Abstract
In this paper I examine whether external forcing of the solar dynamo on long timescales can produce detectable signal in the form of long term modulation of the magnetic cycle. This task is motivated in part by some recent proposals (Abreu et al., 2012; Astron. Ap., 548, A88; Stefani et al., 2021; Solar Phys., 296, 88), whereby modulation of the solar activity cycle on centennial and millennial timescales, as recovered from the cosmogenic radioisotope record, is attributed to perturbation of the tachocline driven by planetary orbital motions. Working with a two-dimensional mean-field-like kinematic dynamo model of the Babcock-Leighton variety, I show that such an external forcing signal may be detectable in principle but is likely to be obliterated by other internal sources of fluctuations, in particular stochastic perturbations of the dynamo associated with convective turbulence, unless a very efficient amplification mechanism is at play. I also examine the ability of external tidal forcing to synchronize an otherwise autonomous, internal dynamo operating at a nearby frequency. Synchronization is readily achieved, and turns out to be very robust to the introduction of stochastic noise, but requires very high forcing amplitudes, again highlighting the critical need for a powerful amplification mechanism.
Highlights
The Sun’s 11-year magnetic cycle modulates the frequency of all geoeffective solar eruptive phenomena, and is seen as a key element of space weather research
Based on the results presented in the preceding sections, it is clear that a very strong amplification mechanism is required for planetary gravitational/tidal influences to impact magnetic activity
In this paper paper I have examined to what degree an externallyimposed harmonic forcing operating on timescales much longer than the solar magnetic cycle can leave a detectable signal in time series of magnetic activity proxies, despite the internal “reprocessing” associated with the operation of the dynamo
Summary
The Sun’s 11-year magnetic cycle modulates the frequency of all geoeffective solar eruptive phenomena, and is seen as a key element of space weather research. Variations of the solar cycle amplitude on much longer timescales, from centuries to millenia, are known to take place, and their impact on planetary atmospheres and interplanetary environment define what is known as space climate. The coincidence of the 1645–1715 Maunder Minimum, a period of strongly suppressed solar activity (Eddy, 1976), with the deepest part of the so-called Little Ice Age well known in climatology, continues to fuel speculations regarding the possible influence of solar activity on Earth’s climate. Investigating such questions on a truly physical basis clearly requires a detailed understanding of the mechanism powering the solar magnetic activity cycle
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