Ulysses Mission-Long Study of Radial Heliospheric Magnetic Fields in Solar Wind Rarefaction Regions

Details Ulysses Mission-Long Study of Radial Heliospheric Magnetic Fields in Solar Wind Rarefaction Regions Ulysses Mission-Long Study of Radial Heliospheric Magnetic Fields in Solar Wind Rarefaction Regions

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsh23a..07l&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #SH23A-07

Physics

2134 Interplanetary Magnetic Fields, 2162 Solar Cycle Variations (7536), 2164 Solar Wind Plasma

Scientific paper

We examine the distribution of directions of the heliospheric magnetic field (HMF) measured by the Ulysses spacecraft during its two and a half high-latitude orbits over the poles of the Sun (from 1992 to the end of 2007). We are looking in particular for the nearly radial field orientations that were first reported by Jones et al. [1998] and analyzed in greater detail by several subsequent authors. As they all noted, such radial fields are commonly observed in the mid-latitude region (~10°-40°), when the observed solar wind velocity shears from the high-latitude fast solar wind toward the low-latitude slow solar wind (i.e. in rarefaction regions where the solar wind velocity decreases monotonically over several days). In contrast to ICMEs where the HMF may be enhanced, the rarefaction regions with radial magnetic fields tend to have extremely low values of the HMF. Within these rarefaction regions, there are "dwells" in the coronal source longitude of the measured solar wind [Nolte and Roelof, 1973] in which the time dependence of the solar wind velocity is given to very good approximation by V(t)=r/(t-t0), where r is the radius of the spacecraft. We have therefore compiled distributions within the dwells of the parameter |Br/B| which is the cosine of the cone angle of the field from the radial direction. We study the mid-latitude transits that occurred during different phases of solar activity: 1992 (decline), 1996 (rise), 2002 (maximum), and 2005 (minimum). During three out of these four periods, the distributions of the HMF tended to be rather uniform in all HMF directions (0<|Br/B|<1), but during 2002, when the dwells were more frequent and of shorter duration, the distributions tended more strongly toward |Br/B|~1. Such distributions were observed up to very high latitudes (65°N). We discuss these observations in terms of the large-scale topology of the HMF as described by an analytic function that includes as special cases both a temporal variation in the coronal solar wind velocity [Gosling and Skoug, 2002] and a longitudinal velocity gradient at the eastern edge of a solar wind source region [Schwadron and McComas, 2005].

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