The Ulysses spacecraft is in a polar orbit around the sun, allowing complete three-dimensional observations of the solar wind and the near-solar region to be made. An instrument called Solar Wind Observations Over the Poles of the Sun (SWOOPS) records the solar wind’s “dynamic pressure,” a measure of its kinetic energy. The outer white circle around the sun represents a pressure of five nanopascals, or billionths of a pascal; the inner one is one nanopascal. (A pascal, of course, is a force of one newton per square meter. But you knew that.) The colored lines trace the dynamic pressures observed.
The background images of the sun are from NASA’s Solar and Heliospheric Observatory (SOHO).

Becalmed?

The solar wind has apparently become just a solar breeze. New data from the Ulysses spacecraft shows that the solar wind has lost power, which has exposed the solar system to more cosmic rays. The data also reveals that the wind and the sun’s magnetic field are more intimately related than previously thought, shedding light on how the wind is produced. “Ulysses has provided a new constraint on the origin of the solar wind,” says JPL’s Ed Smith, project scientist for the mission. “The data provides us with a new view of what’s going on at the source.”

Made of charged particles gushing from the sun’s outer atmosphere—called the corona—at hundreds of kilometers per second, the solar wind reaches billions of kilometers away. The sun’s magnetic field, however, keeps the particles trapped within the corona at first, preventing the wind from escaping. Scientists used to think that the pressure of the wind would grow until it broke free from the magnetic field, like a flock of sheep escaping by pushing open the gate to their pen.

But Ulysses is finding that the solar-wind flux—that is, how many particles spew out per second—is proportional to the strength of the sun’s magnetic field. This relationship suggests a different understanding of how the wind blows. “The magnetic field plays not only an important role, but a dominant role,” Smith explains. Magnetic field lines emanate from the sun and curve back toward it, forming loops that hold in the wind’s charged particles. The sun’s field is irregular and dynamic, however, and sometimes those loops break. When they do, they release the wind into space. In other words, the gate opens by itself to let the sheep roam free. The solar-wind flux is analogous to the number of sheep, and the strength of the magnetic field is analogous to how wide the gate opens. The correlation between the wind and magnetic field must now be taken into account in future computer models, Smith says.

Once released, the wind reaches far beyond the edge of the solar system, where it slams into particles from other stars—the interstellar wind—forming the boundary of a huge bubble called the heliosphere. Because it’s kept inflated by the solar wind, the heliosphere shields the solar system from cosmic rays.

In September, Ulysses scientists announced that the pressure of the solar wind has waned 20 percent since the mid-1990s. The wind hasn’t slowed down much, losing only 3 percent of its speed, but it’s 13 percent cooler and 20 percent less dense. The lack of pressure causes the heliosphere to deflate and weaken, allowing more cosmic rays to pass through. The sun’s magnetic field has also diminished by 30 percent, further crippling the heliosphere.

We’re protected by Earth’s atmosphere and magnetic field, so those of us on the ground don’t have anything to worry about. But a surge in cosmic rays could pose a threat to astronauts, who would need more protection against the damaging radiation, as would spacecraft and satellites in high-Earth orbit.

A shrunken heliosphere also explains Voyager 2’s findings. In the beginning of September 2007, the spacecraft crossed the heliosphere boundary, called the termination shock, earlier than researchers had anticipated.

It’s unclear what a quieter solar wind means. After all, scientists have only been studying the wind since the dawn of the space age a mere 50 years ago. Launched in 1990 and operated from JPL by NASA and ESA, Ulysses circles the sun, studying how solar activity changes along different solar latitudes, from pole to pole. —MW