Computer simulation of the three-dimensional decay of thin collisionless current sheets
Computer-Vision计算机视觉英文ppt
Its mainstream research is divided into three stages:
Stage 1: Research on the visual basic method ,which take the model world as the main object;
Stage 2: Research on visual model ,which is based on the computational theory;
the other is to rebuild the three dimensional object according to the two-dimensional projection images .
History of computer vision
1950s: in this period , statistical pattern recognition is most applied in computer vision , it mainly focuse on the analysis and identification of two-dimensional image,such as: optical character recognition, the surface of the workpiece, the analysis and interpretation of the aerial image.
Application and Analysis of Computer Virtual Simulation Technology in Three-dimensional An
Management Science and ResearchVolume 8(1), 2019, PP.19-21 Application and Analysis of Computer Virtual Simulation Technology in Three-dimensional Animation ProductionLe QiuZIBO Vocational Institute, Zibo Shandong 255314, ChinaAbstractWith the continuous promotion of computer technology, the application system of virtual simulation technology has been further optimized and improved, and has been widely used in various fields of social development, such as urban construction, interior design, industrial simulation and tourism teaching. China's three-dimensional animation production started relatively late, but has achieved good results with the support of related advanced technology in the process of development. Computer virtual simulation technology is an important technical support in the production of three-dimensional animation. In this paper, firstly, the related content of computer virtual simulation technology was introduced. Then, the specific application of this technology in the production of three-dimensional animation was further elaborated, so as to provide some reference for the improvement of the production effect of three-dimensional animation in the future.Keywords: Computer; Virtual Simulation Technology; Three-dimensional Animation Production; Application1.IntroductionComputer virtual simulation technology is one of the key technologies in three-dimensional animation production, which plays an important role in the whole process of three-dimensional animation production. It effectively combines many modern computer technologies to form a comprehensive technology to provide solid technical support for three-dimensional animation. Therefore, the analysis of the characteristics, advantages and related applications of computer virtual simulation technology has an important role in promoting the development of three-dimensional animation production.2.Overview of computer virtual simulation technology2.1Definition of virtual simulation technologyThe definition of virtual simulation technology can be understood and analyzed in both narrow and broad sense. In the narrow sense, virtual simulation technology mainly refers to a kind of experimental research technology developed with computer technology and network technology, which can be used to realize the comprehensive construction of virtual space [1]. However, the generalized virtual simulation technology has always been applied in the process of human understanding the world and exploring nature [2]. Especially in today's continuous development of computer technology and network technology, virtual simulation technology has its own system, which takes simulators and simple mathematical models as entities, and fully reflects the real characteristics of the objective world in the virtual environment [3].2.2Characteristics of virtual simulation technologyThe characteristics of virtual simulation technology can be summarized in four aspects [4]. (1) Immersion. Virtual simulation system can provide users with visual, auditory, tactile, olfactory, and motion sensations, so that users can get a sense of immersion in the virtual environment. (2) Interactivity. Interaction here is mainly the interaction between people and environment. In virtual system, users can control some elements in virtual environment through some actions. At the same time, the corresponding elements in the environment will make corresponding changes to control behavior [5]. For example, when launching missiles, users press the launch button, the virtual environment will appear. In the picture of missile launching, fire and debris will also appear when the missile touches the target and explodes [6]. (3) Illusiveness. The environment constructed in the virtual system is simulated by human using computer technology. The simulated environment may have existed in the past, may be forthcoming in the future, or may not happen. Therefore, the virtual system is not real and has some illusion. (4) Fidelity. Fidelity is also a main characteristic of virtual simulation technology, which is embodied in two aspects: firstly, the environment built in virtual environment can give people a very realistic feeling, just like the real world; secondly, when