无碰撞激波中的离子动力学和结构演化

发布时间：2018-07-12 20:42

本文选题：准垂直激波 + 准平行激波　；参考：《中国科学技术大学》2016年博士论文

【摘要】：无碰撞激波是宇宙空间中常见的物理现象,并且是有效的高能粒子加速器,特别是在宇宙中的大尺度高强度的无碰撞激波可以将带电粒子加速到相当高的能量,形成银河宇宙射线、异常宇宙射线等等。在本文中,我们利用二维混合模拟研究了准垂直激波和准平行激波中波动的激发、结构的演化和粒子动力学过程。具体结论如下：1.低马赫数准垂直激波情形下游粒子环状速度分布低马赫数准垂直激波情形下,上游质子穿越激波面后会形成环状速度分布,环状速度分布所带来的平均速度的扰动,使得磁场也会产生对应的扰动来保证总压力平衡。我们利用二维混合模拟得到了这样的结果,并同时加入了4%的氮离子,来研究氮离子的环状速度分布对磁场结构的影响。发现由于粒子数比较小,氦离子并没有对质子的环状速度分布和磁场结构带来明显的影响：并且,由于氦离子的荷质比较小，，氦离子在速度相空间中的环状速度分布的半径要比质子的大,同时持续的时间要比质子的更久。2.准垂直激波下游波动和粒子速度分布的演化利用二维混合模拟,我们研究了准垂直激波下游波动的激发。低马赫数激波情形下,质子和氦离子穿越激波面之后均会在相空间中形成环状速度分布；质子的环状速度分布很快被磁场的扰动所破坏,不能激发离子回旋波；而对于氦离子,其破坏过程较慢,直到较远的下游,氦离子才开始激发回旋波,这些波动将氮离子自身散射成为球壳状速度分布,并最终散射成为双麦克斯韦分布；质子和氦离子的等离子体β较小,所以没有磁镜波的激发。中等马赫数激波情形下,质子和氦离子穿越激波面后同样出现了环状速度分布,同时也出现了少量的反射质子；质子和氮离子均在激波面下游激发了离子回旋波,但是质子是从激波面就开始激发离子回旋波,而氮离子直到较远的下游才开始激发离子回旋波,这些波动将氮离子散射成为了球壳状速度分布,并最终散射成为了双麦克斯韦分布；同样由于较小的等离子体夕,并没有磁镜波的激发。高马赫数激波情形下,质子和氮离子也同样出现了环状速度分布,但是很快即被破坏掉,同时也出现较大比例的反射粒子：质子和氮离子均从激波面即开始激发离子回旋波；氮离子也同样在下游被散射成为了球壳状速度分布,并最终被散射成为了双麦克斯韦分布,但是却不能肯定是来自氦离子激发的离子回旋波的贡献,即不能肯定氮离子激发的离子回旋波在下游是占主导的；粒子在下游加热较快,等离子体β增长也较快,下游有磁镜波的激发。3.二维准平行激波中的粒子动力学和下游高速流的产生准平行激波中运动到上游的反射粒子与上游入射粒子相互作用可以激发上游低频波动。这些上上游波动会与激波面相互作用,使得激波面变得并不平整,即布满了涟漪。上游波动,或者说涟漪,使得沿着激波面的重构过程并不协调统一,同时也使得沿激波面的粒子动力学过程也变得不同。在激波面涟漪的两侧,由于电磁场结构的不同,粒子动力学过程也不同,在涟漪的下半侧,会有较多的粒子被反射和加速,而在涟漪上半侧,粒子更倾向于直接穿越激波面,并形成下游冷的高速粒子束流。并且,从我们详细研究了我们模拟中出现的下游高速流,发现模拟中出现的下游高速流与地球舷激波磁鞘中观测到的高速流的特征是相似的。4.二维准平行激波中的粒子加速过程利用二维混合模拟,我们发现,被激波反射的粒子首先会紧贴激波面运动,并同时被加速,这是第一阶段的加速；之后,粒子获得了较大的能量,可以到达靠近激波面的上游,并被束缚在上游波动和激波面之间被加速,这是第二阶段的加速。经过两个加速阶段,部分粒子运动到激波远下游,部分运动到远上游。还有部分粒子可经历第三个阶段的加速过程,即粒子在上下游波动之间来回弹跳并被加速,这一加速过程类似激波扩散加速；经过三个加速阶段的粒子的最终能量是明显大于经过两个阶段加速的粒子能量的,并且经过三个阶段的粒子也有两个部分,一部分运动到远下游,另一部分运动到远上游。
[Abstract]:Collisionless shock is a common physical phenomenon in cosmic space, and is an effective high-energy particle accelerator. In particular, large scale and high intensity collisionless shock waves in the universe can accelerate charged particles to quite high energy, form galactic cosmic rays, abnormal cosmic rays and so on. In this paper, we use two-dimensional mixed simulation. The excitation, structure evolution and particle dynamics process of quasi vertical shock and quasi parallel shock wave are studied. The concrete conclusions are as follows: 1. low Maher number quasi vertical shock waves in the case of low Maher number quasi vertical shock waves in the lower reaches of the downstream particles, when the upstream protons cross the shock surface will form a circular velocity distribution, and the annular velocity can be divided. The disturbance caused by the average velocity of the cloth makes the magnetic field produce corresponding disturbances to ensure the balance of the total pressure. We get this result by a two-dimensional mixed simulation and add 4% of the nitrogen ions to study the ring velocity distribution of nitrogen ions on the structure of the magnetic field. There is a significant effect on the annular velocity distribution and the structure of the magnetic field. And because the charge of helium ions is smaller, the radius of the annular velocity distribution of helium ion in the velocity phase is larger than that of the proton, while the lasting time is longer than the proton, and the evolution of the.2. quasi vertical shock wave and the particle velocity distribution is better than that of the proton. In the two-dimensional mixed simulation, we study the excitation of the downstream wave in the quasi vertical shock wave. In the case of low Maher number shock, the circular velocity distribution of the proton and helium ions across the shock surface will be formed in the phase space; the circular velocity distribution of the proton is quickly destroyed by the disturbance of the magnetic field and can not excite the ion cyclotron wave; and for the helium ion, The damage process is slow, until the farther downstream, the helium ions begin to excite the cyclotron wave, which scatters the nitrogen ions themselves into the spherical shell velocity distribution, and eventually scatters into a double Maxwell distribution; the plasma beta of the proton and helium ions is