Multiple spacecraft survey of the north‐south asymmetry of the interplanetary magnetic field

Abstract

Twenty‐three years of spacecraft observations spanning 22 AU of heliocentric distance have been used in an examination of the north‐south asymmetry of the interplanetary magnetic field. Using the observed sector structure of the solar wind, we have examined the winding of the Parker spiral both north and south of the heliospheric current sheet as observed by the Pioneer‐Venus Orbiter and numerous near‐Earth spacecraft and conclude that a significant and persistent asymmetry exists between the two hemispheres at 0.7 and 1 AU. In keeping with previous analyses, we find that the Parker spiral north of the current sheet is more tightly wound than the Parker spiral south of the current sheet. Several definitions of the average winding angle are all consistent with an asymmetry of 2.4° ± 0.9° at 1 AU using 193 solar rotations. The asymmetry at 0.7 AU for the period 1979‐1987 is somewhat smaller, 2.1° ± 1.0°, but is consistent with 1 AU values during this time period. We conclude that neither the observed asymmetry of the solar rotation nor the observed asymmetry of the solar wind speed can account for the observed winding angle asymmetry. We evaluate the north‐south asymmetry of the large‐scale field beyond 1 AU as measured by Voyager 1 and 2 and Pioneer 10 and 11. The computed asymmetry in the winding angle of the field reaches very large values of the order of 30°, but the quantitative agreement among the four data sets is poor. This and other characteristics of the data suggest that residual zero level errors (at the ∼0.05 nT level) in the radial component of the field may be producing spuriously large asymmetries in the winding angle. In contrast, we find strong indications for a genuine asymmetry in the azimuthal component of the field, with all data sets examined in good agreement. The observations are consistent with a stronger azimuthal component in the northern hemisphere than in the southern hemisphere for the range in heliocentric distance from 0.7 to 10 AU.