people manipulate the virtual environment, the environment will make corresponding changes according to the control behavior, which can give people a sense of immersion [7].3.Application of computer virtual simulation technology in three-dimensional animation productionThe application of computer virtual simulation technology in three-dimensional animation production is shown in Fig.1.Figure 1 Application of computer virtual simulation technology in three-dimensional animation production3.1Application of stereo display technologyStereo display technology is a technology that can make three-dimensional animation vivid display, which can combine visual effects with audience aesthetics [8]. In practice, there are two ways to display virtual scene realistically through stereo display technology: Firstly, simultaneous display method, that is, display the image corresponding to the viewer's eyes while playing three-dimensional animation, and achieve the actual viewing effect by wearing three-dimensional glasses; secondly, serial stereo display method, that is, display two animated video images alternately through a certain frequency to achieve the real effect of viewing animation directly with naked eyes [9].3.2Application of three-dimensional virtual sound technologyIn the virtual scene of three-dimensional animation, the real-time sound should be in line with the auditory perception of the audience in the real world, while allowing the audience to perceive the location of the sound source in the virtual environment. The technology of interaction nesting between the virtual environment and the real sound is called "three-dimensional virtual sound technology". Through three-dimensional virtual sound technology, audiences can truly feel the corresponding sound from real life in the virtual environment of three-dimensional animation, which strengthens the authenticity of film and television animation, and enables audiences to devote themselves wholeheartedly to the psychological changes and behavioral motivations of the protagonists in film and television animation. In practice, three-dimensional virtual sound technology is usually embodied in the following aspects: Firstly, three-dimensional virtual sound technology will provide another way for audiences to interact with things in movies in the virtual environment. In the future, audiences will communicate directly with people and things in virtual scene through advanced computer virtual simulation technology interactive equipment, so as to promote virtual reality technology when providing more real feelings for audiences; Secondly, through three-dimensional virtual sound technology, part of the information in the virtual environment of three-dimensional animation can be transmitted to facilitate the audience's understanding of the content of the film, and even the whole film's artistic ideas and related aesthetic values; Thirdly, the spatial information of three-dimensional virtual sound technology is more extensive than visual information, which increases the diversity of film and television scenes, provides rich pictures for the audience, and at the same time increases the visual effect.3.3Application of environmental modeling technologyIn order to create a real three-dimensional model, it is necessary to attach importance to the virtual environment and image. The main purpose of three-dimensional modeling of virtual environment and virtual image in film and television animation is to obtain data reference in real life. According to the audiences' aesthetic rules and characteristics, the virtual environment model with visual consumption function is established by using three-dimensional data, so as to achieve the best state in visual effect. At the same time, the use of virtual environment modeling technology is mainly in three-dimensional movies, or in science fiction films that need to show magnificent scenes, while ordinary plot films and two-dimensional movies do not need too much technology as support. Therefore, in practice, environmental modeling technology will play a more important role with the development of three-dimensional animation.3.4Application of virtual image technologyThe emergence of Star Wars series has enabled the development of virtual image technology in films. Since then, digital virtual image technology has been used in more 3D animation production. Its advanced computer technology is beyond the traditional virtual image skills, such as digital technology, virtual technology, synthesis technology. In a sense, virtual image coversa wide range, including pre-creation process, on-site shooting process and post-production process.4.Technical example and function of computer virtual simulation technologyComputer virtual simulation technology is widely used in the actual situation and plays an important role in film and television animation production. In this paper, the examples of computer virtual simulation technology were listed and its role was briefly discussed: First of all, it is the virtual image capture photography