组合磁场对热驱动风的影响I：偶极场和四极场的相互作用
The Effect of Combined Magnetic Geometries on Thermally Driven Winds I: Interaction of Dipolar and Quadrupolar Fields
摘要：
观测到具有外对流包层的冷恒星具有各种几何形状的磁场，这些磁场在大尺度上由最低阶场的组合（例如偶极、四极和八极模）主导。磁化的恒星风外流是这些天体在主序期间角动量损失的主要原因。先前的工作已经表明，恒星风制动机制的有效性降低，越来越复杂，但奇异的磁场几何形状。在本文中，我们量化的影响，混合偶极和四极场上的自旋下扭矩使用50 MHD模拟与混合场，随着10个纯几何。模拟的风包括一个广泛的磁场强度，并驻留在慢旋转制度。我们发现，从我们的组合几何形状的情况下，恒星风制动扭矩被很好地描述了一个破碎的幂律行为，其中的扭矩缩放与场强可以预测的偶极分量单独或四极缩放利用总的场强。模拟结果可以缩放，并适用于所有的主序星冷。对于太阳参数，场的最低阶分量（本文中的偶极子）在确定角动量损失方面是最重要的。
英文摘要：
Cool stars with outer convective envelopes are observed to have magnetic fields with a variety of geometries, which on large scales are dominated by a combination of the lowest order fields such as the dipole, quadrupole and octupole modes. Magnetised stellar wind outflows are primarily responsible for the loss of angular momentum from these objects during the main sequence. Previous works have shown the reduced effectiveness of the stellar wind braking mechanism with increasingly complex, but singular, magnetic field geometries. In this paper, we quantify the impact of mixed dipolar and quadrupolar fields on the spin-down torque using 50 MHD simulations with mixed field, along with 10 of each pure geometries. The simulated winds include a wide range of magnetic field strength and reside in the slow-rotator regime. We find that the stellar wind braking torque from our combined geometry cases are well described by a broken power law behaviour, where the torque scaling with field strength can be predicted by the dipole component alone or the quadrupolar scaling utilising the total field strength. The simulation results can be scaled and apply to all main-sequence cool stars. For Solar parameters, the lowest order component of the field (dipole in this paper) is the most significant in determining the angular momentum loss.
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