Space current sheets: structure, stability and evolution

在宇宙中热而稀的无碰撞等离子体中，电流片（CSs）是积累的磁能释放的地方，通常是通过重联爆炸释放，导致等离子体加热、等离子体体流和粒子加速。因此，对于天体物理等离子体的能量来说，CSs是仅次于冲击波的最重要的能量源。尽管它们具有相关性，但关于天体物理CSs的结构、稳定性和演化的一些关键问题仍未得到解决。其中包括离子组成的影响、各向异性的影响以及等离子体湍流的作用。原因是大多数天体物理CSs是无法直接观测的。幸运的是，宇宙CSs可以在日球层中进行原位研究，例如在行星磁尾和太阳风中。这些研究揭示了由于离子比质子重、粒子速度空间分布的各向异性以及小到电子尺度的湍流而导致的多尺度CS结构。我们计划对三个行星磁尾（地球、火星和水星）和太阳风中的CS的这些特性进行系统分析，利用丰富的现有卫星数据，从统一的角度基于CS结构、稳定性和能量释放的模型，我们将开发这些模型，考虑到离子的丰度、比质子重、各向异性分布函数和类似于观测到的湍流。我们的比较研究考虑到，尽管表征空间等离子体中CSs的绝对参数有很大差异，但物理相关的量，归一化到等离子体条件下，表明了相似的物理情况。为此，我们将利用多航天器对地球磁尾的观测，在离子尺度上使用CLUSTER，在电子尺度上使用MMS。对于火星磁尾中的CSs，我们将使用MAVEN的观测数据，对于Hermean磁尾，我们将使用MESSENGER的观测数据，对于太阳风中的CSs，我们将使用日球层航天器STEREO A和B、wind、ACE、ULYSSES、HELIOS 1和2、CASSINI、旅行者1和2。我们将根据观察到的环境条件开发和应用CS结构的分析模型，从理论上和数值模拟的方式研究它们的稳定性，利用测试粒子、混合动力学和pic代码数值模拟，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。在重离子、各向异性分布函数和湍流的影响下，我们将通过空间观测和高能粒子光谱验证模型对多尺度CS结构及其演化的预测，以便能够对无法直接进行现场观测的遥远天体物理CS得出结论。

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.