磁场重联中的电子加速机制的数值模拟研究

在应用2.5维混合模拟方法研究Petschek模型磁场重联的基础上,考察了试验电子被加速的特征. 模拟结果表明,稳态的低频重联场能将少量试验电子加速到高能,电子的能谱为幂律谱,但总体分布函数未发生显著变化. 电子在整个加速过程中被束缚在低磁场的加速区内,由重联产生的感应电场Ey分量对其直接加速,根据加速时间和加速区域可以将这些电子分为两种情况：初始位于加速区和漂移到加速区被加速.     

磁重联与等离子体波动

磁场重联与等离子体波动之间存在显著的联系.其中准静态波动、激波和动力学阿尔芬波的本征模是磁重联结构的重要组成部分.而其它的高频波动,不仅能吸收粒子的自由能,还可以导致粒子的加热和反常电阻的产生.这些多尺度的波动过程揭示了磁重联中能量转换的多尺度性质.本文探讨了地球磁层磁重联过程中的各种等离子体波动,涵盖了动力学阿尔芬波、低混杂波、哨声波、静电孤波、离子声波以及电子尺度的高频静电波,并分析了它们在磁重联中的特性和作用.近期的研究成果表明,动力学阿尔芬波（kinetic Alfven wave, KAW）能够描述磁重联区域的微观结构,其中包括霍尔磁场、霍尔电场、平行电场、霍尔电流以及场向电流.在这一过程中,霍尔电场作用于离子,有助于提高重联速率.低混杂波主要在电流片的密度梯度较大处被激发,并对电子进行平行加热.而哨声波是由朗道共振和回旋共振机制驱动的.据当前研究所示,低混杂波和哨声波对于重联过程中的反常电阻效应的影响是次要的.静电孤波多在磁重联分界线区域出现,其对等离子体的加热效应仍需进一步研究.与此同时,关于高频静电波（例如高混杂波和电子伯恩斯坦波）的研究重点在于磁重联的扩散区,这些波被...     

地球磁层顶磁场重联的动力学过程

用三维可压缩MHD数值模拟研究了在磁场重联过程中电子压力梯度项的效应研究结果发现在较高等离子体β,较小离子惯性尺度条件下,广义欧姆定理中压力梯度项在重联过程的作用不可忽略.在磁重联过程中,压力梯度项虽然没有明显改变磁场拓扑结构和重联速度,但它使电子和离子速度明显增大.由于在离子惯性尺度下,离子和电子运动解耦,电子是电流的主要载流子,所以场向电流也增大,并导致核心磁场明显增大.考虑到场向电流是磁层电离层耦合的一个重要因素,所以电子压力梯度项的引入加强了行星际磁场南向期间磁层电离层的耦合.电子压力梯度项还在重联区激发了波动,该波动可向重联区外传播.     

地球磁层顶磁场重联的动力学过程

用三维可压缩MHD数值模拟研究了在磁场重联过程中电子压力梯度项的效应研究结果发现在较高等离子体β,较小离子惯性尺度条件下,广义欧姆定理中压力梯度项在重联过程的作用不可忽略.在磁重联过程中,压力梯度项虽然没有明显改变磁场拓扑结构和重联速度,但它使电子和离子速度明显增大.由于在离子惯性尺度下,离子和电子运动解耦,电子是电流的主要载流子,所以场向电流也增大,并导致核心磁场明显增大.考虑到场向电流是磁层电离层耦合的一个重要因素,所以电子压力梯度项的引入加强了行星际磁场南向期间磁层电离层的耦合.电子压力梯度项还在重联区激发了波动,该波动可向重联区外传播.     

ESW在磁尾磁场重联耗散区分形线附近的观测特性及其作用

磁场重联是空间能量释放和转换的重要机制.静电孤立波(ESW)虽然在空间中有广泛观测,但在磁场重联附近少有直接观测,对它在磁场重联附近的特性了解甚少.通过Geotail卫星对一个磁场重联事件的观测,仔细分析了其边界层上观测到的静电孤立波的特性,并讨论了它对磁场重联的影响.研究表明,亚暴期间在磁尾发生磁场重联,重联区域的分形线附近观测到了大量的静电孤立波,其特性与在其他地方观测到的并没有显著差别,但具有更明显的非线性和孤立性的特征.它们对电子加速和能量耗散有促进作用,加速磁场重联的进程.     

磁岛合并研究

本文采用二维全粒子模拟来研究无碰撞等离子体中的磁岛合并过程.结果表明,磁岛合并分为两个阶段,在第一个阶段,两个磁岛因同向电流丝之间的吸引力而缓慢地相互靠近,在这个过程中,合并线附近的电子被面外电场加速,形成薄电流片,同时电流片两侧形成磁场堆积.第二个阶段为快速重联阶段,合并线附近的电磁场结构和以Harris电流片为初态的重联扩散区的电磁场结构很相似,其中最显著的特点为面外磁场的四极型结构.     

