The hemispheric asymmetry of solar activity during the last century and the solar dynamo

Abstract

We believe the Babcock–Leighton process of poloidal field generation to be
the main source of irregularity in the solar cycle. The random nature of this process may
make the poloidal field in one hemisphere stronger than that in the other hemisphere at
the end of a cycle. We expect this to induce an asymmetry in the next sunspot cycle. We
look for evidence of this in the observational data and then model it theoretically with
our dynamo code. Since actual polar field measurements exist only from the 1970s, we
use the polar faculae number data recorded by Sheeley (1991, 2008) as a proxy of the
polar field and estimate the hemispheric asymmetry of the polar field in different solar
minima during the major part of the twentieth century. This asymmetry is found to have
a reasonable correlation with the asymmetry of the next cycle. We then run our dynamo
code by feeding information about this asymmetry at the successive minima and compare
the results with observational data. We find that the theoretically computed asymmetries
of different cycles compare favorably with the observational data, with the correlation coefficient
being 0.73. Due to the coupling between the two hemispheres, any hemispheric
asymmetry tends to get attenuated with time. The hemispheric asymmetry of a cycle either
from observational data or from theoretical calculations statistically tends to be less
than the asymmetry in the polar field (as inferred from the faculae data) in the preceding
minimum. This reduction factor turns out to be 0.43 and 0.51 respectively in observational
data and theoretical simulations.