system. Usually two cameras take pictures of objects at the same time, focusing on a stronger three-dimensional effect, but also won't make the audience dizzy in the process of watching, to a large extent, in line with the aesthetic law of the public. Ghosts of the Abyss was the first film in the world to use this technology, since then this technology was more and more widely used. Many films were produced and filmed using this technology to produce more aesthetic effect images, such as: Journey to the Center of the Earth, Spy Kids and so on; Secondly, it is implementation preview. In the process of filming, in order to pursue the perfect combination of real scene and virtual environment, Cameron, a famous American director, developed a kind of virtual camera together with the creative team of the corresponding technical department. The camera can effectively combine the physical background and the characters with the virtual background in the process of filming, and complete the real-time preview, which saves the time of combining the two, and also effectively controls the shooting process. For example, in the production of the movie Avatar, actors need to wear tight pants full of motion capture points in the process of shooting, so that the camera can capture them synchronously and facilitate the automatic synthesis of virtual cameras. Thirdly, it is motion capture. In the process of filming, many scenes and actions need to be taken by real people.At this time, digital virtual technology should be applied to capture actions, digitalize actions, and then present a perfect state. For example, in Hollywood science fiction action movies, many of them use virtual motion capture technology to increase the visual effect of the film and enhance its attractiveness, such as King Kong, Rise of the Planet of the Apes, etc. The rich and vivid expressions and extraordinary movements in the film are shot through the above technology, and the effect is more realistic and natural.5.ConclusionWith the continuous expansion of the application scope of three-dimensional animation in recent years, computer virtual simulation technology as an important technology of three-dimensional animation production has been given great attention. The application of computer virtual simulation technology in the production of three-dimensional animation has great practical significance. The application of computer virtual simulation technology in the production of three-dimensional animation is conducive to better understanding of animation works and interpretation of real scenes. At the same time, three-dimensional animation production plays an important role in restoring ancient times and looking forward to the future. In summary, with the rapid development of three-dimensional animation production, the application of computer virtual simulation technology in this field will inevitably be highly valued. From the analysis of this paper, it can be seen that the application of virtual simulation technology in the production of three-dimensional animation can not only make abstract things concrete and clear, but also make the scenes and characters in animation more realistic, and improve the animation animation vividness and visual impact. In the future, with the continuous expansion of the application scope of three-dimensional animation, the optimization and improvement of computer virtual simulation technology is bound to be very important. Only by constantly improving the virtual simulation technology, can better three-dimensional animation be produced to better play its role.References[1]Zeng Longkai. Application of computer virtual simulation technology in three-dimensional animation production. Moderninformation technology, 2019,08:84-85+88.[2]Wei Qingzhong, Yi Hongchi, Zhao Wentao. Virtual simulation of orthopaedic manipulation model of distal radius extensionfracture for teaching. China Digital Medicine, 2018, 1307:6-9.[3] Liu Qingming. Application of Virtual Reality Technology in Film and Television Animation Production. TV Guide, 2017, 21:174-175.[4] Meng Fanjun. Research on the Application of Director-based Sports Simulation Training Technology in the Referee of SportsEvents. Television Technology, 2018,4210:65-69.[5] Anxiufang. Application of computer virtual simulation technology in three-dimensional animation production. Information andcomputer (theoretical version), 2016, 19:31-32.[6] Wang Xiao. Application of virtual reality technology in three-dimensional animation production. Information and computer(theoretical version), 2017,02:86-87.[7] Ye Hongling, Wang Pengfei. Construction of Teaching Resource Bank of automobile construction course based on computersimulation technology. Science and technology innovation report, 2017, 1417:238-242.[8] Xu Dan. Prospects for the development of the combination of animation and VR technology.Journal of Jiamusi VocationalCollege, 2017,02:418-419.[9] Ma Qianqian. Application of virtual reality technology in three-dimensional animation production. Think tank era, 2017,06:210-211.。
