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排序方式: 共有119条查询结果,搜索用时 46 毫秒
81.
82.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2020,65(1):129-137
The nature of ionospheric losses from Venus is of essential importance for understanding the ionosphere dynamics of this unmagnetized planet. A plausible mechanism that can explain the escape of charged particles involves the solar wind interaction with the upper atmospheric layers of Venus. The hydrodynamic approach proposed for plasma expansion in the present study comprises two populations of positive ions and the neutralizing electrons, which interact with the solar wind electrons and protons. The fluid equations describing the plasma are solved numerically using a self-similar approach. The behavior of plasma density, velocity, and electric potential, as well as their reliance upon solar wind parameters have been examined. It is found that for noon midnight sites, the oxygen ion-to-electron relative density may be the main factor to enhance the ionic loss. However, the other parameters, like hydrogen density and solar wind density and velocity seem to do not stimulate the runaway ions. For lower dawn-dusk region, the plasma are composed of hydrogen and oxygen ions as well as electrons, but for higher altitudes only hydrogen ions and electrons are encountered. All ionic densities play an important role either to reduce or boost the ionic loss. The streaming solar wind velocity has no effect on the plasma escaping for lower altitudes, but it reduces the expansion at higher altitudes. 相似文献
83.
84.
S.A. Elwakil E.M. Abulwafa E.K. El-Shewy H.M. Abd-El-Hamid 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
A theoretical investigation has been made for electron acoustic waves propagating in a system of unmagnetized collisionless plasma consists of a cold electron fluid and ions with two different temperatures in which the hot ions obey the non-thermal distribution. The reductive perturbation method has been employed to derive the Korteweg–de Vries equation for small but finite amplitude electrostatic waves. It is found that the presence of the energetic population of non-thermal hot ions δ, initial normalized equilibrium density of low temperature ions μ and the ratio of temperatures of low temperature ions to high temperature ions β do not only significantly modify the basic properties of solitary structure, but also change the polarity of the solitary profiles. At the critical hot ions density, the KdV equation is not appropriate for describing the system. Hence, a new set of stretched coordinates is considered to derive the modified KdV equation. In the vicinity of the critical hot ions density, neither KdV nor modified KdV equation is appropriate for describing the electron acoustic waves. Therefore, a further modified KdV equation is derived. An algebraic method with computerized symbolic computation, which greatly exceeds the applicability of the existing tanh, extended tanh methods in obtaining a series of exact solutions of the various KdV-type equations, is used here. Numerical studies have been reveals different solutions e.g., bell-shaped solitary pulses, singular solitary “blowup” solutions, Jacobi elliptic doubly periodic wave, Weierstrass elliptic doubly periodic type solutions, in addition to explosive pulses. The results of the present investigation may be applicable to some plasma environments, such as Earth’s magnetotail region. 相似文献
85.
重装空投动力学分析与控制方案研究 总被引:1,自引:0,他引:1
针对重装空投任务,基于拉格朗日分析力学,建立了货物在机舱内滑行阶段飞机-货物两体系统的动力学模型.给出了一种控制补偿方案,用以抵消或削弱货物运动对飞机的影响.通过数值仿真,观察货物运动和飞机运动之间的耦合关系,分析了牵引伞尺寸对飞机和货物运动参数的影响.仿真结果表明,所给出的控制补偿方案能够有效改善飞机飞行品质,提高飞... 相似文献
86.
Panagiota Petkaki Alexander L. MacKinnon 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. Collisionless magnetic reconnection could provide both the energy release mechanism and the particle accelerator in space plasmas. Here we studied particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbance. Our results are compared to particle acceleration from constant electric field superposed on an X-type magnetic field. The constant electric field configuration represents the effects of steady state magnetic reconnection. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For frequencies (ω in dimensioless units) in the range 0.5 ? ω ? 1.0 a substantial fraction (20%–30%) of the proton distribution is accelerated to gamma-ray producing energies. For frequencies in the range 1 ? ω ? 100.0 the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas. 相似文献
87.
