Constraining non-linear dynamo models using quasi-biennial oscillations from sunspot area data

Abstract

Context. Solar magnetic activity exhibits variations with periods between 1.5-4 years, the so-called quasi-biennial oscillations (QBOs), in addition to the well-known 11-year Schwabe cycles. Solar dynamo is thought to be the responsible mechanism for generation of the QBOs. Aims. In this work, we analyse sunspot areas to investigate the spatial and temporal behaviour of the QBO signal and study the responsible physical mechanisms using simulations from fully nonlinear mean-field flux-transport dynamos. Methods. We investigated the behaviour of the QBOs in the sunspot area data in full disk, and northern and southern hemispheres, using wavelet and Fourier analyses. We also ran solar dynamos with two different approaches to generating a poloidal field from an existing toroidal field, Babcock-Leighton and turbulent α mechanisms. We then studied the simulated magnetic field strengths as well as meridional circulation and differential rotation rates using the same methods. Results. The results from the sunspot areas show that the QBOs are present in the full disk and hemispheric sunspot areas and they show slightly different spatial and temporal behaviours, indicating a slightly decoupled solar hemispheres. The QBO signal is generally intermittent and in-phase with the sunspot area data, surfacing when the solar activity is in maximum. The results from the BL-dynamos showed that they are neither capable of generating the slightly decoupled behaviour of solar hemispheres nor can they generate QBO-like signals. The turbulent α-dynamos, on the other hand, generated decoupled hemispheres and some QBO-like shorter cycles. Conclusions. In conclusion, our simulations show that the turbulent α-dynamos with the Lorentz force seems more efficient in generating the observed temporal and spatial behaviour of the QBO signal compared with those from the BL-dynamos.

Does the mean-fieldαeffect have any impact on the memory of the solar cycle?

“…Most of the dynamo-based prediction models completely ignore the contribution of distributed mean-field α effect; they consider the Babcock-Leighton mechanism as the only poloidalfield-generation mechanism for their prediction models. However, some studies indicate that the mean-field α effect plays an important role in solar dynamo models and is necessary to recover the solar cycle from grand-minima-like episodes (Pipin & Kosovichev 2011;Pipin et al 2013;Passos et al 2014;Hazra et al 2014a;Inceoglu et al 2019). Recently, Bhowmik & Nandy (2018) considered both the Babcock-Leighton mechanism and mean-field α effect as poloidal-field-generation mechanisms in their model to predict the strength of solar cycle 25.…”

The QBO-type signals in the subsurface flow fields during solar cycles 23 and 24

“…The amplitude of the QBOs are in phase with the Schwabe cycle, attaining their highest amplitude during solar cycle maxima and become weaker during solar cycle minima. The QBOs are observed to behave differently in each solar hemisphere while showing the same intermittency (Bazilevskaya et al 2014;Inceoglu et al 2019). Together with exhibiting signals distributed over all solar latitudes in the magnetic synoptic maps (Vecchio et al 2012), they are also reported to be present in the high solar latitudes (≥ 60 • ) using monthly values of the polar faculae data recorded between 1951 and 1998 (Deng et al 2020).…”

Causal Interaction between the subsurface rotation rate residuals and radial magnetic field in different timescales

“…The tail-like structure, indicating the overlapping period between two consecutive cycles, in the flow field can be explained by flux transport dynamos with Babcock-Leighton (BL) mechanism together with turbulent α-effect operating throughout the convection zone are the main source for the generation of the poloidal field from a pre-existing toroidal field (Passos et al 2014;Simoniello et al 2016). Solar dynamos that use only the BL mechanism as well as only thin-shell dynamos with turbulent α-effect, on the other hand, tend to generate longer overlapping periods between the cycles (Dikpati & Gilman 2001;Dikpati et al 2005;Bushby 2006;Karak & Miesch 2017;Inceoglu et al 2017Inceoglu et al , 2019. An interesting feature that can be observed is the absence of this behavior at depths below ∼0.95R .…”

“…The QBOs are shown to be more intermittent signals and the variations in their amplitude are inphase with the Schwabe cycle, meaning they attain their highest (lowest) amplitude during the solar cycle maxima (minima) (Bazilevskaya et al 2014). Together with exhibiting signals over all solar latitudes (Vecchio et al 2012), they are also shown to behave differently in each solar hemisphere (Gurgenashvili et al 2017;Inceoglu et al 2019). The QBOs are found to be present from the subsurface layers to the surface of the Sun, and they can even be identified in the neutron counting rates measured on Earth as indicators of the Galactic Cosmic Ray intensities (Benevolenskaya 1998;Kudela et al 2010;Simoniello et al 2012;Vecchio et al 2012).…”

“…Causal interaction between the flow and magnetic fields 3 Among the physical mechanisms that were proposed to explain the existence and spatio-temporal behaviors of the QBOs, we can include spatio-temporal fragmentation of radial profiles of the rotation rates (Simoniello et al 2013), 180 • shifting of the active longitudes (Berdyugina & Usoskin 2003), a secondary dynamo operating in the subsurface shear layer at 0.95R (Benevolenskaya 1998), instability of the magnetic Rossby waves in the tachocline (Zaqarashvili et al 2010), and tachocline nonlinear oscillations (TNOs) through periodic energy exchange between the Rossby waves, differential rotation, and the present toroidal field (Dikpati et al 2018). In addition, based on results from a fully nonlinear flux transport dynamos, Inceoglu et al (2019) proposed that there are indications for the QBOs to be generated via interplay between the flow and magnetic fields, where the turbulent α-mechanism working in the lower half of the solar convection zone, which extends from 0.70R to the surface. The bottom of the convection zone overlaps with the region of strong radial shear.…”