J. Geophys. Res., 101, 395-403, 1996
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
helio-graphic 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 south-ward of the
maximum latitude of the heliospheric current sheet at about 30°S it became
pos-sible to study the distribution of the azimuth angle in purely inward
polarity southern hemi-sphere 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 out-ward 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.
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