BENCHMARKING FAST-TO-ALFVÉN MODE CONVERSION IN A COLD MAGNETOHYDRODYNAMIC PLASMA
BENCHMARKING FAST-TO-ALFVÉN MODE CONVERSION IN A COLD MAGNETOHYDRODYNAMIC PLASMA
复制标题
冷磁流体动力学等离子体中快速到 ALFVÉN 模式转换的基准测试
DOI：
--
复制
发表时间：
2011
期刊：
影响因子：
--
通讯作者：
Shelley C. Hansen
中科院分区：
文献类型：
--
作者：
P. Cally;Shelley C. Hansen
alfvn波可能是由快速磁声波在太阳大气中反射水平附近的模式转换产生的，这对日冕振荡和活动区日震学都有影响。在活动区域，这种反射通常发生在足够高的地方，以至于阿尔夫萨芬速度a大大超过声速c，远远高于快、慢模式相互作用的a = c水平。因此，为了把重点放在快速/阿尔夫萨默转换的基本特征上，去掉不必要的细节，采用重力分层冷磁流体力学模型c→0来冻结慢速模式是有用的。这为在更复杂的环境中快速到alvvac模式转换提供了基准。假设有均匀的倾斜磁场和密度标度高度为h的指数alfv录影带速度剖面，则alfv录影带转换系数仅取决于三个变量：分层方向的无量纲横向波数κ = kh，从分层方向的磁场倾角θ，以及波矢量相对于包含分层和磁场方向的平面的极化角φ。我们在这个参数空间中对模式转换进行了广泛的探索，并得出结论，对于小θ和大φ（80°-90°），几乎完全转换为向外传播的alfv<s:1> n波通常发生，尽管当θ为零（垂直场）时完全不存在。对于感兴趣的日震波数，转换区域足够宽，可以覆盖整个色球层。
Alfvén waves may be generated via mode conversion from fast magnetoacoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helioseismology. 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 magnetohydrodynamic model c → 0. 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°–90°), 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.