多层电流片中撕裂模不稳定性的数值研究

应用二维磁流体动力学模拟方法数值研究了三层电流片中电阻撕裂模不稳定性的特征及磁场重联过程．结果表明,这是一种复杂的非稳态磁场重联．在初期阶段,三个电流片中分别由撕裂模不稳定性引起磁场重联,形成薄而长的磁岛．随着撕裂模不稳定性的非线性发展,每个磁岛的宽度都逐步增大,以至导致新的磁场重联发生．同时,三个电流片的强度都逐渐减弱,且原中心反向电流区最终消失．部分磁能不断地转化为等离子体的热能和动能,引起等离子体的加热和加速．多层电流片中撕裂模不稳定性引起的自发重联,可能对太阳耀斑、日冕加热、太阳风与磁层耦合等有重要影响．     

多层电流片中撕裂模不稳定性的数值研究

应用二维磁流体动力学模拟方法数值研究了三层电流片中电阻撕裂模不稳定性的特征及磁场重联过程．结果表明，这是一种复杂的非稳态磁场重联．在初期阶段，三个电流片中分别由撕裂模不稳定性引起磁场重联，形成薄而长的磁岛．随着撕裂模不稳定性的非线性发展，每个磁岛的宽度都逐步增大，以至导致新的磁场重联发生．同时，三个电流片的强度都逐渐减弱，且原中心反向电流区最终消失．部分磁能不断地转化为等离子体的热能和动能，引起等离子体的加热和加速．多层电流片中撕裂模不稳定性引起的自发重联，可能对太阳耀斑、日冕加热、太阳风与磁层耦合等有重要影响．     

密度非对称的二维无碰撞磁场重联

使用二维粒子模拟（PIC）的方法研究了在电流片两侧具有不同温度或密度情况下的无碰撞磁场重联过程.在初始等离子体密度非对称的情况下,发现重联区等离子体流场结构、电磁场结构以及重联过程与对称情况下的结果有明显不同.通过对电流片两侧温度比取不同的参数Tm/Ts=1,2,5进行模拟(其中Tm和Ts分别代表磁层侧和磁鞘侧的温度),结果分析发现,(1)在密度非对称系统中,出流区电子沿着分离面出现一个整体的从高密度区向低密度区的流动,并围绕磁岛形成一个电流环;(2)在高温低密度一侧,在重联过程中,分离面两侧将出现很强的电荷分离并产生一个基本垂直于分离面的强度较大的电场Ez,其幅度和空间尺度与温度梯度近似地成线性正比和反比关系.在初始电流片两侧温度之比取Tm/Ts=5的情况下,Ez的幅度将达到0.71,其空间尺度与局地电子惯性长度de同一量级,这一结果与观测相吻合;(3)重联率随着温度梯度增大而下降.     

太阳宇宙线高能电子能谱的形成

提出了一个太阳脉冲和经变耀斑中高能太阳宇宙线电子能谱的形成模型,探讨了高能电子通过日冕捕获区的库仑损失、轫致辐射和同步辐射等物理过程,首次研究了日冕等离子体尾场对太阳宇宙线电子的加速及其能谱的形成．所得结果和观测谱能很好地符合,从而较合理地阐明了脉冲耀斑和经变耀斑两类太阳宇宙线高能电子谱的结构．     

Magnetohydrodynamic simulation of a solar flare: 2. Flare model and simulation using active-region magnetic maps

The results of a three-dimensional MHD simulation and data obtained using specialized spacecraft made it possible to construct an electrodynamic model of solar flares. A flare results from explosive magnetic reconnection in a current sheet above an active region, and electrons accelerated in field-aligned currents cause hard X rays on the solar surface. In this review, we considered works where the boundary and initial conditions on the photosphere were specified directly from the magnetic maps, obtained by SOHO MDI in the preflare state, in order to simulate the formation of a current sheet. A numerical solution of the complete set of MHD equations, performed using the new-generation PERESVET program, demonstrated the formation of several current sheets before a series of flares. A comparison of the observed relativistic proton spectra and the simulated proton acceleration along a magnetic field singular line made it possible to estimate the magnetic reconnection rate during a flare (∼10⁷ cm s⁻¹). Great flares (of the X class) originate after an increase in the active region magnetic flux up to 10²² Mx.     

Numerical MHD simulations of the appearance of a series of current sheets above the active region AR 0365

The first attempt at numerical MHD simulations of the appearance of several current sheets above an active region before a series of elementary flares is described. Energy accumulates in the field of each sheet that can be released during one of the flares. The computations started three days before the appearance of a series of flares, i.e., before the emergence of a new magnetic flux in the active region. The initial (potential) magnetic field was calculated by solving the Laplace equation with an oblique derivative. The boundary conditions on the photosphere were specified from maps of the measured magnetic field in the active region for various instants of time. The Peresvet program solving the full system of MHD equations with dissipative terms was used in the computations. An absolutely implicit scheme conservative relative to the magnetic flux was used. The problem of properly choosing the size of the computational domain and finding the positions of singular magnetic field lines is discussed.     

