A coupled 2 x 2D Babcock-Leighton solar dynamo model. II Reference dynamo solutions
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Astrophysical journal ; vol. 834, no. 2.Publisher(s)
American Astronomical SocietyAbstract(s)
In this paper we complete the presentation of a new hybrid 2 × 2D flux transport dynamo (FTD) model of the solar
cycle based on the Babcock–Leighton mechanism of poloidal magnetic field regeneration via the surface decay of
bipolar magnetic regions (BMRs). This hybrid model is constructed by allowing the surface flux transport (SFT)
simulation described in Lemerle et al. to provide the poloidal source term to an axisymmetric FTD simulation
defined in a meridional plane, which in turn generates the BMRs required by the SFT. A key aspect of this
coupling is the definition of an emergence function describing the probability of BMR emergence as a function of
the spatial distribution of the internal axisymmetric magnetic field. We use a genetic algorithm to calibrate this
function, together with other model parameters, against observed cycle 21 emergence data. We present a reference
dynamo solution reproducing many solar cycle characteristics, including good hemispheric coupling, phase
relationship between the surface dipole and the BMR-generating internal field, and correlation between dipole
strength at cycle maximum and peak amplitude of the next cycle. The saturation of the cycle amplitude takes place
through the quenching of the BMR tilt as a function of the internal field. The observed statistical scatter about the
mean BMR tilt, built into the model, acts as a source of stochasticity which dominates amplitude fluctuations. The
model thus can produce Dalton-like epochs of strongly suppressed cycle amplitude lasting a few cycles and can
even shut off entirely following an unfavorable sequence of emergence events.
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