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 inferred from Ulysses cosmic-ray observations, is investigated using simultaneous Ulysses and Wind magnetic field observations. Such an asymmetry, presumably associated with a southward displacement of the heliospheric current sheet (HCS), is expected to produce significantly different magnitudes of the radial field component, |BR|, in the Sun's north and south magnetic hemispheres or, alternatively, in the positive and negative magnetic sectors. Ulysses, while at high latitudes, spends time predominantly in first one and then the other hemisphere. As a consequence, measurements in the positive sector are obtained several months later than measurements made in the negative sector, making comparisons susceptible to temporal changes. To address this ambiguity, the fields in both sectors observed by the Wind spacecraft in the ecliptic were compared. A large difference in |BR| of ≈30% was observed at Wind between 1994 December and 1995 April, with |BR| larger in the south than in the north. Subsequent measurements show a gradual increase in the north (outward) radial component and a decrease in the south (inward) component, ending in only a small difference by 1995 June. Thus, the Wind observations are consistent with a southward displacement of the HCS of ≈10° and with the energetic particle observations. The secular time variation, which occurred as the spacecraft transited from the south to the north hemisphere, explains why a significant north-south difference in |BR| was not evident in the Ulysses measurements. The current sheet configuration and various questions and implications associated with these results are also discussed.