The Sun at solar minimum: North ‐ south asymmetry of the polar coronal holes

Abstract

Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic charts derived from Kitt Peak magnetograms are used to compare the south and north polar coronal holes which existed during the declining/minimum phase of the solar activity cycle from 1992 to 1997. The kinetic properties of the solar wind emanating from the two polar coronal holes, as represented by solar wind speed, do not differ significantly. However, the electron temperature in the two coronal holes inferred from ionic charge composition data, namely the O7+/O6+ ratio, show consistent differences, with the south polar hole being 10 to 15% hotter. The ground‐based magnetograms show that the north polar coronal hole covers a larger part of the solar surface than the southern one. The total magnetic flux and, specifically, the flux density of the north polar coronal hole is considerably lower for the whole interval of time between 1992 and 1997. This strongly indicates that the difference in coronal hole temperature between the southern and northern coronal hole is intrinsic and is not due to the fact that the Ulysses observations in the south and north coronal hole streams were made at different phases of the solar cycle. Thus the differences found represent a real north‐south asymmetry during this time period.