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

Thursday, July 25, 2019

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. The solar dynamo is thought to be the responsible mechanism for the generation of the QBOs.

Graphic depicting continuous wavelet spectra of the sunspot area (SSA)
Continuous wavelet spectra of the sunspot area (SSA) data in full disk (a), northern hemisphere (b), southern hemisphere (c). Thick red contours indicate the significance level of 0.05. The main period in the SSA data sets is 11 years corresponding to the Schwabe cycle. Additionally, there are shorter periodicities present in the SSA data below 5 years, which are not the harmonics of the main period of 11 years and are significant at the 0.05 level. These shorter periods indicate the presence of the QBOs in hemispheric and full disk SSA data. The temporal behaviour of the QBO signal in the wavelet spectrum of the full disk SSA data shows in-phase relationship with the Schwabe cycle, where the power of the QBO signal is more prominent during the Schwabe cycle maxima, whereas the QBO signal vanishes during the Schwabe cycle minima.

In this work, we analyze sunspot areas to investigate the spatial and temporal behavior of the QBO signal and study the responsible physical mechanisms using simulations from fully nonlinear mean-field flux-transport dynamos. We investigated the behavior 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 alpha-mechanisms. We then studied the simulated magnetic field strengths as well as meridional circulation and differential rotation rates using the same methods. 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 behaviors, 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 behavior of solar hemispheres nor can they generate QBO-like signals. The turbulent alpha-dynamos, on the other hand, generated decoupled hemispheres and some QBO-like shorter cycles. In conclusion, our simulations show that the turbulent alpha-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.

Publication Name: Astronomy & Astrophysics
First HAO Author's Name: Matthias Rempel