Variabilities in the polar field and solar cycle due to irregular properties of Bipolar Magnetic Regions
arxiv(2024)
摘要
Decay and dispersal of the tilted Bipolar Magnetic Regions (BMRs) on the
solar surface are observed to produce large-scale poloidal field, which acts as
the seed for the toroidal field and, thus, the next sunspot cycle. However,
various properties of BMR, namely, the tilt, time delay between successive
emergences, location, and flux, all have irregular variations. Previous studies
show that these variations can lead to changes in the polar field. In this
study, we first demonstrate that our 3D kinematic dynamo model, STABLE,
reproduces the robust feature of the surface flux transport (SFT) model, namely
the variation of the generated dipole moment with the latitude of the BMR
position. Using STABLE in both SFT and dynamo modes, we perform simulations by
varying the individual properties of BMR and keeping their distributions the
same in all the cycles as inspired by the observations. We find that randomness
due to the distribution in either the time delay or the BMR latitude produces
negligible variation in the polar field and the solar cycle. However,
randomness due to BMR flux distribution produces substantial effects, while the
scatter in the tilt around Joy law produces the largest variation. Our
comparative analyses suggest that the scatter of BMR tilt around Joy law is the
major cause of variation in the solar cycle. Furthermore, our simulations also
show that the magnetic field-dependent time delay of BMR emergence produces
more realistic features of the magnetic cycle, consistent with observation.
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