ROTATION OF THE PHOTOSPHERIC MAGNETIC-FIELDS - A NORTH-SOUTH ASYMMETRY

During most of solar cycle 21 the first Fourier component of the photospheric field rotates more rapidly in the northern hemisphere than in the southern. Analysis of the frequency intervals near the fundamental and first harmonic of the solar rotation frequency shows that the large-scale northern field rotated with a 26.9 day period (synodic), was centered at 15° N, and covered a latitude zone ∼24° wide. The corresponding southern field rotated with a periodicity of 28.1 days, was centered at 26° S, and covered a latitude ∼32° wide. By separately analyzing the first Fourier components of the field we impose a severe low-pass spatial filter to the solar data; but one which is of interest because the scale corresponds to the effective filtering that occurs in the solar corona and is observed in the interplanetary magnetic field (IMF). Our analysis showed rotational power at only a few discrete latitudes and frequencies in each hemisphere. The center of each peak lies near the sunspot differential rotation curve. The strength of the first harmonic of the northern field suggests that this structure may be related to the four-sector pattern observed in the IMF polarity. The southern field had much lower power at the first harmonic of the solar rotation rate and so would contribute only to a two-sector structure in the IMF. These results were discovered in Fourier analysis of photospheric synoptic charts obtained at the Wilcox Solar Observatory from 1976 to 1986 and confirmed in higher resolution maps from the National Solar Observatory. Mount Wilson magnetic field measurements from solar cycle 20 shows a similar north-south asymmetry.