Does Nonaxisymmetric Dynamo Operate in the Sun?

Abstract

We explore effects of random non-axisymmetric perturbations of kinetic helicity (the α effect) and diffusive decay of bipolar magnetic regions on generation and evolution of large-scale non-axisymmetric magnetic fields on the Sun. Using a reduced 2D nonlinear mean-field dynamo model and assuming that bipolar regions emerge due to magnetic buoyancy in situ of the large-scale dynamo action, we show that fluctuations of the α effect can maintain the non-axisymmetric magnetic fields through a solar-type α 2 Ω dynamo process. It is found that diffusive decay of bipolar active regions is likely to be the primary source of non-axisymmetric magnetic fields observed on the Sun. Our results show that non-axisymmetric dynamo models with stochastic perturbations of the α effect can explain periods of extremely high activity ('super-cycle' events) as well as periods of deep decline of magnetic activity. We compare the models with synoptic observations of solar magnetic fields for the last four activity cycles, and discuss implications of our results for interpretation of observations of stellar magnetic activity.

Evolution of Magnetic Helicity in Solar Cycle 24

“…We use results of the non-axisymmetric α 2 Ω dynamo model developed recently by Pipin & Kosovichev (2018). The model simulates the solar-type dynamo, in which the surface magnetic activity is governed by the dynamogenerated axisymmetric toroidal magnetic field.…”

“…In order to mimic emergence of solar active regions, we take into account the Parker's magnetic buoyancy instability, which produces bipolar regions from the toroidal magnetic field at random latitudes and random moment of time under the condition that the magnetic field strength exceeds a critical threshold. The reader can find a detailed description of the model and the code in Pipin & Kosovichev (2018) and Pipin (2018). Figure 1 shows snapshots of the magnetic field components, as well as the magnetic and current helicity densities.…”

The magnetic helicity density patterns from non-axisymmetric solar dynamo

“…We find that the emerging active regions can create a bias in our conclusions about the helical properties of the dynamo inside the convection zone. This study is based on the numerical simulations of the non-axisymmetric dynamo model, which was suggested recently by Pipin & Kosovichev (2018). The next section describes the model.…”

Shape of Solar Cycles and Mid-Term Solar Activity Oscillations

“…Выбор модели динамо определялся необходимостью воспроизведения одновременно симметричных и асимметричных деталей магнитного поля. В работе использован упрощенный вариант модели, предложенной в [3]. Динамо возбуждается в тонком слое в глубине конвективной зоны, возникновение биполярных активных областей воспроизводится с помощью магнитной плавучести.…”