The Galileo satellite, on its tour of Jupiter's moons, has provided a remarkable bonus for solar physicists. In January, the Sun obstructed our line of sight to the distant probe. By monitoring radio signals from Galileo as they passed through the Sun's corona, astronomers have solved an old problem about the origin of the solar wind. It has long been known that the wind has two distinct components, slow and fast. But where on the surface of the Sun do they originate?

This image (Habbal, S. R. et al. Astrophys. J. 489, L103-L106; 1997) is a white-light view obtained using the Solar and Heliospheric Observatory (SOHO). The solar disk is blanked out, revealing the hot, inner coronal regions and filamentary structures known as streamers. Jupiter is the bright point to the left of the solar disk.

Galileo's apparent passage behind the Sun is marked by the straight black lines, which show where the slit of a spectrometer on SOHO was positioned to make ultraviolet measurements of the corona, simultaneous with the radio transmissions. The UV spectra give a rough indication of the Solar wind speed, and the 94 km s-1 contour is shown in white. Scintillation of Galileo's radio signal also indicates wind speed, importantly with very high spatial resolution: on one occasion, the scintillation increased markedly (a sign of the slow wind), and, by no coincidence, a streamer stalk intercepted the line of sight to the probe at the same time. It seems that the slow wind comes from streamers, and the fast wind comes from the whole surface — not, as had been thought, only from the polar regions.

Eventually, these results may help solve the biggest mystery of the solar corona — the nature of its heating mechanism.