新核心3第1、2、3、4、5、7课文翻译
Unit 1Main Reading拥有自己头脑的机器如果任其自由发展，有些机器可以学得更聪明，在一些最需要脑力的任务方面甚至会超越人类。
人类能否建造出可以演变得更好并可以超出人们想象而发明解决方案的机器吗？利用计算蛮力方法，计算机现在可以进行通行的国际象棋游戏。
1997年，IBM的一款名为深蓝的超级计算机击败了卡斯帕罗夫。
世界冠军认为这次经历如同与顶尖的人类挑战者对抗一样艰难。
阿蓝图灵，战时英国谜团破译密码工作背后的数学天才，于20世纪50年代设立了人工智能的标准，而深蓝的行为至少达到了其中的一个。
然而，深蓝的成功并没有给人工智能界留下深刻的印象，那是因为这台机器的创举仅仅在于运算速度快于其他任何以前的计算机。
巨大的处理能力可以使它预测到向前推进的棋步多达30个，而且它聪明的编程可以计算出数百万的可能的棋步中哪一步会加强它的位置。
但就本身而言，深蓝所能做的，而且出色完成的仅仅是数学。
它不能为象棋游戏制定自己的战略。
但是如果深蓝被赋予一种演变的能力，使用反复试验的经历学会完善自身，会怎么样呢？一种名为“演化硬件”的新技术正试图这么做。
和深蓝一样，演化硬件也是通过尝试几十亿个不同的可能，寻求解决方案。
区别在于，和深蓝不同，演化硬件不停地调整和完善它的搜索算法，而这也正是找到解决方案所需的逻辑步骤。
它每次都选择最好的，并加以尝试。
而且，它所作的一切不是根据编好的指令，而都是自动完成的，。
传统观念长期认为一个机器的能力是受限于创造者的想象力。
但是在过去的几年里，演化硬件的前驱已经成功地建造了一些可以自行调整并且表现更佳的设备。
有些情况下，后来出现的机器甚至超出了创造者的能力。
例如，在电路设计领域，对几十年来人类束手无策的一些问题，演化硬件却找出了创造性的解决方案。
演化硬件首先需要硬件可以重新配置。
如果一个设备不能调整形状或调整做事方法，它是不可能演变的。
拿一把瑞士军刀为例，如果要完成开启瓶子的任务，使用者要确认刀具中合适的工具，然后打开刀具，再把设备转变成一个可以敲开瓶盖的用具。
三维解析仿真的英语作文
三维解析仿真的英语作文Three-Dimensional Computational Modeling.Three-dimensional (3D) computational modeling is the process of creating a mathematical representation of a three-dimensional object. This representation can be used to simulate the behavior of the object under different conditions. 3D computational modeling is used in a wide variety of fields, including engineering, medicine, and manufacturing.In engineering, 3D computational modeling is used to simulate the behavior of structures and machines. This information can be used to design structures that are safe and efficient. In medicine, 3D computational modeling is used to simulate the behavior of organs and tissues. This information can be used to diagnose diseases and develop new treatments. In manufacturing, 3D computational modeling is used to simulate the behavior of products during the manufacturing process. This information can be used tooptimize the manufacturing process and reduce product defects.There are many different types of 3D computational modeling software available. The type of software used will depend on the specific application. Some of the most popular 3D computational modeling software programs include ANSYS, COMSOL, and Siemens NX.3D computational modeling is a powerful tool that can be used to simulate the behavior of objects in a variety of different fields. This information can be used to design safer and more efficient structures, diagnose and treat diseases, and optimize the manufacturing process.Benefits of 3D Computational Modeling.There are many benefits to using 3D computational modeling. Some of the most notable benefits include:Increased accuracy: 3D computational models are more accurate than traditional 2D models. This is because 3Dmodels can take into account the effects of all three dimensions of space.Reduced time and cost: 3D computational modeling can save time and cost by reducing the need for physical testing. Physical testing can be expensive and time-consuming, and it is not always possible to test all possible scenarios.Improved communication: 3D computational models can be used to communicate complex designs and concepts more easily. This can help to reduce errors and improve collaboration between different teams.Applications of 3D Computational Modeling.3D computational modeling is used in a wide variety of applications, including:Engineering: 3D computational modeling is used to simulate the behavior of structures and machines. This information can be used to design structures that are safeand efficient.Medicine: 3D computational modeling is used to simulate the behavior of organs and tissues. This information can be used to diagnose diseases and develop new treatments.Manufacturing: 3D computational modeling is used to simulate the behavior of products during the manufacturing process. This information can be used to optimize the manufacturing process and reduce product defects.Future of 3D Computational Modeling.The future of 3D computational modeling is bright. As computer hardware and software continue to improve, 3D computational models will become even more accurate and sophisticated. This will open up new possibilities for using 3D computational modeling in a wide variety of applications.One of the most exciting developments in 3Dcomputational modeling is the use of artificialintelligence (AI). AI can be used to automate the process of creating and running 3D computational models. This will make it easier for engineers, scientists, and other professionals to use 3D computational modeling in their work.Another exciting development in 3D computational modeling is the use of virtual reality (VR). VR can be used to create immersive 3D environments that allow users to interact with 3D computational models. This can make it easier to understand complex designs and concepts.3D computational modeling is a powerful tool that is transforming the way we design, build, and heal. As computer hardware and software continue to improve, 3D computational modeling will become even more powerful and versatile. This will open up new possibilities for using 3D computational modeling in a wide variety of applications.。
