The α effect with imposed and dynamo-generated magnetic fields
The α effect with imposed and dynamo-generated magnetic fields
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外加磁场和发电机产生的磁场的 α 效应
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
U. Helsinki
中科院分区：
文献类型：
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作者：
A. Hubbard;F. Sordo;P. Kapyla;A. B. Nordita;U. Helsinki
本文用两种不同的方法对外加磁场下螺旋湍流中的非线性α效应进行了估算：外加场法，其中使用外加场产生的电动势;测试场法，其中对一组不同的测试场求解单独的演化方程。这两种方法都适用于较强的磁场，但对于较弱的磁场存在明显的差异，这可以通过发电机产生的磁场对被称为中尺度磁场的域的尺度的影响来解释。例子进行了讨论，这些中尺度场可以导致大大高估和低估的α值相比，运动学的情况下。结果表明，通过在一定的时间间隔内将脉动磁场复位到零，可以恢复运动学值。它的结论是，这是首选的技术都强加场和测试场的方法。
Estimates for the non-linear α effect in helical turbulence with an applied magnetic field are presented using two different approaches: the imposed-field method where the electromotive force owing to the applied field is used, and the test-field method where separate evolution equations are solved for a set of different test fields. Both approaches agree for stronger fields, but there are apparent discrepancies for weaker fields that can be explained by the influence of dynamo-generated magnetic fields on the scale of the domain that are referred to as meso-scale magnetic fields. Examples are discussed where these meso-scale fields can lead to both drastically overestimated and underestimated values of α compared with the kinematic case. It is demonstrated that the kinematic value can be recovered by resetting the fluctuating magnetic field to zero in regular time intervals. It is concluded that this is the preferred technique both for the imposed-field and the test-field methods.