Solar Cycle Variability Induced by Title Angle Scatter in a Babcockl-Leighton Solar Dynamo Model

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Saturday, April 15, 2017

We present results from a three-dimensional Babcock–Leighton dynamo model that is sustained by the explicit emergence and dispersal of bipolar magnetic regions (BMRs). On average, each BMR has a systematic tilt given by Joy’s law. Randomness and nonlinearity in the BMR emergence of our model produce variable magnetic cycle.

Variability in sunspot number (SSN) in STABLE dynamo simulations of the solar cycle image
Variability in sunspot number (SSN) in STABLE dynamo simulations of the solar cycle using the observed random scatter in sunspot pair tilt angle of 15 degrees. (a) 19 magnetic cycles are highlighted with red and blue representing the northern and southern hemispheres respectively. Red shaded areas indicate periods when the SSN in the north exceeds that in the south and blue shaded areas indicate the opposite. (b) Long-term SSN variability in the same simulation, exhibiting extended periods of low and high activity analogous to grand minima and maxima. Black and red indicate northern and southern hemispheres and the dotted line shows the observed SSN for the Sun, averaged over the last 13 cycles, for comparison.

However, when we allow for a random scatter in the tilt angle to mimic the observed departures from Joy’s law, we find more variability in the magnetic cycles. We find that the observed standard deviation in Joy’s law of 15 degrees produces a variability comparable to observed solar cycle variability of ∼ 32%, as quantified by the sunspot number maxima between 1755–2008. We also find that tilt angle scatter can promote grand minima and grand maxima. The time spent in grand minima for a scatter of 15 degrees is somewhat less than that inferred for the Sun from cosmogenic isotopes (about 9% compared to 17%). However, when we double the tile angle scatter to 30 degrees, the simulation statistics are comparable to the Sun (∼18% of the time in grand minima and ∼ 10% in grand maxima). Though the Babcock–Leighton mechanism is the only source of poloidal field, we find that our simulations always maintain magnetic cycles even at large fluctuations in the tilt angle. Furthermore, we find that moderate scatter in the tilts can enhance the efficiency of the dynamo, giving rise to stronger mean fields. We also demonstrate that tilt quenching is a viable and efficient mechanism for dynamo saturation; a suppression of the tilt by only 1-2 degrees is sufficient to limit the dynamo growth. Thus, any potential observational signatures of tilt quenching in the Sun may be subtle.

Submitted to Astrophysical Journal March 8, 2017.