[论文解读] Theories of the solar cycle : a critical view

H. C. Spruit|arXiv (Cornell University)|Apr 26, 2010

Solar and Space Plasma Dynamics参考文献 5被引用 5

一句话总结

本文主张太阳周期主要由磁场自身的对流不稳定性驱动，而非传统假设的湍流对流。它挑战了平均场发电机理论，认为差速旋转将偶极磁场拉伸为环形磁场，后者因浮力而爆发——该观点得到活动区浮现和纬向漂移的观测证据支持。

ABSTRACT

Some established views of the solar magnetic cycle are discussed critically, with focus on two aspects at the core of most models: the role of convective turbulence, and the role of the `tachocline' at the base of the convection zone. The standard view which treats the solar cycle as a manifestation of the interaction between convection and magnetic fields is shown to be misplaced. The main ingredient of the solar cycle, apart from differential rotation, is instead buoyant instability of the magnetic field itself. This view of the physics of the solar cycle was already established in the 1950s, but has been eclipsed mathematically by mean field turbulence formalisms which make poor contact with observations and have serious theoretical problems. The history of this development in the literature is discussed critically. The source of the magnetic field of the solar cycle is currently assumed to be located in the `tachocline': the shear zone at the base of the convection zone. While the azimuthal field of the cycle is indeed most likely located at the base of the convection zone, it cannot be powered by the radial shear of the tachocline as assumed in these models, since the radiative interior does not support significant shear stresses. Instead, it must be the powered by the latitudinal gradient in rotation rate in the convection zone, as in early models of the solar cycle. Possible future directions for research are briefly discussed.

研究动机与目标

• 批判性地重新审视将太阳周期视为由湍流对流和平均场发电机过程驱动的标准观点。
• 主张观测证据（尤其是活动区浮现与向赤道漂移）表明，磁浮力才是主要机制。
• 质疑热壳层作为发电机源点的作用，断言那里的径向剪切无法维持显著的剪切应力。
• 重申早期基于纬向差速旋转与磁浮力不稳定的模型的重要性，这些模型曾被平均场形式化理论所掩盖。
• 指出磁质稳定性、混合极性磁通量退火以及10⁵ G磁场强度下的热力学平衡等未解问题。

提出的方法

• 分析活动区浮现的观测数据，包括倾角、向赤道漂移和向极区传播，以推断磁浮力的作用。
• 使用简单的差速旋转模型，模拟偶极磁场的缠绕，预测环形磁场增长的纬向依赖性，与观测到的活动区漂移一致。
• 评估在对流层底界10⁵ G磁场的热力学稳定性，发现实现中性浮力需温度降低约100倍于典型对流涨落。
• 批判性地审视平均场湍流形式化的局限性，特别是其与观测的脱节及理论上的不一致。
• 回顾3D辐射磁流体动力学模拟，其成功再现了磁通量浮现与黑子结构，支持浮力驱动模型。
• 提出表面混合极性磁通量的退火过程可能是一个关键但尚不明确的机制，可能影响总太阳辐照度。

实验结果

研究问题

• RQ1 为何太阳周期表现出活动区向赤道漂移？其背后的物理机制是什么？
• RQ2 观测到的活动区浮现（包括倾角与磁极性分离）能否由磁浮力而非湍流对流解释？
• RQ3 为何热壳层虽存在径向剪切，却不能作为驱动环向磁场的可行来源？
• RQ4 如何使约10⁵ G的磁场在对流层底部稳定分层而不迅速发生浮力上升？
• RQ5 分散的混合极性磁通量退火过程在太阳周期中扮演何种角色？它是否可能影响总太阳辐照度？

主要发现

• 太阳周期由环形磁场的浮力不稳定性驱动，而非湍流对流或平均场发电机过程。
• 活动区向赤道漂移及活动区向极区传播的观测与模型一致：差速旋转导致场强随纬度增加，约在60°纬度处首先达到不稳定性。
• 热壳层无法通过径向剪切为环向磁场供能，因为辐射层无法维持显著剪切应力。
• 偶极磁场的主要来源很可能是对流层内旋转速率的纬向梯度，如早期模型所提出的。
• 约10⁵ G的磁场需温度降低约100倍于典型对流涨落，才能实现中性浮力，构成显著的热力学挑战。
• 近期的3D辐射磁流体动力学模拟成功再现了磁通量浮现与黑子结构，为浮力驱动模型提供了可信支持，并对过时的平均场方法构成挑战。