The underlying Parker spiral structure in the Ulysses magnetic field observations, 1990–1994

Abstract

The fundamental aim of the Ulysses space mission is to extend our understanding of the heliosphere into three dimensions. By April 1994, the spacecraft had reached a heliographic latitude of 60°S. Hourly averages of the Ulysses heliospheric magnetic field observations have been analyzed to determine to what extent the underlying field direction within 60° of the heliographic equator can be described by the Parker spiral model. At all latitudes from near the ecliptic southward to 60°S, the most probable value of the azimuthal orientation of the field lines remained in approximate agreement with the Parker model. Once Ulysses passed southward of the maximum latitude of the heliospheric current sheet at about 30°S it became possible to study the distribution of the azimuth angle in purely inward polarity southern hemisphere fields without the confusion between the northern and southern hemisphere sectors. This distribution is revealed to be highly asymmetric with a greater probability of observing field lines with an azimuth angle less tightly wound than the most probable angle. Comparison with near‐ecliptic data showed a similar asymmetry in inward polarity field sectors while in outward polarity sectors there was an asymmetry in the opposite sense. We suggest that the asymmetry in the >30°S azimuth angle distribution arises due to the presence of long‐period radially outward propagating Alfvén waves in the solar wind flows originating from the southern polar coronal hole. In addition, from studying the meridional (north‐south) orientation of the field lines we find that at south heliographic latitudes within about 25° of the equator there is a tendency in the Ulysses data set for the field lines to be on average deflected equatorward of their expected direction, most likely due to flow deflections associated with the interaction of high‐ and low‐speed solar wind streams.