太阳宇宙线高能电子能谱的形成

提出了一个太阳脉冲和经变耀斑中高能太阳宇宙线电子能谱的形成模型，探讨了高能电子通过日冕捕获区的库仑损失、轫致辐射和同步辐射等物理过程，首次研究了日冕等离子体尾场对太阳宇宙线电子的加速及其能谱的形成．所得结果和观测谱能很好地符合，从而较合理地阐明了脉冲耀斑和经变耀斑两类太阳宇宙线高能电子谱的结构．     

Particle acceleration and transport in the inner heliosphere

In the solar system, our Sun is Nature’s most efficient particle accelerator. In large solar flares and fast coronal mass ejections (CMEs), protons and heavy ions can be accelerated to over ~GeV/nucleon. Large flares and fast CMEs often occur together. However there are clues that different acceleration mechanisms exist in these two processes. In solar flares, particles are accelerated at magnetic reconnection sites and stochastic acceleration likely dominates. In comparison, at CME-driven shocks, diffusive shock acceleration dominates. Besides solar flares and CMEs, which are transient events, acceleration of particles has also been observed in other places in the solar system, including the solar wind termination shock, planetary bow shocks, and shocks bounding the Corotation Interaction Regions (CIRs). Understanding how particles are accelerated in these places has been a central topic of space physics. However, because observations of energetic particles are often made at spacecraft near the Earth, propagation of energetic particles in the solar wind smears out many distinct features of the acceleration process. The propagation of a charged particle in the solar wind closely relates to the turbulent electric field and magnetic field of the solar wind through particle-wave interaction. A correct interpretation of the observations therefore requires a thorough understanding of the solar wind turbulence. Conversely, one can deduce properties of the solar wind turbulence from energetic particle observations. In this article I briefly review some of the current state of knowledge of particle acceleration and transport in the inner heliosphere and discuss a few topics which may bear the key features to further understand the problem of particle acceleration and transport.     

Solar flare model: Comparison of the results of numerical simulations and observations

The electrodynamic flare model is based on numerical 3D simulations with the real magnetic field of an active region. An energy of ∼10³² erg necessary for a solar flare is shown to accumulate in the magnetic field of a coronal current sheet. The thermal X-ray source in the corona results from plasma heating in the current sheet upon reconnection. The hard X-ray sources are located on the solar surface at the loop foot-points. They are produced by the precipitation of electron beams accelerated in field-aligned currents. Solar cosmic rays appear upon acceleration in the electric field along a singular magnetic X-type line. The generation mechanism of the delayed cosmic-ray component is also discussed.     

Powerful nonstationary processes on the sun: Spatio-temporal structure and efficient particle acceleration

Data on high-energy processes on the Sun are summarized. We refine the classification of flares and substantiate the view that a coronal mass ejection and a flare proper are manifestations of the same common process, at least for the most powerful events. Next, we analyze data on the acceleration of electrons (RHESSI, Mars Odyssey) and protons. The existence of two peaks of hard X-ray emission spaced 10–20 min apart and the evolution of its spectra are shown to be indicative of two acceleration episodes. We have analyzed the spectra of 172 proton increases identified with the ratio of the proton fluxes at energies above 10 and 100 MeV near the Earth. These spectra turn out to be virtually the same for most of the large flares under favorable conditions for the escape of particles from the corona and their propagation in the interplanetary space. This is an argument for the invariance of the main features of efficient particle acceleration in powerful events. This process takes place at the explosive phase of a flare and its source is located low, immediately above the chromosphere, in the region adjacent to sunspots. There is a reason to believe that, in this case, a rapid simultaneous acceleration of electrons and protons takes place with the capture of some fraction of the particles into magnetic traps. However, there exist a few events in which an additional number of protons with energies as high as 10–30 MeV escape from the corona at the post-eruptive phase of flare development. Analysis of these cases with softer particle spectra more likely suggests an additional particle acceleration at coronal heights (about 30 000 km) than the facilitation of particle escape from magnetic traps. We estimate the contribution from the proton flux at an energy above 10 MeV arising at the post-eruptive phase of a flare to the total particle flux at the maximum of a proton increase and discuss possible particle acceleration mechanisms at significant coronal heights.     

Modeling of Physical Processes by Analysis of Hard X-Ray and Microwave Radiations in the Solar Flare of November 10, 2002

For electron acceleration during solar flares, it is very important to determine the pitch-angle and energy dependences of the electron distribution function. At present, this cannot be done directly from observations. Therefore, it is necessary to perform a numerical simulation of the propagation of accelerated electrons in the magnetic field of the flare loop (loops) and calculate the X-ray and radio emissions. For the solar flare of November 10, 2002, we have obtained qualitative and quantitative agreements of modeled X-ray and radio maps with the RHESSI satellite and Nobeyama Radioheliograph data. We have determined the flare model parameters that agree with observations. The pitch-angle anisotropy of electrons determined by highly directional functions of the S(α) = cos⁸(α) type, the energy spectrum consist of two electron populations, the low-energy part of the spectrum up to an energy of break of 350 keV is characterized by a power law with the exponent δ₁ = 2.7–2.9, and the energy spectrum is more rigid above 420 keV (δ₂ = 2–2.3).