A Star with two North Poles
Sometimes the Sun's magnetic field goes haywire,
April 22, 2003: 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.
Right: An ultraviolet image of the Sun at solar maximum. Image credit: SOHO. [more]
"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."
Ordinarily, the current sheet circles the Sun's equator like a wavy skirt around a ballerina's waist. But during the double north pole event of March 2000, the current sheet was radically altered: The waviness increased. Irregularities appeared. Its topology "morphed" from a ballerina's skirt to a giant seashell.
Interesting to a solar physicist, perhaps...
...but ordinary people should care about this, too. First because of energetic cosmic rays: The current sheet acts as a barrier to cosmic rays traveling through the heliosphere. Cosmic rays can't cross the sheet; instead they flow along it. The shape of the current sheet therefore determines how many cosmic rays strike Earth.
Space weather is another reason: As Earth orbits the Sun, it dips in and out of the undulating current sheet. On one side the Sun's magnetic field points north (toward the Sun), on the other side it points south (away from the Sun). South-pointing solar magnetic fields tend to cancel Earth's own magnetic field. Solar wind energy can then penetrate the local space around our planet and fuel geomagnetic storms.
Below: These auroras appeared over Alaska's Knik Valley during a strong geomagnetic storm on April 8, 2003. Photo credit and copyright: LeRoy Zimmerman.
Geomagnetic storms are both good and bad -- bad because they can cause electronics on satellites to short circuit and power grids on Earth to fail; good because they spark auroras, which sky watchers enjoy. "If we could make an accurate daily map of the current sheet, then we could do a better job predicting the onset of these storms."
There's a problem, though: the current sheet is invisible. "We can't see it through an optical telescope," he says, "which means we have to calculate where it is." Riley and his colleagues have developed a computer program to do that. The input data are measurements of the Sun's surface magnetic field; these are taken daily by telescopes on Earth. The program applies the equations of resistive magnetohydrodynamics to calculate how the electrified solar wind drags that magnetic field through the solar system. A supercomputer -- Riley uses the Blue Horizon IBM SP3 at the San Diego Supercomputing Center--is required to execute the code.
The episode of the double north pole provided a key test of their software. "We calculated the shape of the current sheet for a Sun with two north poles," recalls Riley. "The result looked like a conch shell ... more than a billion kilometers wide."
Left: The shape of the current sheet in March 2000 as calculated by the Blue Horizon supercomputer. [more]
But how could he check his results?
NASA's Ulysses spacecraft provided the crucial data. In early 2000, Ulysses was about 600 million km from the Sun -- perfect for testing the conch model. As the spacecraft cruised through space at 10 km/s it crossed the current sheet twice, once in March and again in April 2000. Onboard magnetometers recorded the crossings, which were in good agreement with Riley's predictions.
Using only measurements of the Sun's surface magnetic field, his software had successfully predicted magnetic fields in interplanetary space 600 million km away. Impressive.
Below: Ulysses observations of the Sun's magnetic field in March 2000 overlaid on Riley's magnetohydrodynamic (MHD) calculations. [more]
"It has taken us ten years to develop this capability," says Riley. "We would like to improve it even more by including measurements of the temperature, density and speed of the solar wind -- parameters that we merely estimate now. Our ultimate goal is to provide up to 4 days advance warning of geomagnetic storms."
Testing that next-generation software will require more data from Ulysses. The spacecraft follows a high-looping orbit where it can see the Sun's polar regions, something no other spacecraft can do. "This unique trajectory has allowed scientists for the first time to fully explore the heliosphere in three dimensions," says Riley.
A supercomputer on Earth. A spacecraft hundreds of millions of kilometers away. Working together they're getting us ready for the next time the Sun sprouts an extra north pole ... or something stranger yet.