Hot wind. The solar corona, as seen by instruments aboard SOHO. White line marks boundary between slow wind (near streamers) and fast wind.

Stalking the Solar Wind

Astronomers have narrowed down the sources of the solar wind, the stream of particles that starts somewhere in the tangle of magnetic fields and turbulent gases near the sun's surface and scours the entire solar system. By probing the sun's atmosphere with the help of several different spacecraft, the researchers found one component of the wind--a fitful slow wind--dribbling from long, narrow structures called stalks, which stick up from the equator. The findings, reported this month in Astrophysical Journal Letters, also hint that the other, fast component of the wind emerges in patches over most of the sun rather than just from near the poles, as astronomers had thought.

In earlier work, Richard Woo of the Jet Propulsion Laboratory in Pasadena, California, and collaborators had probed for the source of the slow wind by monitoring the communications beams of interplanetary spacecraft as they dipped behind the sun. The researchers looked for "scintillation," or scattering, of the signals close to the visible edge of the sun. "It's like a headlight scattered by fog: The headlight looks bigger," says Woo. The scintillation should pinpoint regions of unsteady flow in the gases near the solar surface. Woo and his colleagues measured the strongest effects where the radio waves passed through the stalks, suggesting that the ragged, unsteady slow wind originates there.

To prove that suspicion, Shadia Habbal of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, Woo, Silvano Fineschi of CfA and others measured the wind speed with an instrument aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. The instrument collected light that is emitted by oxygen ions deep in the solar atmosphere, then scattered in all directions off other oxygen ions flowing outward in the solar wind. The faster the wind, the weaker the scattering. By correlating their speedometer readings with images of the magnetic arches and stalks and with scintillation measurements from the Galileo probe orbiting Jupiter, the team was able to confirm that the slow wind was leaking out of the stalks.

The new measurements also show that when the fast wind leaves the sun it blows from a wide range of latitudes, rather than just near the poles. That conclusion is controversial--"revolutionary," as a skeptical Jack Gosling of Los Alamos National Laboratory in New Mexico puts it--because the sun's magnetic field near the surface was thought to bottle up the fast wind except at the poles. But most researchers say the mystery of the slow wind is as good as solved. "It's a giant step forward," says Alan Title, a solar physicist at the Stanford-Lockheed Institute for Space Research.