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arXiv 2407.09042astro-ph.SR
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存在小尺度发电机时大尺度发电机转变附近的磁滞现象

Hysteresis near the transition of the large-scale dynamo in the presence of the small-scale dynamo

Vindya Vashishth

备注：
11 pages, 9 figures, Under review in Solar Physics journal
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AI总结：

本研究通过含螺旋驱动湍流的αΩ发电机模型模拟，证实存在小尺度发电机时大尺度发电机仍有亚临界分支与磁滞行为，支持慢速旋转恒星亚临界区存在大尺度发电机的可能。

摘要：

在太阳与类太阳恒星中，大尺度发电机的运行存在一个临界发电机数，低于该值时发电机将停止运作，这一区域被称为亚临界区。此前研究表明，在不存在小尺度发电机的情况下，类太阳大尺度（全球）发电机有可能在亚临界区运行。由于在太阳对流区中，大尺度与小尺度发电机预计会在同一时间、同一位置运行，我们在一个能同时激发大、小尺度发电机的数值模型中，检验了此前识别出的亚临界发电机分支的稳健性。为此，我们使用Pencil Code（铅笔代码），构建了一个带有均匀剪切和螺旋驱动湍流的αΩ发电机模型。我们开展了几组不同相对螺旋度的模拟，以探究存在小尺度发电机时大尺度振荡场的产生情况。我们发现，在部分参数区间内，发电机表现出磁滞行为，即根据所用初始参数的不同，可能存在两种发电机解：当初始磁场较弱时，发电机呈衰减解；当初始磁场较强时，则呈强振荡解。由此证实，在存在小尺度发电机的情况下，大尺度发电机的亚临界分支确实存在。不过，与不存在小尺度发电机的情况相比，磁滞区间相当狭窄。我们的研究为慢速旋转恒星的亚临界区中可能存在大尺度发电机提供了支持。

英文摘要：

In Sun and solar-type stars, there is a critical dynamo number for the operation of a large-scale dynamo, below which the dynamo ceases to operate. This region is known as the subcritical region. Previous studies showed the possibility of operating the solar-like large-scale (global) dynamo in the subcritical region without a small-scale dynamo. As in the solar convection zone, both large- and small-scale dynamos are expected to operate at the same time and location, we check the robustness of the previously identified subcritical dynamo branch in a numerical model in which both large- and small-scale dynamos are excited. For this, we use the {\sc Pencil Code} and set up an $αΩ$ dynamo model with uniform shear and helically forced turbulence. We have performed a few sets of simulations at different relative helicity to explore the generation of large-scale oscillatory fields in the presence of small-scale dynamo. We find that in some parameter regimes, the dynamo shows hysteresis behavior, i.e., two dynamo solutions are possible depending on the initial parameters used. A decaying solution when the dynamo was started with a weak field and a strong oscillatory solution if the dynamo was initialized with a strong field. Thus, the existence of the sub-critical branch of the large-scale dynamo in the presence of small-scale dynamo is established. However, the regime of hysteresis is quite narrow with respect to the case without the small-scale dynamo. Our work supports the possible existence of large-scale dynamo in the sub-critical regime of slowly rotating stars.

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