On the Origin of Long-term Modulation in the Sun’s Magnetic Activity Cycle

One of the most striking manifestations of orderly behavior emerging out of complex interactions in any astrophysical system is the 11 yr cycle of sunspots. However, direct sunspot observations and reconstructions of long-term solar activity clearly exhibit amplitude fluctuations beyond the decadal timescale, which may be termed as supradecadal modulation. Whether this long-term modulation in the Sun’s magnetic activity results from nonlinear mechanisms or stochastic perturbations remains controversial and a matter of active debate. Utilizing multimillennial-scale kinematic dynamo simulations based on the Babcock–Leighton paradigm—in the likely (near-critical) regime of operation of the solar dynamo—we demonstrate that this supradecadal modulation in solar activity cannot be explained by nonlinear mechanisms alone; stochastic forcing is essential for the manifestation of observed long-term fluctuations in the near-critical dynamo regime. Our findings substantiate some independent observational and theoretical investigations and provide additional insights into temporal dynamics associated with a plethora of natural phenomena in astronomy and planetary systems arising from weakly nonlinear, nondeterministic processes.