This review is devoted to ponderomotive forces and their importance for the acceleration of charged particles by electromagnetic
waves in space plasmas. Ponderomotive forces constitute time-averaged nonlinear forces acting on a media in the presence of
oscillating electromagnetic fields. Ponderomotive forces represent a useful analytical tool to describe plasma acceleration.
Oscillating electromagnetic fields are also related with dissipative processes, such as heating of particles. Dissipative
processes are, however, left outside these discussions. The focus will be entirely on the (conservative) ponderomotive forces
acting in space plasmas.
The review consists of seven sections. In Section 1, we explain the rational for using the auxiliary ponderomotive forces
instead of the fundamental Lorentz force for the study of particle motions in oscillating fields. In Section 2, we present
the Abraham, Miller, Lundin–Hultqvist and Barlow ponderomotive forces, and the Bolotovsky–Serov ponderomotive drift. The hydrodynamic,
quasi-hydrodynamic, and ‘`test-particle’' approaches are used for the study of ponderomotive wave-particle interaction. The
problems of self-consistency and regularization are discussed in Section 3. The model of static balance of forces (Section
4) exemplifies the interplay between thermal, gravitational and ponderomotive forces, but it also introduces a set of useful
definitions, dimensionless parameters, etc. We analyze the Alfvén and ion cyclotron waves in static limit with emphasis on
the specific distinction between traveling and standing waves. Particular attention has been given to the impact of traveling
Alfvén waves on the steady state anabatic wind that blows over the polar regions (Section~5). We demonstrate the existence
of a wave-induced cold anabatic wind. We also show that, at a critical point, the ponderomotive acceleration of the wind is
a factor of 3 greater than the thermal acceleration. Section 6 demonstrates various manifestations of ponderomotive forces
in the Earth's magnetosphere, for instance the ionospheric plasma acceleration and outflow. The polar wind and the auroral
density cavities are considered in relation to results from the Freja and Viking satellites. The high-altitude energization
and escape of ions is discussed. The ponderomotive anharmonicity of standing Alfvén waves is analyzed from ground based ULF
wave measurements. The complexity of the many challenging problems related with plasma processes near the magnetospheric boundaries
is discussed in the light of recent Cluster observations. At the end of Section 6, we consider the application of ponderomotive
forces to the diversity of phenomena on the Sun, in the interstellar environment, on newborn stars, pulsars and active galaxies.
We emphasize the role of forcing of magnetized plasmas in general and ponderomotive forcing in particular, presenting some
simple conceivable scenarios for massive outflow and jets from astrophysical objects. 相似文献
88.
Jian He Qingguo Zhang 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
In this paper, using the intensity ratio of carbon ions emission lines, we determined the optical depths at the line center of the CI at about 165.7 nm, CII at about 133.5 nm, CIII at about 117.5 nm and CIV at about 154.9 nm emission lines by escape factor treatment. For CI and CII emission lines, we discuss the intensity ratio of two lines arising from the common upper levels; while for CIII and CIV emission lines, we discuss the intensity ratio of two lines arising from the common lower levels. By introducing the measured abundance of carbon and the results of ionization balance calculations, we make an estimate of the line-of-sight physical thickness of the regions of carbon ions. This discussion will provide some results in the discussion of opacity on the solar ultraviolet (UV) or extreme-ultraviolet (EUV) spectrum. 相似文献
89.
90.
重离子在生物组织中的阻止本领 总被引:2,自引:0,他引:2
根据中能重离子与靶物质相互作用的特点,将参数化的阻止本领计算方法进行了改进和推广,给出了计算重离子在人体组织及生物等效介质中阻止本领的方法。作为带电离子适形放疗的基础,计算了中能碳离子在肌肉、骨胳、脂肪、水及其他生物等效介质中的阻止本领,Bragg峰、射程等参量,并对计算方法的可靠性及在临床肿瘤治疗剂量控制方面的应用进行了分析。参数化的计算方法能满足实际应用的要求。 相似文献