The numerical computation of turbulent flows
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4 P 0, Oti
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of turbulence energy von Karman’s constant appearing in (2.1 - 11) Molecular viscosity Turbulent viscosity Kinematic viscosity A generalized dependent variable Density Effective turbulent Prandtl number Effective turbulent Prandtl number for transport Molecular Prandtl number Shear stress
Nomen constant Curte t number defined by (3.1 - 1) Coefficients in approximated turbulent transport equations Specific heat at constant pressure Diffusion coefficient for quantity (p Rate of diffusive transport of Reynolds stress Constant in near-wall description of velocity profile (- 9) Functional defined by (2.2 - 6) Turbulence kinetic energy uiuj/2 Length of energy containing eddies Fluctuating component of static pressure Heat flux Radius Reynolds number in pipe flow based on bulk velocity and pipe diameter Rate of redistribution of Reynolds stress through pressure fluctuations Turbulent Reynolds number k2/ve Temperature Fluctuating component of velocity in direction xi Mean component of velocity in direction Xi Streamwise velocity nondimen~onalized by T,JP Mean streamwise velocity on axis Change in mean velocity across shear flow ‘Vorticity’ fluctuations squared Cartesian space coordinate
新概念第四册课文翻译及学习笔记【Lesson31、32、33】
【导语】新概念英语作为⼀套世界闻名的英语教程，以其全新的教学理念，有趣的课⽂内容和全⾯的技能训练，深受⼴⼤英语学习者的欢迎和喜爱。
为了⽅便同学们的学习，为⼤家整理了⾯的新概念第四册课⽂翻译及学习笔记，希望为⼤家的新概念英语学习提供帮助!Lesson31 【课⽂】 First listen and then answer the following question. 听录⾳，然后回答以下问题。
What do you have to be able to do to appreciate sculpture? Appreciation of sculpture depends upon the ability to respond to form in three dimension. That is perhaps why sculpture has been described as the most difficult of all arts; certainly it is more difficult than the arts which involve appreciation of flat forms, shape in only two dimensions. Many more people are 'form-blind' than colour-blind. The child learning to see, first distinguishes only two-dimensional shape; it cannot judge distances, depths. Later, for its personal safety and practical needs, it has to develop (partly by means of touch) the ability to judge roughly three-dimensonal distances. But having satisfied the requirements of practical necessity, most people go no further. Though they may attain considerable accuracy in the perception of flat form, they do not make the further intellectual and emotional effort needed to comprehend form in its full spatial existence. This is what the sculptor must do. He must strive continually to think of, and use, form in its full spatial completeness. He gets the solid shape, as it were, inside his head-he thinks of it, whatever its size, as if he were holding it completely enclosed in the hollow of his hand. He mentally visualizes a complex form from all round itself; he knows while he looks at one side what the other side is like, he identifies himself with its centre of gravity, its mass, its weight; he realizes its volume, as the space that the shape displaces in the air. And the sensitive observer of sculpture must also learn to feel shape simply as shape, not as description or reminiscence. He must, for example, perceive an egg as a simple single solid shape, quite apart from its significance as food, or from the literary idea that it will become a bird. And so with solids such as a shell, a nut, a plum, a pear, a tadpole, a mushroom, a mountain peak, a kidney, a carrot, a tree-trunk, a bird, a bud, a lark, a ladybird, a bulrush, a bone. From these he can go on to appreciate more complex forms or combinations of several forms. HENRY MOORE The Sculptor Speaks from The Listener 【New words and expressions ⽣词和短语】 auditory adj. 听觉的 colour-blind adj. ⾊盲的 perception n. 知觉 comprehend v. 理解 spatial adj. 空间 visualize v. 使具形象，设想 reminiscence n. 回忆，联想 tadpole n. 蝌蚪 mushroom n. 蘑菇 carrot n. 胡萝⼘ bud n. 花蕾 lark n. 云雀 ladybird n. 瓢⾍ bulrush n. 芦苇 【课⽂注释】 1.respond to 响应，对 … 起反应 例句：He resolved to respond to the call of the Party. 他决⼼响应党的号召。
基于卷积神经网络的改进机械臂抓取方法
CAI Chen WEI Guoliang （School of Optical-Electrical and Computer Engineering，University of Shanghai for Science and Technology，Shanghai 200093）
Abstract In order to improve the accuracy of manipulator grasping，an efficient framework is proposed for detect the optimal position of robotic grasping based on Mask R-CNN. The architecture，which uses RGB-D images as input，makes an accurate in⁃ stance segmentation to determine the category，location and mask information of grabbing objects. Then the weighted depth coordi⁃ nates of the center point are obtained from the denoised depth map by the inverse distance weighted method to form the three-dimen⁃ sional target position. And the final optimal grasping position is obtained by coordinate transformation. As such，the performance can be effectively improved by considering the posture and edge information of target object. Finally，some experiments for grasping network on Universal Robot 3 are utilized to demonstrate the effectiveness of the proposed framework.
