Nonlinear Evolution of Short-wavelength Torsional Alfvén Waves
Nonlinear Evolution of Short-wavelength Torsional Alfvén Waves
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DOI：
10.3847/1538-4357/aa6c65
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发表时间：
2017-05
期刊：
影响因子：
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通讯作者：
S. Shestov;V. Nakariakov;A. Ulyanov;A. Reva;S. Kuzin
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文献类型：
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作者：
S. Shestov;V. Nakariakov;A. Ulyanov;A. Reva;S. Kuzin
我们分析了填充低β等离子体和周围有低密度等离子体的直磁通管中扭转Alfvén波的非线性演化。具体地说，这种磁管模拟日冕环或极地羽流的一段。波长与管的半径相当。我们用理想磁流体力学对波的传播进行了数值模拟。我们发现，扭转波非线性地诱导了三种压缩流：构成沿磁场的整体等离子体运动的Alfvén速度的平行流、管波以及与香肠快速磁声模式相关的径向横向流。此外，非线性扭转波变陡，传播速度加快。后一种效应导致扭转波前的渐进性失真，即非线性混频。由于扭转波幅沿圆管半径方向的固有不均匀性，非线性效应在波幅较大的区域更为明显。它们总是不在通量管的轴线上。在波幅呈线性径向分布的情况下，非线性效应局限在管子边界附近的环空区域。因此，太阳和恒星日冕中由扭转Alfvén波驱动的平行压缩流在垂直方向上基本上是不均匀的。附加波能汇的存在降低了扭转Alfvén波中非线性平行叶栅的效率。
We analyze nonlinear evolution of torsional Alfvén waves in a straight magnetic flux tube filled in with a low-β plasma, and surrounded with a plasma of lower density. Such magnetic tubes model, in particular, a segment of a coronal loop or a polar plume. The wavelength is taken comparable to the tube radius. We perform a numerical simulation of the wave propagation using ideal magnetohydrodynamics. We find that a torsional wave nonlinearly induces three kinds of compressive flows: the parallel flow at the Alfvén speed, which constitutes a bulk plasma motion along the magnetic field, the tube wave, and also transverse flows in the radial direction, associated with sausage fast magnetoacoustic modes. In addition, the nonlinear torsional wave steepens and its propagation speed increases. The latter effect leads to the progressive distortion of the torsional wave front, i.e., nonlinear phase mixing. Because of the intrinsic non-uniformity of the torsional wave amplitude across the tube radius, the nonlinear effects are more pronounced in regions with higher wave amplitudes. They are always absent at the axes of the flux tube. In the case of a linear radial profile of the wave amplitude, the nonlinear effects are localized in an annulus region near the tube boundary. Thus, the parallel compressive flows driven by torsional Alfvén waves in the solar and stellar coronae, are essentially non-uniform in the perpendicular direction. The presence of additional sinks for the wave energy reduces the efficiency of the nonlinear parallel cascade in torsional Alfvén waves.