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arXiv 1105.5754astro-ph.SR
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冷MHD等离子体中快-阿尔芬模转换的基准测试

Benchmarking Fast-to-Alfven Mode Conversion in a Cold MHD Plasma

Paul S. Cally, Shelley C. Hansen

备注：
14 pages plus supplementary tables. Astrophys J (accepted 25 May 2011). Two ancillary animations (animated gif) attached
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摘要：

阿尔文波可能是由快磁声波在太阳大气中反射层附近通过模式转换产生的，这对日冕振荡和活跃区日震学都有影响。在活跃区域，这种反射通常发生得足够高，以至于阿尔文速度$a$大大超过声速$c$，远高于快模式和慢模式相互作用的$a=c$水平。为了集中讨论快/阿尔文转换的基本特性，去掉不必要的细节，采用重力分层冷MHD模型$c\to0$来冻结慢模是有用的。这为在更复杂的大气中快速到阿尔文模式转换提供了基准。假设一个均匀的倾斜磁场和一个指数阿尔文速度分布与密度标度高度$h$，阿尔文转换系数只取决于三个变量;无量纲的横向于分层的波数$κ=kh$、磁场相对于分层方向的倾斜度$θ$以及波矢量相对于包含分层和磁场方向的平面的偏振角$ϕ$。我们提出了一个广泛的探索模式转换在这个参数空间，并得出结论，几乎完全转换为向外传播的阿尔文波通常发生在小$θ$和大$ϕ$（$80^\circ$ - $90^\circ$），虽然它是完全不存在时，$θ$正好为零（垂直场）。对于日震感兴趣的波数，转换区域足够宽，可以覆盖整个色球层。

英文摘要：

Alfvén waves may be generated via mode conversion from fast magneto-acoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helio-seismology. In active regions this reflection typically occurs high enough that the Alfvén speed $a$ greatly exceeds the sound speed $c$, well above the $a=c$ level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfvén conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold MHD model $c\to0$. This provides a benchmark for fast-to-Alfvén mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfvén speed profile with density scale height $h$, the Alfvén conversion coefficient depends on three variables only; the dimensionless transverse-to-the-stratification wavenumber $κ=kh$, the magnetic field inclination from the stratification direction $θ$, and the polarization angle $ϕ$ of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfvén waves typically occurs for small $θ$ and large $ϕ$ ($80^\circ$--$90^\circ$), though it is absent entirely when $θ$ is exactly zero (vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.

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