Space current sheets: structure, stability and evolution

在宇宙中热的和稀薄的无碰撞等离子体中，电流片(CS)是积累的磁能被重新连接释放的位置，通常是爆炸性的，导致等离子体加热、等离子体整体流动和粒子加速。因此，共聚焦对于天体物理等离子体的能量激发仅次于冲击波。尽管它们具有相关性，但关于天体物理共聚焦的结构、稳定性和演化的一些关键问题仍然没有得到解决。其中包括离子组成的影响、各向异性的影响和等离子体湍流的作用。原因是，大多数天体物理条件空间都无法进行直接观测。幸运的是，可以在日光层，例如在行星磁尾和太阳风中，对宇宙CS进行现场研究。这些研究揭示了由于比质子重的离子、粒子速度空间分布的各向异性和最小的电子尺度的湍流而产生的多尺度CS结构。我们计划系统地分析三个行星磁尾(地球、火星和水星)和太阳风中的CS的这些特性，利用现有的丰富的卫星数据，从统一的角度出发，基于CS结构、稳定性和通过它们释放能量的模型，我们将开发这些模型，考虑到比质子更重的丰富的离子、各向异性分布函数和与观测到的相似的湍流。我们的比较研究考虑到，尽管空间等离子体的绝对参数有很大的不同，但归一化到等离子体条件的物理相关量表明相似的物理情况。为此目的，我们将利用地球磁尾的多航天器观测，在离子尺度上按星团进行观测，在电子尺度上利用MMS观测。为了研究火星磁尾中的CS，我们将使用MAVEN观测，对于信使的Herean磁尾，对于太阳风中的CS，日球层航天器STEREO A和B，WIND，ACE，Ulysses，HELIOS 1和2，Cassini，Voyager 1和2。我们将根据观测的环境条件建立和应用CS结构的分析模型，从理论上和通过数值模拟来研究CS结构的稳定性，利用测试粒子、混合动力学和PIC代码的数值模拟来探索不稳定结构演变的非线性后果以及通过它们重新连接的非线性后果。我们将通过空间观测验证多尺度CS结构的模型预测及其在重离子、各向异性分布函数和湍流影响下的演化，以及高能粒子的光谱，以便能够得出无法直接用于现场观测的远程天体物理CS的结论。

In the hot and dilute collisionless plasmas of the Universe, current sheets (CSs) are the sites at which accumulated magnetic energy is released, often explosively, by reconnection, causing plasma heating, plasma-bulk-flow and particle acceleration. Hence, CSs are next to shock waves most important for the energization of astrophysical plasmas.Despite their relevance a number of crucial questions about structure, stability and evolution of astrophysical CSs are still unsolved. Among them are the influence of the ion composition, of anisotropies and the role of the plasma turbulence. The reason is that most astrophysical CSs are out-of reach for direct observations. Fortunately, cosmic CSs can be in-situ investigated in the heliosphere, e.g. in planetary magnetotails and in the solar wind. These investigations have revealed multi-scale CS structures due to ions heavier than protons, anisotropies in the particles velocity space distribution and turbulence down to the smallest, the electron scales.We plan systematical analysis of those properties of CS in three planetary magnetotails (Earth, Mars and Mercury) and in the solar wind, utilizing the wealth of existing satellite data from a unified point of view based on models of CS structure, stability and energy release through them, which we are going to develop, taking into account the abundance of ions, heavier than protons, anisotropic distribution functions and turbulence similar to the observed ones.Our comparative investigations take into account that, although the absolute parameters characterizing CSs in space plasmas considerably differ, the physically relevant quantities, normalized to the plasma conditions, indicate similar physical situations. For this purpose we will utilize multi-spacecraft observations in the Earth's magnetotail by CLUSTER at ion scales and by MMS at electron scales. For the investigations of CSs in the Martian magnetotail we will use MAVEN observations, for the Hermean magnetotail those of MESSENGER, for CSs in the solar wind the heliospheric spacecraft STEREO A and B, WIND, ACE, ULYSSES, HELIOS 1 and 2, CASSINI, VOYAGER 1 and 2. We will develop and apply analytical models of CS structure in dependence on the observed environmental conditions, investigate their stability theoretically and by means of numerical simulations, explore the nonlinear consequences of the evolution of the unstable structures as well as of reconnection through them utilizing test-particle, hybrid-kinetic and PIC-code numerical simulations. We will verify the model predictions of multi-scale CS structures and their evolution under the influence of heavy ions, anisotropic distribution functions and turbulence by means of space observations, as well as the spectra of energetic particles, in order to be able to draw conclusions also for remote astrophysical CSs, not directly accessible for in situ observations.