Abstract:
We have defined the duration of polar magnetic activity as the time interval between two
successive polar reversals. The epochs of the polarity reversals of the magnetic field at the poles of
the Sun have been determined (1) by the time of the final disappearance of the polar crown filaments
and (2) by the time between the two neighbouring reversals of the magnetic dipole configuration
(l = 1) from the Hα synoptic charts covering the period 1870–2001. It is shown that the reversals
for the magnetic dipole configuration (l = 1) occur on an average 3.3 ± 0.5 years after the sunspot
minimum according to the Hα synoptic charts (Table I) and the Stanford magnetograms (Table III). If
we set the time of the final disappearance of the polar crown filaments (determined from the latitude
migration of filaments) as the criterion for deciding the epoch of the polarity reversal of the polar
fields, then the reversal occurs on an average 5.8 ± 0.6 years from sunspot minimum (last column
of Table I). We consider this as the most reliable diagnostic for fixing the epoch of reversals, as the
final disappearance of the polar crown filaments can be observed without ambiguity. We show that
shorter the duration of the polar activity cycle (i.e., the shorter the duration between two neighbouring
reversals), the more intense is the next sunspot cycle.We also notice that the duration of polar activity
is always more in even solar cycles than in odd cycles whereas the maximum Wolf numbers Wmax
is always higher for odd solar cycles than for even cycles. Furthermore, we assume there is a secular
change in the duration of the polar cycle. It has decreased by ∼1.2 times during the last 120 years