Lesson 22 Computer Simulation
Lesson 22 Computer SimulationComputer simulation as a powerful analytic tool widely used in scientific research and engineering design demonstrates unrivalled advantages. With computer simulation, scientists and engineers do not have to build real primary prototypes when they observe an known phenomenon, analyse a complex process, design a machine or a building, etc. Computer simulation is particularly significant when the object under study and examination is costly or even impossible to be built into a real model. For example, to study the cause of engine malfunction that has led to a series of supersonic plane crashed, or to examine the impact on passengers when an airplane crashes, researchers may have to repeat simulati ng the calamities over and over again before they can find out what they need to reach a conclusion. Obviously, these can only be simulated by running computer simulation programs or the like, rather than replicating the tragedies. Another example is engineering design, in which engineers have to try many schemes and parameters before they can come up with a satisfactory design. Using computer simulation programs, engineers can accomplish that iterative process each time by inputting different schemes and parameters into their computer models, rather than building many differentreal models.Virtually, computer simulation is based on mathematical models representing the nature of the object under study or examination. The mathematical model comprises a series of equations that depict the inherent processes of the object in mathematical terms. A computer simulation program includes algorithms that are derived from those equations. The outcome of simulation is usually expressed in rather abstract forms, for example, 2-D diagrams curves, tables and figures.Over the past years, computer graphics techniques have helped computer simulation by creating realistic 3-D images to depict the object to be simulated and the environment around it or the effect imposed on it. Sophisicated computer simulation package capable of providing real-time interactive moving images have emerged, although still beyond the reach of most industries. Meanwhile, many CAD systems have incorporated visual modules to enable engineers to interactively “walk”through their 3-D pseudo models on screen to review their designs and present them to their clients. This convenience is particularly important to architects.Here is an example. To analyse the distribution of stress in a fuselage when the plane is flying, the computer simulationpackage will first set up a mathematical model for this specific theme which comprises equations derived from aerodynamics, elasticity, structural mechanics, then implement a series of computation on a simplified structure of the plane, based on the finite element stress analysis, finally, it gives the outcome which will be clusters of curves spread over the simplified structure of the plane, each indicating the locations in the fuselage that suffer stress of a uniform value. The accuracy of the simulation depends on the accuracy of mathematical model, that is , how closely the model is built to represent the real plane and its environment in terms of mathematics, geometry and mechanics.Computer simulation seems to be the only choice for analysing disasters, calamities and accidents, Quite a lot of impressive simulation have been made on some potentially threatening disasters and calamities, for example, the aftermaths of a global nuclear war, the disastrous effect on global climate and environment as a result of worldwide greenhouse effect, the probability of collision between an invading meteorite and the earth, etc. .Other application of computer simulation technique can be weather forecasting, simulation of combat actions in battlefield, flight programs for training future pilots, human models fortesting new medicines and bus operator’s workstation (as shown in Fig.22-1)New Words and Expressionssimulation [ˌsimjuˈleiʃən]仿真, 模拟demonstrate[ˈdemənstreit]示范, 证明, 论证prototype [ˈprəutətaip]原型phenomenon [fiˈnɔminən]现象malfunction[ˌmælˈfʌŋkʃən]故障supersonic [ˌsju:pəˈsɔnik]超音速的; 超声波crash [kræʃ]碰撞impact [ˈimpækt]碰撞, 冲击, 影响, 效果calamity[kəˈlæmiti]灾难, 不幸事件replicate [ˈreplikeit]复制iterative[ˈitərətiv]重复的, 反复的depict [diˈpikt]描写,描述CAD(Computer Aided Design) 计算机辅助设计pseudo [ˈsju:dəu]假的, 冒充的architect [ˈɑ:kitekt]建筑师fuselage[ˈfju:zilɑ:ʒ]机身aerodynamics [ˌɛərəudaiˈnæmiks]空气动力学elasticity[ilæsˈtisiti]弹力, 弹性geometry[dʒiˈɔmitri]几何学aftermath[ˈɑ:ftəmæθ]结果,后果greenhouse effect [iˈfekt]温室效应collision[kəˈliʒən]碰撞, 冲突meteorite[ˈmi:tjərait]陨星comprise[kəmˈpraiz]包含, 由….组成。
