【作者】 张健；

【导师】 黄狮勇；

【作者基本信息】 武汉大学 ， 空间物理学， 2024， 博士

【摘要】 等离子体湍流广泛分布于宇宙空间中,其表现为等离子体中物理量无规则的随机扰动,并且磁场、等离子体速度、密度等物理量的功率谱密度呈现出类似于流体湍流的幂律谱分布特征。由于湍流在诸如日冕加热、太阳风加速/加热等许多的重大物理问题中都扮演着重要角色,对等离子体湍流的研究一直都是空间物理学的重要课题。前人对空间等离子体湍流的研究主要聚焦于太阳风,然而对太阳风湍流的观测通常只有单颗卫星提供数据,并且数据分辨率较低,这导致太阳风湍流研究主要关注湍流的大尺度时空特性。而在小尺度上,等离子体湍流中发生的能量耗散及拓扑结构的演化等过程对于理解粒子的加热或加速等问题具有重要意义。近年来,随着磁层多尺度卫星(Magnetospheric Multiscale Misson,MMS)计划的成功发射,其高时空分辨率的多卫星数据使得开展湍流小尺度观测研究成为可能。在地球空间等离子体环境中,地球磁鞘和磁尾等离子体片高速流(Bursty Bulk Flows,BBFs)是研究空间等离子体湍流的理想天然实验室,并且MMS卫星在这些区域中积累了大量的观测数据,因此本文将利用MMS卫星在地球磁鞘和磁尾高速流中的观测数据对空间等离子体湍流的能量耗散和磁场的拓扑结构展开研究,本文的主要研究成果及意义主要包括以下几个方面:1.本文系统地比较了磁鞘和磁尾高速流中湍流能量耗散与不同尺度上的能量传输过程。我们发现磁鞘和磁尾高速流中湍流在含能尺度上的能量注入率与惯性尺度上的能量串级率相当,并且显著高于动理学尺度上的非线性相互作用能量传输率;压缩性会促进磁鞘和磁尾高速流湍流的能量传输,并且在磁鞘中这一现象更加显著;相比1 AU处太阳风湍流,磁鞘和磁尾高速流湍流的能量传输都要更为强烈。这些结果给出了磁鞘和磁尾高速流湍流中能量传输率与耗散率的理论计算结果,也是对太阳风湍流相关研究的一大补充。2.本文将中性流体中流场拓扑分类的方法学引入到MMS卫星的观测数据中,并对磁鞘和磁尾高速流湍流在动理学尺度上的磁场拓扑分布进行了研究。结果表明磁鞘和磁尾高速流湍流中动理学尺度上的“O”型磁场拓扑结构的比例大约是“X”型结构的三倍,这一比例会随着洛伦兹力的增加而有所下降;相比“X”型磁场拓扑,“O”型拓扑中分布着更多强的电流结构。这些结果为等离子体湍流中动理学尺度的磁场拓扑提供了新的观测证据。3.本文发现在地球磁鞘和磁尾高速流湍流中,湍流的能量耗散呈现出间歇性分布的特点,具体表现为在强电流或强PVI指数的区域中存在更强的电场做功以及电子温度的增强现象,而对于存在强压强张量做功的强涡度区域,电场做功的增强效果不显著;此外还发现“O”型磁场拓扑结构对湍流系统总能量耗散的贡献明显大于“X”型磁场拓扑结构。这些结果为等离子体湍流中不同类型间歇结构或磁场拓扑对能量耗散的贡献提供了观测依据。更多还原

【Abstract】 Plasma turbulence is widely distributed in space,manifesting as irregular random disturbances in physical quantities within plasma.In addition,the power spectral density(PSDs)of physical quantities such as magnetic field and plasma velocity and densities exhibits power-law spectral distribution similar to fluid turbulence.Due to its significant role in some vital physical phenomena such as coronal heating and solar wind heating,the study of plasma turbulence has always been an important topic in space physics.Previous research on space plasma turbulence has mainly focused on the solar wind.However,observations for solar wind turbulence are usually implemented by single spacecraft with low-resolution data,leading to a focus on large-scale spatiotemporal characteristics for solar wind turbulence.On small scales,processes such as energy dissipation and evolution of topological structures in plasma turbulence are of crucial importance for understanding issues like particle heating or acceleration.In recent years,with the launch of the Magnetospheric Multiscale(MMS)Mission,high spatiotemporal resolution data from multiple spacecraft are provided,which makes it possible to conduct observational researches on small-scale plasma turbulence.Considering that the Earth’s magnetosheath and magnetotail bursty bulk flows(BBFs)provide the ideal natural laboratories for studying the space plasma turbulence and many data are recorded by MMS in these regions,this paper will utilize data from the MMS within the Earth’s magnetosheath and magnetotail BBFs to study the energy dissipation and the topological structure of magnetic fields of plasma turbulence.The main findings of this paper include the following aspects:1.This paper systematically compares the energy dissipation and energy transfer processes at different scales of turbulence in the magnetosheath and magnetotail BBFs.We find that the energy injection rate at energy-containing scales and the energy cascade rate at inertial scales in both magnetosheath and magnetotail BBFs are comparable,significantly higher than the energy transfer rate of nonlinear interactions at kinetic scales,but lower than the energy dissipation of turbulence.The plasma density perturbations promote energy transfer in magnetosheath and magnetotail BBFs turbulence,with this phenomenon being more pronounced in magnetosheath turbulence.Compared to solar wind turbulence,the energy transferring in magnetosheath and magnetotail BBFs is more significant.These results provide theoretical calculations of the energy transfer and dissipation rates of turbulence within the magnetosheath and magnetotail BBFs,and they also serve as a significant complement to related studies on solar wind turbulence.2.This paper introduces the methodology of classifying flow field topology in neutral fluids into the observational data from MMS spacecraft and studies the distribution of magnetic field topology in turbulence within magnetosheath and magnetotail BBFs.We find that the proportion of"O"-type magnetic field topology structures at kinetic scales in magnetosheath and magnetotail BBFs turbulence is approximately three times that of"X"-type structures,and this ratio decreases with increasing Lorentz force.Compared to"X"-type magnetic field topology,"O"-type topology contains more intense current structures.These results provide new observational evidence for the magnetic field topology at kinetic scales in plasma turbulence.3.In turbulence within Earth’s magnetosheath and magnetotail BBFs,energy dissipation is found to exhibit intermittent distribution characteristics,specifically manifested as stronger electric field work and enhanced electron temperature in regions with strong currents or high PVI index,while the enhancement of electric field work in regions with strong vorticity accompanied with stronger pressure-strain interactions is not significant.Additionally,we find that the contribution of"O"-type magnetic field topology structures to the total energy dissipation of the turbulent system is significantly greater than that of"X"-type magnetic field topology.These results provide observational evidence for the contribution of different types of intermittent structures or magnetic field topologies to energy dissipation in plasma turbulence.更多还原