A Star With Two North Poles

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 Centre. - 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 kilometres 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.

The shape of the current sheet in March 2000 as calculated by the Blue Horizon supercomputer.

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 kilometres away. Working together they're getting us ready for the next time the Sun sprouts an extra north pole ... or something stranger yet.