Asymmetric Solar Polar Field Reversals

The solar polar fields reverse because magnetic flux from decaying sunspots moves towards the poles, with a preponderance of flux from the trailing spots. If there is a strong asymmetry in the sense that most activity is in the Northern Hemisphere, then that excess flux will move to the North Pole and reverse that pole first. If later on, there is more activity in the South, then that flux will help reverse the South Pole. In this way, we get (in the ideal case) two humps in solar activity and a corresponding difference in time of reversals. Such difference was first noted in the very first observation of polar field reversal just after the maximum of the strongly asymmetric solar Cycle 19, when the Southern Hemisphere was most active before sunspot maximum and the South Pole duly reversed first, followed by the Northern Hemisphere more than a year later, when that hemisphere became most active. Solar cycles since then have had the opposite asymmetry, with the Northern Hemisphere being most active before solar maximum. We show that polar field reversals for these cycles have as expected all happened first in the North. This is especially noteworthy for the present solar Cycle 24. We suggest that the association of two or more peaks of solar activity when separated by hemispheres with correspondingly different times of polar field reversals is a general feature of the cycle, and that asymmetric polar field reversals are simply a consequence of asymmetry of solar activity. Subject headings: Sun: surface magnetism — Sun: activity — Sun: dynamo