磁层顶边界层剪切流引起的MHD波模转化

采用非本征模方法（non-modal analysis）研究了磁层顶边界层中剪切流导致的MHD波模转化及其与背景流场的能量交换过程.发现在分别代表磁层顶内、外边界层及过渡区的均匀剪切流场中，初始设定的Alfvén波扰动可部分转化为快、慢磁声波.而且，在不同区域，根据等离子体参数的不同，发生的波模转化过程也不相同.在外边界，Alfvén波主要转化为慢磁声波；在内边界，Alfvén波则主要转化为快磁声波；而在二者之间的过渡区中，Alfvén波可同时转化为两种类型的磁声波.我们还发现，含有较强快波分量的扰动可从磁层顶剪切流场中获得能量而得到线性放大.上述物理过程可能对解释磁鞘至磁层的能量及动量异常输运现象有所帮助.

Wave transformation induced by shear flow in the magnetopause boundary layer

We study wave transformation induced by shear flow and energy exchange between the magnetohydrodynamic waves and mean flow in the magnetopause boundary layer (BL) with the non modal approach. The magnetopause is assumed to be consisting of three BLs: the inner, outer layers and the transition region with uniform velocity shear and different plasma parameters. It is found that an initially given Alfvénic perturbation experiences distinct changes in different regions. It is mainly transformed into the slow mode in the outer part of the magnetopause BL and fast mode in the inner part, while in the transition region both modes with comparative amplitudes can be obtained. We also find that the fluctuations with stronger fast mode oscillations can gain more energy from the background shear flow and get amplified as a result. The physical processes described above may be useful in understanding the anomalous transport phenomena of energy and momentum from the magnetosheath to the magnetosphere.