ALFVEN WAVES IN THE SOLAR CORONA AND SOLAR-WIND

Alfven waves have long been known to be a major component of the turbu lence measured in situ in the interplanetary medium. Until recently, h owever, observations had been limited to the ecliptic plane, where the solar wind structure is complicated by the interaction of fast and sl ow solar wind streams, the Alfvenic turbulence being essentially limit ed to high-speed streams in well defined magnetic sectors. The Ulysses spacecraft has shown how this structure disappears with increasing la titude, leading to a relatively constant high-speed (similar or equal to 750 km s(-1)) stream originating from polar coronal holes. Within t his region the radial magnetic field appears to be relatively constant with latitude, and the fluctuations are everywhere dominated by large -amplitude Alfven waves propagating away from the sun, covering a broa d band of wavelengths. Here we discuss the origin and evolution of sol ar wind Alfven waves; the possible role played by such fluctuations in the heating of the corona and acceleration of the high-speed wind is explored in the light of both analytical models and numerical simulati ons.