smaller, so there is no excitation of the magnetic mirror wave. In the middle Maher shock case, protons When the helium ions pass through the shock surface, there is a circular velocity distribution as well as a small amount of reflected protons. Both proton and nitrogen ion excite the ion cyclotron wave at the lower reaches of the shock surface, but the proton begins to excite the ion cyclotron wave from the shock surface, and the nitrogen ions start to excite the ion cyclotron waves until the farther downstream. The wave scatters the nitrogen ions into the spherical shell velocity distribution, and eventually scatters into a double Maxwell distribution; also because of the small plasma Eve, there is no excitation of the magnetic mirror wave. In the high Maher shock case, the proton and nitrogen ions also appear ring velocity distribution, but they are quickly destroyed and also appear more. A large proportion of the reflected particles: the proton and the nitrogen ion all start to excite the ion cyclotron wave from the shock surface, and the nitrogen ions are also scattered into the spherical shell velocity distribution in the downstream, and eventually scattering into the double Maxwell distribution, but it is not sure that the contribution of the ion cyclotron waves from the helium ion excitation is not sure. The ion cyclotron waves excited by nitrogen ions are dominant in the lower reaches; the particles are heated downstream faster, the plasma beta increases faster, and the downstream magnetic mirror waves are excited by the.3. two-dimensional quasi parallel shock wave and the downstream high-speed flow produces the interaction between the reflected particles moving to the upstream and the upstream incident particles in the quasi parallel shock wave. The upstream fluctuations can stimulate the low frequency fluctuations in the upstream. These upstream waves interact with the shock surface, making the shock surface uneven, that is, ripples. The upstream fluctuations, or ripples, make the process of the reconstruction of the shock surface uncoordinated and the particle dynamics along the exciting surface becomes different. On both sides of the ripples, the particle dynamics process is different because of the difference in the structure of the electromagnetic field. In the lower half of the ripples, more particles will be reflected and accelerated, and in the upper half of the ripples, the particles tend to cross the shock surface and form the downstream cold high speed particle beam. And we have studied the bottom of our simulation in detail. It is found that the characteristics of the high velocity flow of the downstream high velocity flow in the simulation and the high velocity flow observed in the earth's starboard shock magnetic sheath are similar to the particle acceleration in the.4. two-dimensional quasi parallel shock wave. We find that the particles reflected by the shock wave first adhere to the excited wave surface and are accelerated at the same time. This is the first stage. After that, the particle gets greater energy and reaches the upstream of the shock surface and is bound between the upstream wave and the shock surface is accelerated. This is the acceleration of the second stage. After two acceleration stages, some particles move to the far downstream of the shock wave, and the part moves to the far upstream. And some particles can go through third stages. The acceleration process, that is, the particle bounces back and forth between the upstream and downstream waves, and is accelerated. This acceleration is similar to the acceleration of shock wave diffusion; the final energy of the particles passing through the three accelerating stages is obviously larger than the particle energy that is accelerated by the two stages, and there are two parts of the particles passing through the three stages, and a part of the particles moving far away. Downstream, the other part moves to the far upper.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P353

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