【作者】 柴立晖；

【导师】 李毅； J(?)rg B(?)chner；

【作者基本信息】 中国科学技术大学 ， 空间物理， 2013， 博士

【摘要】 本文主要研究磁场重联中的低频波动和绝热过程中的动力学Alfven孤立波。1.在磁场重联低频波动方面的主要工作包括：磁场重联在空间等离子体中扮演重要角色,它可以有效地转移和转化等离子体的物质、动量和能量。空间等离子体中的许多爆发现象以及太阳风和地球磁层的相互作用都与磁场重联有关。磁场重联与波动有紧密联系。波动可能是触发重联的因素之一,而重联过程中磁力线拓扑结构的改变和高速粒子的产生都是激发波动的重要来源。在本文中,主要研究重联激发的低频波动特性。本文采用二维三分量混杂模拟程序模拟了重联过程。在重联达到准稳态后的随体坐标中,利用快速傅里叶分析法研究了波动的频谱特性,利用最小方差分析法研究了波动的传播方向和偏振特性。通过研究不同重联区域的波动,给出了波动的空间分布特性。并结合观测讨论了本文的研究结果。本文研究结果表明：重联区的低频波动以Alfven离子回旋波为主,其频率主要集中在0-1个离子回旋频率之间,偏振特性为左旋。其中入流区波动以小振幅的Alfven离子回旋波为主,传播方向以沿磁场方向为主,频率较高,其主峰常常高于0.5个当地离子回旋频率；而出流区波动以大尺度的湍动为主,振幅较大,频率较低,主要集中在0-0.6个当地离子回旋频率之间,其旋转特性相对较杂乱。由此推测重联可以产生左旋低频Alfven波,并提供了一些磁尾和极区的观测证据。本文还结合其他采用混杂模拟研究波动传播的文章,讨论了本文研究结果的可靠性,解释了本文结果与观察出现不同的原因。另外本文的研究结果还表明出流区大尺度的湍动会引起部分Hall重联四极结构的分布出现部分翻转,这可以解释卫星观测到的一些与Hall理论相反的数据点。2.在绝热动力学Alfven孤立波方面的主要工作包括：动力学Alfven孤立波(SKAWs)伴随有密度扰动和平行于磁场方向的电场扰动,是空间等离子体物理的重要研究内容之一。SKAWs可以解释空间等离子体中观测到的强电磁扰动,其平行电场对等离子体中波粒相互作用和磁层-电离层耦合有重要意义。已有大量文章从观测、理论和数值模拟方面对SKAWs进行了研究。为了简化数学分析,以往许多关于SKAWs的研究都采用了等温假设。但是事实上在不同的等离子体中热力学过程并不相同。许多研究表明在不同的空间等离子体区域,绝热指数的取值各不相同,甚至在同一地区的不同方向绝热指数也会不同。因此,研究不同热力学过程中绝热指数对SKAWs的影响是有必要的。为此本文在绝热条件下对SKAWs进行了研究。本文从双流体模型出发,采用Sagdeev势方法,在小振幅近似下得到了SKAWs的解析解。并在相同参数下,数值分析了SKAWs的波形和扰动场随绝热指数的变化情况。通过对比绝热过程和等温过程中SKAWs的差别大小,分析了绝热指数对SKAWs的修正程度。本文的研究结果表明随着绝热指数的增加,SKAWs波形的振幅变小、宽度变大,其平行方向的扰动电场变小,垂直方向的扰动电磁场变大。而且相比于等温过程,绝热过程的修正是显著的。因而在太阳风或磁层极区的热等离子体中,需考虑绝热指数对SKAWs的影响,特别在估算平行电场对电子的加速效应时应考虑这一修正。更多还原

【Abstract】 This thesis is focused on the the low-frequency waves in magnetic reconnection and solitary kinetic Alfven waves in an adiabatic process.1.Low-frequency waves in magnetic reconnection:Magnetic reconnection plays an important role in space plasma physics. It can ef-ficiently transfer and transform the material, momentum, and energy of plasmas.There are lots of eruptive activities in space plasmas and interactions between the solar wind and Earth’s magnetosphere that relate to magnetic reconnection. Plasma waves are im-portant to magnetic reconnection. Because there is a possibility that magnetic recon-nection is triggered by some plasma waves, also lots of plasma waves can be excited during the magnetic reconnection process. This thesis focuses on the characteristics of the low-frequency waves that are produced by magnetic reconnection.A two-dimensional hybrid simulation code is carried out to simulate the magnetic reconnection process. In the coordinate moving with fluid, wave spectrums are obtained by the fast Fourier transformation of magnetic field component which are perpendicular to the magnetic reconnection plane, and wave propagation directions and polarizations are determined by the minimum variance analysis of the electric field.After the recon-nection becoming quasi-steady, the space distributions of waves are studied.The results show that low-frequency Alfven ion-cyclotron waves are dominating in reconnection area. The frequencies of these waves are between0and1local proton gyrofrequency, the polarizations are all left-handed. In the inflow regions the dominant waves are Alfven ion-cyclotron waves with smaller amplitudes and propagation direc-tions mainly along the ambient magnetic field, these waves have higher frequencies, the main peaks of spectrums are usually higher than half of the local proton gyrofrequency. The large amplitude turbulence with frequency of0-0.6local proton gyrofrequency and isotropic propagation direction dominates in the outflow regions. We believe that the magnetic reconnection can produce Alfven waves, and some observational evidences are presented. A comparison of our results with another paper which investigates wave propagation with hybrid code is carried out to prove the correction of our results and explain the observations.The large-scale turbulence in the outflow regions can affect the Hall quadrupole structure distribution and produce some inverted distribution which agrees with some of the observations.2. Solitary kinetic Alfven waves in an adiabatic process:Solitary Kinetic Alfven Waves (SKAWs) are important in the field of space plas-ma physics because of their nonzero parallel electrical fields and density fluctuations. SKAWs play significant roles in wave-particle interaction and magnetosphere-ionosphere coupling. They have been investigated extensively through observation and theoretic-s with a focus on charged particle acceleration and heating. However, those studies were done under the simplifying assumption that the whole process was an isothermal process.In reality, the adiabatic index varies significantly under different plasma con-ditions. Therefor it is necessary to investigate the influence of changing adiabatic index on the SKAWs.Under different thermodynamic processes, SKAWs with the limit of small ampli-tudes are studied analytically and numerically by the method of the Sagdeev potential. The results show that as the adiabatic index increases, the amplitude of the solitary struc-ture and perturbed electric field along the background magnetic field direction reduce, the width of the solitary structure and perturbed electromagnetic fields which are per-pendicular to the background magnetic field direction increase. The results also show that the modifications of an adiabatic process to the isothermal process is significan-t. Therefor it is necessary to consider the modifications in plasmas where the electron thermal effect is much stronger than electron inertial effect, such as in the solar wind and in the pole regions of the Earth’s magnetosphere.更多还原