On large-scale dynamos with stable stratification and the application to stellar radiative zones
On large-scale dynamos with stable stratification and the application to stellar radiative zones
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DOI：
10.1093/mnras/stac2676
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发表时间：
2022-03
期刊：
影响因子：
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通讯作者：
V. Skoutnev;J. Squire;Ashis Bhattacharjee
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文献类型：
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作者：
V. Skoutnev;J. Squire;Ashis Bhattacharjee
我们对恒星辐射区大尺度磁场的理解仍然是支离破碎和不完整的。这种磁场必须由某种形式的发电机机制产生，被认为是主宰角动量传输的磁场，使它们对恒星演化至关重要。一个主要的困难是稳定层结的影响，这通常会抑制发电机的作用。我们探讨了稳定层结对平均场发电机理论的影响，特别关注了一种称为磁切变电流效应的非螺旋大规模发电机(LSD)机制。我们发现，只要满足其操作的原始要求，该机制对于增加稳定层结是稳健的：剪切和非螺旋磁涨落的来源(例如，来自小型发电机)。两者似乎都源于差异旋转的存在。在平均场理论的支持下，我们的理想化直接数值模拟证明了近等分大尺度环向场的产生。此外，对磁雷诺数的扫描显示，LSD的增长或饱和没有变化，这为发电机机制对灾难性猝灭的弹性提供了很好的数值证据，这一直是螺旋发电机的一个问题。这些特性--没有灾难性的猝灭和对稳定层结的稳健性--使该机制成为在恒星辐射区产生就地大范围磁场的合理候选者。
Our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. Such magnetic fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution. A major difficulty is the effect of stable stratification, which generally suppresses dynamo action. We explore the effects of stable stratification on mean-field dynamo theory with a particular focus on a non-helical large-scale dynamo (LSD) mechanism known as the magnetic shear-current effect. We find that the mechanism is robust to increasing stable stratification as long as the original requirements for its operation are met: a source of shear and non-helical magnetic fluctuations (e.g. from a small-scale dynamo). Both are plausibly sourced in the presence of differential rotation. Our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. Additionally, a scan over magnetic Reynolds number shows no change in the growth or saturation of the LSD, providing good numerical evidence of a dynamo mechanism resilient to catastrophic quenching, which has been an issue for helical dynamos. These properties -- the absence of catastrophic quenching and robustness to stable stratification -- make the mechanism a plausible candidate for generating in-situ large-scale magnetic fields in stellar radiative zones.