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NASA Science News for April 22, 2003

A Star with two North Poles

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

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. 

"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. 

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." 

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. 

"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. 

Credits & Contacts
Author: Dr. Tony Phillips
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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