Propagation of non-linear circularly polarised Alfvén waves in a homogeneous medium
Propagation of non-linear circularly polarised Alfvén waves in a homogeneous medium
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非线性圆偏振阿尔文波在均匀介质中的传播
DOI：
10.1051/0004-6361:20031167
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发表时间：
2003
影响因子：
6.5
通讯作者：
U. Torkelsson
中科院分区：
文献类型：
--
作者：
R. Turkmani;U. Torkelsson
通过数值求解一维磁流体力学（MHD）方程，研究了非线性圆极化Alfven波的演化。我们研究了波的行为，发现不同的物理机制与不同的β范围有关。在低β等离子体中，波可能经历参数衰减。这是因为波激发了密度增强，其传播速度比波本身慢，因此与波相互作用。当$\beta \ge 1$时，密度增强不会与波相互作用，也不会发生衰减，相反，阿尔芬波会被密度增强反射。反射区以速度$\frac{1}{n} \，v_{\rm A}$传播。由于磁通量是守恒的，这导致振荡磁场放大了一个因子$\frac{1}{n}$。我们发现n依赖于β，特别是对于$\beta \sim 1$的值，它≤ 1，对于$\beta \gg 1$，它≥ 1。我们讨论了这些机制的相关性，太阳风的加速，并在极风区的MHD湍流的触发。特别是这些模拟可以解释存在向内传播的阿尔芬波在太阳日冕。
We study the evolution of non-linear circularly polarised Alfven waves by solving numerically the time-dependent equations of magnetohydrodynamics (MHD) in one dimension. We examine the behaviour of the waves and find that different physical mechanisms are relevant in different ranges of β . In a low β plasma the wave may undergo a parametric decay. This is because the wave excites a density enhancement that travels slower than the wave itself and thus interacts with the wave. When $\beta \ge 1$ the density enhancement does not interact with the wave and no decay takes place, instead the Alfven wave is reflected against the density enhancement. The reflection zone propagates with the speed $\frac{1}{n} \, v_{\rm A}$. Because of that the magnetic flux is conserved which results in an amplification of the oscillating magnetic field by a factor $\frac{1}{n}$. We find that n depends on β , and that in particular it is ≤ 1 for values of $\beta \sim 1$ and ≥ 1 for $\beta \gg 1$. We discuss the relevance of these mechanisms to the acceleration of the solar wind, and the triggering of MHD turbulence in the polar wind region. In particular these simulations can explain the presence of inward propagating Alfven waves in the solar corona.