Effect of field divergence on reflectivity of Alfvén waves at the transition region

太阳物理学和天体物理学长期存在的问题之一是大气层的神秘加热，使温度保持在100万K以上。在过去的70年里，人们提出了不同的设想，但没有一个得到证实。阿尔芬波是在二战期间发现的，它代表了磁力线张力和等离子体惯性之间的相互作用。因此，它们类似于弦乐器上的波。以前对阿尔芬波的研究已经证明，它们能够将大量的能量从太阳的可见表面（称为光球层）携带到大气层中。在长途旅行结束时，阿尔芬波可以通过不同的机制将能量转化为热量。例子包括阿尔芬波湍流，相位混合，和冲击加热通过非线性耦合到压缩波。后一种机制已被证明是有效的一维研究。这些研究已经能够模拟阿尔芬波的传播沿着一维环状结构从光球到大气。当波到达太阳的上层大气（称为日冕）时，它们的振幅增加，导致它们转换为压缩的慢波和快波。压缩波迅速变陡并转变为冲击，导致大气加热。一维研究的主要优点是能够采用高分辨率的数值网格，从而有机会研究波的传播、转换和变陡成激波的详细过程。其缺点是缺乏现实的物理，包括一个人工刚性磁场，一个简单的规定的场的几何形状，始终保持在附近的对称轴的loop.We的线提出调查阿尔芬波的传播和他们的作用，通过使用一个3维模型，是基于一个开放的源代码http://pencil-code.nordita.org/在太阳日冕加热。该模型将有能力纳入热传导和辐射、磁场弯曲和膨胀、重力和大气层分层的重要影响。结构几何形状将由完全三维物理模型中的内部和外部磁环境之间的相互作用来确定。它将成为可能解决阿尔芬波的传播和它们的相互作用与纵向以及横向波。我们以前的研究表明，等离子体射流和浪涌复制太阳针状体增强波放大的过程，并提供重要的额外能量的阿尔芬波。因此，我们期望在更复杂的几何中找到类似的贡献。我们将能够在一个更现实的3维模型中讨论针状体在阿尔芬波放大中的作用及其对加热过程的间接贡献。最近，利用Hinode/EIS、SST和其他仪器进行的观测表明，阿尔芬波在太阳大气中无处不在。拟议的研究项目将有助于我们对太阳物理的理解。它将阐明阿尔芬波在太阳大气动力学和能量学中的作用。这项工作将使用威尔士国家超级计算研究设施（SCW）在MPI下在大规模并行共享内存计算机上进行

One of the longstanding problems in solar physics and astrophysics is the mysterious heating of the atmosphere that maintains temperatures above 1 million degrees K. Over the past 70 years different scenarios have been put forward none of which has yet been confirmed. Alfven waves that were discovered during WW2 represent an interplay between the tension of the magnetic field lines and plasma inertia. They therefore resemble waves on a string instrument.Previous studies of Alfven waves have demonstrated their ability to carry significant amounts of energy from the visible surface of the sun known as the photosphere into the atmosphere. Towards the end of their long distance journey Alfven waves can transfer their energy into heating through different mechanisms. Examples include Alfven waves turbulence, phase mixing, and shock heating through nonlinear coupling to compressional waves. The latter mechanism has been shown to be efficient in 1 dimensional studies. These studies have been able to model the propagation of Alfven waves along 1 dimensional loop like structures from the photosphere into the atmosphere. As the waves reach the upper atmosphere of the sun known as the corona, their amplitudes increase which results in their conversion into compressional slow and fast waves. The compressional waves rapidly steepen and turn into shocks resulting in heating of the atmosphere. The energy of the Alfven waves is thus converted into heating.The main advantage of the 1 dimensional studies is their ability to employ a high resolution numerical grid that gives the opportunity to investigate the detailed process of wave propagation, conversion, and steepening into shocks. The drawback is the lack of realistic physics that includes an artificially rigid magnetic field, a simple prescribed field geometry with field lines that always remain in the vicinity of the symmetry axis of the loop.We propose to investigate the propagation of Alfven waves and their role in heating the solar corona by using a 3 dimensional model that is based on an open source code http://pencil-code.nordita.org/. The model will have the ability to incorporate the important effects of thermal conduction and radiation, magnetic field curvature and expansion, gravity and stratification in the atmosphere. The structure geometry will be determined by the interaction between the internal and external magnetic environments in a fully three dimensional physical model. It will become possible to address the propagation of the Alfven waves and their interactions with the longitudinal as well as the transverse waves. The resulting heating of the atmosphere through the processes of wave coupling will be investigated.Our previous studies have demonstrated that plasma jets and surges replicating solar spicules enhance the process of wave amplification and provide important additional energy to the Alfven waves. We would therefore expect to find a similar contribution in a more complex geometry. We will be able to address the role of spicules in Alfven wave amplification and their indirect contribution to the heating process in a more realistic 3 dimensional model. Recent observations with Hinode/EIS, SST, and other instruments have demonstrated the ubiquity of the Alfven waves in the solar atmosphere.The proposed research project will contribute to our understanding of the physics of the sun. It will clarify the role of Alfven waves in the dynamics and energetics of the solar atmosphere. The work will be carried out under MPI on massively parallel shared memory computers using the national supercomputing research facility of Wales (SCW)