Evidence of a north-south asymmetry in the heliosphere associated with a southward displacement of the heliospheric current sheet

Evidence of a north-south asymmetry in the global heliosphere, first inferr ed from Ulysses cosmic-ray observations, is investigated using simultaneous Ulysses and Wind magnetic field observations. Such an asymmetry, presumabl y associated with a southward displacement of the heliospheric current shee t (HCS), is expected to produce significantly different magnitudes of the r adial field component, /B-R/, in the Sun's north and south magnetic hemisph eres or, alternatively, in the positive and negative magnetic sectors. Ulys ses, while at high latitudes, spends time predominantly in first one and th en the other hemisphere. As a consequence, measurements in the positive sec tor are obtained several months later than measurements made in the negativ e sector, making comparisons susceptible to temporal changes. To address th is ambiguity, the fields in both sectors observed by the Wind spacecraft in the ecliptic were compared. A large difference in /B-R/ of approximate to 30% was observed at Wind between 1994 December and 1995 April, with /B-R/ l arger in the south than in the north. Subsequent measurements show a gradua l increase in the north (outward) radial component and a decrease in the so uth (inward) component, ending in only a small difference by 1995 June. Thu s, the Wind observations are consistent with a southward displacement of th e HCS of approximate to 10 degrees and with the energetic particle observat ions. The secular time variation, which occurred as the spacecraft transite d from the south to the north hemisphere, explains why a significant north- south difference in /B-R/ was not evident in the Ulysses measurements. The current sheet configuration and various questions and implications associat ed with these results are also discussed.