Ulysses encounters a different solar wind
5 October 2000Its really exciting how different the solar wind is this time compared with the first orbit, David McComas from the Los Alamos National Laboratory told the 34th ESLAB symposium on the heliosphere yesterday morning. His observation was repeated by many of the speakers: however you look at the solar wind or corona, theres evidence of far more solar activity now than during Ulysses first south polar passage in 1994.
Were not seeing the fast and slow wind we saw then. Now its intermediate in speed, said McComas. During the 1994 south polar fly-by, the Sun was at a minimum in its 11- year activity cycle. Then, the Ulysses spacecraft detected a slow-moving solar wind at low latitudes near the Suns equator and a fast wind coming from high latitudes near the pole. This summer, as the Sun approaches maximum activity, the fast wind has all but disappeared. We detected it just once at about 65 degrees south, said McComas. The intermediate-speed wind could be a mixture of fast and slow winds emanating from the more highly variable solar corona, he said - or it could be a new type of wind originating from a different source.
The different source idea, however, found little favour with other speakers. By classifying the solar wind by composition or even temperature, its possible to distinguish fast and slow wind even at solar maximum when both types emanate from all regions of the Sun, said Rudi von Steiger from the International Space Science Institute, Bern, Switzerland. Fast and slow wind are composed of the same ions, but in different proportions. Fast wind originates from coronal holes, which concentrate around the pole at solar minimum, but appear at all latitudes at maximum. These are cooler areas of the corona where magnetic fields stretch out away from the Sun. The rest of the Suns surface is characterised by closed magnetic loops which re-direct escaping material back to the Suns surface.
Slow wind is released when these closed field lines are cut, said Len Fisk from the University of Michigan. He presented theoretical work, which suggested that this can happen when neighbouring open magnetic fields gradually migrate through closed-loop regions. To thoroughly test this idea, however, requires a more sophisticated spacecraft than Ulysses or its companions, SOHO and Cluster. The proposed Solar Orbiter, now undergoing studies at ESA, could do the job: it would have the ability to distinguish sufficiently small areas on the Suns surface and monitor their progress.