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J. Biichner and J.-P. Kuska
Max-Planck-Institut f'dr Aeronomie, Max-Planck Str. 2, D-37191 Katlenburg-Lindau, Germany
ABSTRACT Recent theoretical investigations and simulations of collisionless space plasma current sheets have claimed their stabilisation against reconnection by finite cross-sheet magnetic field components. However, all these theoretical investigations and simulations were based on two-dimensional models. Currrently we have shown that the energy variations change quite a bit as soon as one considers the problem in three dimensions- the sheet can become unstable in three dimensions where it was stable in two dimensions. Since it is difficult, however, to evaluate the three-dimensional structure of the dynamical current sheet decay analytically, numerical simulations will be helpful. Here in this paper we report results of our numerical simulations of the three-dimensional decay of a thin Harris current sheet. We demonstrate that in three dimensions the decay of a thin collisionless current sheet starts with the unstable growth of a compressional sausage-mode wave, propagating in the current direction. Afterwards reconnection begins at a time scale faster than a tearing mode instability known from the past two-dimensional considerations. INTRODUCTION Reconnection is a very efficient way to transform the energy of plasma flows and magnetic fields to plasma heating and particle acceleration. In space it is supposed to act at interfaces between magnetized plasmas (magnetopauses), during magnetospheric substorms as well as in solar flares and other astrophysical and space plasma energy release processes. From its first suggestion by Giovanelli, Hoyle, Dungey and from its pioneering investigation by Parker, Sweet and Petschek it is clear that reconnection means irreversible energy transformation (see Vasyliunas, 1975 as well as Axford, 1984 and references therein). In coronal and magnetospheric plasmas, which are essentially collisionless, wave-particle resonance processes must provide the mechanism of irreversible energy transfer. Coppi et al. (1966) developed a first appropriate kinetic theory of current sheet reconnection after a so called tearing mode instability. In their theory electron Landau damping provides irreversible wave particle energy transfer for reconnection. Later Biskamp et al. (1970) and Schindler (1974) drew the attention at the more realistic situation of current sheets, whose magnetic fields contain a finite component accross the sheet, which inhibits Landau damping on the electrons. They suggested that in this case Landau damping on ions might provide the necessary dissipation. Galeev and Zelenyi (1976) provided a kinetic analysis of this situation. They pointed out that an ion tearing mode instability might not work due to the stabilizing influence of the reacting electron gas. Lembege and Pellat (1982) claimed the total stability of a sheet which contains a finite normal magnetic field component Bn. Pellat et al. (1991) later confirmed this result on more general grounds. Brittnacher et al. (1994) and Quest et al. (1996) showed that the compression of the plasma frozen in two-dimensional magnetic flux tubes causes the stability of sheets in the presence of a finite Bn. The two-dimensional simulations of Pritchett (1994) confirmed this result (see also Pritchett and Biichner, 1995). All these past models were essentially two-dimensional. This means that the available free energy as well as the irreversible energy transfer are evenly distributed along the invariant, the current direction. And so is the compression of flux tubes, the major stumbling block, inhibiting collisionless reconnection through current sheets penetrated by a finite Bn. The transition to three-dimensional considerations may essentially change the situation. For example the energy necessary to compress a flux tube in order to allow reconnection, changes a lot if one considers a perturbation with a finite wave vector component ky in the current direction in addition to the usual sheet-tearing perturbation modulated perpendicular, in the X-direction (Biichner, 1995; 1996). Unfortunately, it is difficult to describe the three-dimensional dynamics of a current sheet and to understand the dominating mode of a three-dimensional current sheet decay just by analytical methods. In order to find the mode which determines the decay of thin current 177