus (north) to minus (south). "We call it the 'current sheet,'" says Riley, "because an electrical current flows there, about 10^-10 amps/m²." The filament of an ordinary light bulb carries sixteen orders of magnitude (10¹⁶x) more amps/m². But what the current sheet lacks in local amperage, it makes up in sheer size. The sheet is 10,000 km thick and extends from the Sun past the orbit of Pluto. "The entire heliosphere is organized around this giant sheet."