Does the Poleward Migration Rate of the Magnetic Fields Depend on the Strength of the Solar Cycle?

Abstract

We present the pattern of the polar magnetic reversal for cycle 23 derived from Hα synoptic charts and have also included the reversals of the earlier cycles 18–22 for comparison. At the beginning of a new cycle (i.e., soon after the polar reversal) the zonal boundaries of unipolar magnetic regions of opposite polarities (seen as filament bands on the synoptic charts) appear close to and on either side of the equator continuing through the years of minimum indicating the onset of the cancellation of flux at these low latitudes. The cycle thus starts with cancellation of flux close to the equator and ends with the polar reversal or flux cancellation near the poles. The filament bands just below the polemost ones migrate and reach latitudes 35°–45° by the time of polar reversal and become the polemost, once the polar reversal has taken place. During the years of minimum that follow, these filament bands remain more or less stagnant at the latitudes 35°–45° except for occasional slow migration towards the equator. The migration to the poles starts at a low speed of 3 m s⁻¹ only when the spot activity has risen to a significant level and then it accelerates to 30 m s⁻¹ at the peak of the activity. It takes 3–4 years for the polemost bands to reach the poles moving at these high speeds. We quantify this possible cause and effect phenomenon by introducing the concept of the `strength of the solar cycle' and represent this by either of a set of three parameters. We show that the velocity of poleward migration is a linear function of the `strength of the solar cycle'.