Spiral galaxies, including the Milky Way, have large-scale magnetic fields with significant energy densities. The dominant theory attributes these magnetic fields to a large-scale dynamo. We review the current status of dynamo theory and discuss various numerical simulations designed either to explain particular aspects of the problem or to reproduce galactic magnetic fields globally. Our main conclusions can be summarized as follows: ▪ Idealized direct numerical simulations produce mean magnetic fields, whose saturation energy density tends to decline with increasing magnetic Reynolds number. This is still an unsolved problem. ▪ Large-scale galactic magnetic fields of microgauss strengths can probably be explained only if helical magnetic fields of small or moderate length scales can be rapidly ejected or destroyed. ▪ Small-scale dynamos are important throughout a galaxy's life and probably provide strong seed fields at early stages. ▪ The circumgalactic medium (CGM) may play an important role in driving dynamo action at small and large length scales. These interactions between the galactic disk and the CGM may provide important insights into our understanding of galactic dynamos.We expect future research in galactic dynamos to focus on the cosmological history of galaxies and the interaction with the CGM as means of replacing the idealized boundary conditions used in earlier work.

中文翻译：

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银河发电机

包括银河系在内的螺旋星系拥有能量密度极高的大尺度磁场。主流理论将这些磁场归因于大型发电机。我们回顾了发电机理论的现状，并讨论了各种数值模拟，这些模拟旨在解释问题的特定方面或在全球范围内重现银河磁场。我们的主要结论可以概括如下： ▪ 理想化的直接数值模拟产生平均磁场，其饱和能量密度随着磁雷诺数的增加而趋于下降。这仍然是一个未解决的问题。 ▪ 只有小或中等长度尺度的螺旋磁场能够被快速喷射或破坏，才能解释微高斯强度的大尺度银河磁场。 ▪ 小型发电机在星系的整个生命周期中都很重要，并且可能在早期阶段提供强大的种子田。 ▪ 环河介质（CGM）可能在驱动小和大长度尺度的发电机作用方面发挥重要作用。银盘和 CGM 之间的这些相互作用可能为我们对银河发电机的理解提供重要的见解。我们预计银河发电机的未来研究将重点关注星系的宇宙学历史以及与 CGM 的相互作用，作为替代所使用的理想化边界条件的手段在早期的工作中。