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Science Highlights from the Second Solar Orbit

Solar Wind and Magnetic Field

Polar plots of solar wind speed as a function of latitude for Ulysses' first two orbits. Sunspot number (bottom panel) shows that the first orbit occurred through the solar cycle declining phase and minimum while the second orbit spanned solar maximum. Both are plotted over solar images characteristic of resp. solar minimum (17 August 1996) and maximum (07 December 2000).
Courtesy: D. McComas

Ulysses found an entirely different heliosphere at solar maximum compared with that observed near solar minimum. At solar minimum, the heliosphere was dominated by the fast wind from the southern and northern polar coronal holes. In contrast, during solar maximum, the large polar coronal holes had disappeared, and the heliosphere appeared much more symmetric. The solar wind flows measured throughout the south polar pass, and much of the rapid transit from south to north, showed no systematic dependence on latitude. The wind itself was generally slower and much more variable than at solar minimum at all latitudes. Nevertheless, when Ulysses reached high northern latitudes in late 2001, it witnessed the formation and growth of a new polar coronal hole. The solar wind recorded at Ulysses became faster and more uniform, resembling the flows seen over the poles at solar minimum.

Although the solar magnetic field, corona and solar wind were highly variable, the magnetic field at Ulysses (~1.5-2.5 AU from the Sun) maintained a surprisingly simple, dipole-like structure. In contrast to the situation at solar minimum, however, the equivalent magnetic poles were located at low latitudes rather than in the polar caps.

Another phenomenon of great interest was the Sun's magnetic polarity reversal that occurred during the polar passes. It was found that the process of magnetic polarity reversal is a complex one that takes place over a period of several months. The previous finding of Ulysses concerning the lack of dependence of the radial component of the magnetic field on latitude was found to be valid even in the more disturbed conditions at solar maximum. This appears to be a fundamental property.

Last Update: 17 November 2006

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