Study Of Plasma In The Earth’s Magnetotail By Numerical Simulation

Magnetosphere is produced by the interaction of Earth original dipolar magnetic field and solar wind, which is always in company with high speed plasma flow and interplanetary magnetic field (IMF). Magnetotail is the nightside of magnetosphere, in which many space plasma phenomenons are involved. The plasma activity in magnetotail has many relationship with human-beings’living environment, for example, the magnetic storm mainly caused by strong solar wind will bring huge energy passing through magnetotail and injecting into ionosphere, which will destroy communication system and electrical power system of human society. Therefore it is very important to study magnetotail for understand human-beings’living situation.The main target of studying space plasma is particles involved in electromagnetic field in space environment. We have three important methods, such as theory model, spacecraft and groundbase observation, and simulation. Theory will give us the physics’pictures from the most fundamental level, but we observation and simulation result to prove theory prediction; spacecraft and groundbase observation can give us the most direct and accurate data of space plasma, however, the space environment around earth is to big for human-being to set spacecraft everywhere, we can only get data in short time interval or small region. Numerical simulation is a research method developing with the growth of computer technique, by which we can solve the space plasma equations discretely with initial conditions and boundary conditions from observation and theory. Numerical simulation can help us to make up the limitation of in-situ observation, by which we can study some critical issues effectively from both multi time and space scales.For example, the particles are accelerated associated with magnetospheric substorms in magnetotail. Substorm is a large scale process in the region including near-earth tail and mid-tail, with the bursty bulk flow earthward moving in plasma sheet, magnetic filed in magnetotail will change from anti-parallel shape to dipole shape, in this process particles (both electrons and ions) will be accelerated and injected into geostationary orbit, then these high energy particles will drift with magnetic field into polar cusp, and lead to ionization of upper atmosphere in polar region. The whole process will last several hours and cross about20Re distance, satellite in-situ observation can show us the accelerated particles at10Re and6Re, but we still have no idea on where, when and how these particles are accelerated. To solve this problem, we applied global MHD simulation method and Large Scale Kinetic method to study the magnetotail magnetic field during substorm, the bursty bulk flow and particle acceleration mechanism. We also compared the high energy ions in our simulation result with in-situ observation result from THEMIS Mission. By our simulation we found that the earthward moving dipolarization fronts are reproduced approximatively in our MHD frame, which are observed by THEMIS P4in Feb.15,2008event. Meanwhile, the ions which are accelerated to more than100k electron volt in observation are also generated in our simulation result, and the enhancement of high energy ion flux in simulation is consistent with observation as well. At last the single particle kinetic theory is applied to study the acceleration mechanism by dipolarization front by which we found that the non-adiabatic process dominates the ion acceleration. We will introduce substorm mechanism and model in Chapter I, THEMIS dataset and large scale kinetic method in Chapter III, while the simulation result of ions particles acceleration associated with dipolarization front will be discussed in Chapter V.For understanding magnetic reconnection in magnetotail, the particle-in-cell(PIC) method are launched to solve fast collisionless reconnection problem. In this paper we will study the electromagnetic field near and within the reconnection site. Because it is very important to understand Hall electric field near diffusion region for accelerating particles and energy transition, Hall electric filed structures in different initial guide field will be discussed. Because it is hard to distinguish guide field from ambient field in spacecraft observation, and the lack of the third dimension electric field data from in-situ observation, the bipolar Hall field is treated as the typical and unique observed structure when spacecraft passing through reconnection diffusion region in traditional viewpoint. However, we found new tripolar Hall electric field structure in our simulation with big initial guide field(about0.5ambient field), each term in generalized Ohm’s law is studied to find out which mechanism dominates the formation of this new tripolar electric field, we also studied how the existence of guide field directly affects Hall electric field. At last we declare the possibility of bipolar Hall electric field with big guide filed. We will introduce the collisionless reconnection theory in Chapter II, the PIC simulation method in Chapter IV, the simulation result of new Hall elecretic Field structure will be discussed in Chapter IV.Now let us make a conclusion, to make a better understanding of space plasma with many different scales, we used some different simulation method:the global MHD method and Large Scale Kinetic method are combined to study big scale substorm and ion acceleration associated with dipolarazition fronts, while the PIC simulation method are applied to study the small scale Hall electric field in fast collisionless reconnection situation, with different guide fields. Because the future space science projects such as NASA-MMS, ESA-Cross Scale, et al., our simulation work will be compared with these observation result to help us understand the multiscale space plasma process. To forecast the space weather is our final aim, by which we can avoid the space weather disaster. We will discuss this part in Chapter VII.