SWICS/Ulysses observations: The three-dimensional structure of the heliosphere in the declining/minimum phase of the solar cycle



The ESA/NASA spacecraft Ulysses provides the first in-situ observation of the solar wind at high solar latitudes. Data obtained with the Solar Wind Ion Composition Spectrometer (SWICS) during the first orbit of Ulysses around the Sun can be used to obtain a global view of the solar wind pattern within the heliosphere during the declining/minimum phase of the solar activity cycle. This provides information on the solar corona, specifically on any latitudinal variation of coronal parameters as far as they play a role in the solar wind acceleration process. The measurements show: The 3-dim. distribution of the solar wind appears as a pattern of basically two states. The fast solar wind with speeds of about 750 km/s emanating from the polar coronal holes dominates the heliosphere. The slow solar wind from the streamer belts is restricted to a region of about +/−20° around the heliographic equator. The boundary between the two states of the solar wind is sharply defined. The coronal hole streams are remarkably stable and homogeneous on a larger scale. The two independent coronal holes are similar in terms of average speed and spatial extent. In both coronal holes a gradual increase (≤10%) of speed with solar latitude is observed.