Observations of chromospheric and coronal emissions from various solar-type stars show that the stellar magnetic activity varies with the rotation rates of the stars. The faster the star rotates, its magnetic activity gets stronger but activity cycle period does not show a straightforward variation with the rotation rate. For slowly rotating stars, the cycle period decreases with the increase of rotation rate, while for the fast rotators dependency of cycle period on rotation is presently quite complicated. We aim to provide an explanation of these observational trends of stellar magnetic activity using a dynamo model. We construct a theoretical dynamo model for stars of mass 1 $M_\odot$ based on the kinematic flux transport dynamo model including radial pumping near the surface of the stars. The inclusion of this near surface downward radial pumping is found to be necessary to match the observed surface magnetic field in case of the Sun. The main ingredients of our dynamo model, meridional circulation and differential rotation for stars are obtained from a mean-field hydrodynamic model. Our model shows a decrease of cycle period with increasing rotation rate in the slowly rotating regime and a slight increase of cycle period with rotation rate for the rapid rotators. The strength of the magnetic field is found to be increasing as the rotation rate of the star increases. We also find that the parity of the stellar magnetic field changes with rotation. According to our model, the parity flips to quadrupolar from dipolar if the rotation period of the star is less than 17 days.
Over the course of their lifetimes, the rotation of solar-type stars goes through different phases. Once they reach the zero-age main sequence, their global rotation rate decreases during the main sequence until at least the solar age, approximately following the empirical Skumanich's law and enabling gyrochronology. Older solar-type stars might then reach a point of transition when they stop braking, according to recent results of asteroseismology. Additionally, recent 3D numerical simulations of solar-type stars show that different regimes of differential rotation can be characterized with t…
We investigate the effect of the surface bipolar magnetic regions (BMR) on the large-scale dynamo distributed in the bulk of the convection zone. The study employs the nonlinear 3D mean-field dynamo model. We model the emergence of the BMR on the surface through the nonaxisymmetric magnetic buoyancy effect, which acts on the large-scale toroidal magnetic field in the upper half of the convection zone. The nonaxisymmetric magnetic field which results from this mechanism is shallow. On the surface, the effect of the BMR on the magnetic field generation is dominant. {However, because of the shall…
Observations reveal a relatively small but statistically significant North-South (NS) asymmetry in sunspot activity varying on a time scale of several solar cycles. This paper proposes a dynamo model for the phenomenon of long-term NS asymmetry. The model separates dynamo equations for magnetic fields of dipolar and quadrupolar equatorial parity. The NS asymmetry results from the superposition of dipolar and quadrupolar fields. Model computations confirm the formerly proposed excitation of the quadrupolar dynamo mode by a dominant dipolar mode mediated by the equator-symmetric fluctuations in …
Exploring the Role of a Tachocline in M-Dwarf Magnetism
M型恒星正在迅速成为寻找可居住的类地系外行星的最佳候选者，然而许多M型矮星表现出非凡的耀斑事件，这些事件会用电离辐射轰击其他可居住的行星。近年来，观测者发现，表现出显著磁活动的M星的比例从比光谱型M3.5更早（质量更大）的主序星的大约$10\%$（0.35 M $_\odot$）急剧转变为比光谱型M3.5更晚的恒星的接近$90\%$。暗示它也晚于主序星完全对流的M3.5，并且可能不再包含速跃层。使用球形三维MHD模拟代码瑞利，我们比较的峰值场强，拓扑结构和时间依赖性的对流发电机内产生的快速旋转（2 $Ω_\odot$）M2（0.4 M $_\odot$）明星，与计算域终止于对流区的底部或包括速跃层。我们发现，虽然这两种模式产生强大的（$\sim$ 10千克），花环状的环向场表现出极性反转，速跃层模型提供了一个进一步的水库的环向场，减缓了平均反转周期从100旋转到超过220旋转，并增加了近地表径向场的低阶模式的光谱功率的一个因素为4。
The alpha effect is believed to play a key role in the generation of the solar magnetic field. A fundamental test for its significance in the solar dynamo is to look for magnetic helicity of opposite signs in the two hemispheres, and at small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the magnetic field vector from observations of solar spectra, caused by what is known as the "pi ambiguity" of spectropolarimetric observations. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected…
