Solar Cycle Variability Induced by Tilt Angle Scatter in a Babcock–Leighton Solar Dynamo Model

Abstract

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 cycles. 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 σ δ = 15• 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 σ δ = 15• is somewhat less than that inferred for the Sun from cosmogenic isotopes (about 9% compared to 17%). However, when we double the tilt scatter to σ δ = 30• , 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. We also demonstrate that tilt quenching is a viable and efficient mechanism for dynamo saturation; a suppression of the tilt by only 1-2• is sufficient to limit the dynamo growth. Thus, any potential observational signatures of tilt quenching in the Sun may be subtle.

Impact of rogue active regions on hemispheric asymmetry

“…In contrast to the results of Karak and Miesch (2017) we found strong correlation between the hemispheric asymmetry of the polar cap flux of cycle i and the asymmetry of hemispheric activity levels during the subsequent cycle i + 1. The time lag between the hemispheric lag in the onset of cycle i + 1 is also strongly correlated to the asymmetry of the polar cap flux of the preceding cycle.…”

“…According to their results, the intense inflow in one hemisphere leads to stronger toroidal fields and this asymmetry is sustained for more than one cycle. Karak and Miesch (2017), using their 3D surface flux transport and Babcock-Leighton solar dynamo model (Miesch and Dikpati 2014;Miesch and Teweldebirhan 2016), investigated the influence of the tilt angle distribution by adding random scatter on Joy's law and a tilt-angle saturation was also added to the model (on this point see also Lemerle and Charbonneau 2017).…”

Average motion of emerging solar active region polarities

“…This scatter is essential to producing a varying solar cycle amplitude in Babcock-Leighton and surface flux transport models (e.g. Karak & Miesch 2017). We tested this theory by looking at the scatter of the separation between the polarities in high and low flux active regions.…”

Predictability of the Solar Cycle Over One Cycle

“…By including detailed information about the individual tilt angles and magnetic polarities of the bipolar magnetic regions (BMRs) that emerged during cycle 23, the weak polar field around the end of cycle 23 is well reproduced by Jiang et al (2015). Recently series of BL dynamo models successfully reproduce the variability of the solar cycle by introducing random BMR tilts (Jiang et al 2013;Olemskoy & Kitchatinov 2013;Cameron & Schüssler 2017;Nagy et al 2017;Karak & Miesch 2017). The randomness of the sunspot emergence -part of the toroidal-to-poloidal part of the dynamo loop-makes the dynamo a stochastic process.…”