Magnetohydrodynamic discontinuities in space plasmas: Interrelation between stability and structure

It is believed that shock waves and other discontinuous flows form the basis of a wide range of phenomena in space medium. We review the results concerning the particular property of MHD discontinuities, the interrelation between their stability and structure. Such an interrelation is associated primarily with the requirement of their evolutionarity. For a non-evolutionary discontinuity the amplitudes of reflected and refracted waves caused by a small amplitude incident wave are not determined unambiguously from the conservation laws at the discontinuity surface. Since the problem of the further time evolution of the initial small perturbation does not have a unique solution, such a discontinuity cannot exist in a real medium as a single steady-state configuration. Therefore it is unstable with respect to a disintegration into several discontinuities or to a transition to some more general unsteady flow. This is confirmed by the fact that, in the studied cases, the non-evolutionary shock transitions can, while the evolutionary ones cannot, be represented as a set of several discontinuities and waves of rarefaction. One more fundamental property of the non-evolutionary shocks that argues for their non-existence is that they do not have a unique structure for all values of the dissipative transport coefficients. At the same time, the possibility of their existence cannot be excluded when the shock has a unique structure or when the disintegration is forbidden for some reasons. Besides the non-evolutionarity, which is a direct reason for a disintegration, there is an indirect one. It can be shown that the hydrodynamic shocks without magnetic field that are corrugationally unstable also allow the shock transitions through more than one discontinuity. This suggests that the shocks unstable in the ordinary sense, for which the small perturbation grows with time, do not exist at all, but they disintegrate into stable ones. However, the physical mechanism that distinguishes between these two scenarios remains unclear. The interrelation between the stability and structure of MHD discontinuities thus governs their nonlinear evolution. This fact is essential when describing shock waves and other discontinuous flows in the space medium.     

Tearing instability of reconnecting current sheets in space plasmas

Magnetic reconnection provides an efficient conversion of the so-called free magnetic energy to kinetic and thermal energies of cosmic plasmas, hard electromagnetic radiation, and accelerated particles. This phenomenon was found in laboratory and space, but it is especially well studied in the solar atmosphere where it manifests itself as flares and flare-like events. We review the works devoted to the tearing instability — the inalienable part of the reconnection process — in current sheets which have, inside of them, a transverse (perpendicular to the sheet plain) component of the magnetic field and a longitudinal (parallel to the electric current) component of the field. Such non-neutral current sheets are well known as the energy sources for flare-like processes in the solar corona. In particular, quasi-steady high-temperature turbulent current sheets are the energy sources during the main or hot phase of solar flares. These sheets are stabilized with respect to the collisionless tearing instability by a small transverse component of magnetic fiel, normally existing in the reconnecting and reconnected magnetic fluxes. The collision tearing mode plays, however, an important and perhaps dominant role for non-neutral current sheets in solar flares. In the MHD approximation, the theory shows that the tearing instability can be completely stabilized by the transverse fieldB _n if its value satisfies the conditionB _n /BS ^–3/4 B is the reconnecting component of the magnetic field just near the current sheet,S is the magnetic Reynolds number for the sheet. In this case, stable current sheets become sources of temporal spatial oscillations and usual MHD waves. The application of the theory to the solar atmosphere shows that the effect of the transverse field explains high stability of high-temperature turbulent current sheets in the solar corona. The stable current sheets can be sources of radiation in the radio band. If the sheet is destabilized (atB _n /BS ^–3/4) the compressibility of plasma leads to the arizing of the tearing instability in a long wave region, in which for an incompressible plasma the instability is absent. When a longitudinal magnetic field exists in the current sheet, the compressibility-induces instability can be dumped by the longitudinal field. These effects are significant in destabilization of reconnecting current sheets in solar flares: in particular, the instability with respect to disturbances comparable with the width of the sheet is determined by the effect of compressibility.     

Stability and waves of transonic laboratory and space plasmas

The properties of magnetohydrodynamic waves and instabilities of laboratory and space plasmas are determined by the overall magnetic confinement geometry and by the detailed distributions of the density, pressure, magnetic field, and background velocity of the plasma. Consequently, measurement of the spectrum of MHD waves (MHD spectroscopy) gives direct information on the internal state of the plasma, provided a theoretical model is available to solve the forward as well as the inverse spectral problems. This terminology entails a program, viz. to improve the accuracy of our knowledge of plasmas, both in the laboratory and in space. Here, helioseismology (which could be considered as one of the forms of MHD spectroscopy) may serve as a luminous example. The required study of magnetohydrodynamic waves and instabilities of both laboratory and space plasmas has been conducted for many years starting from the assumption of static equilibrium. Recently, there is a outburst of interest for plasma states where this assumption is violated. In fusion research, this interest is due to the importance of neutral beam heating and pumped divertor action for the extraction of heat and exhaust needed in future tokamak reactors. Both result in rotation of the plasma with speeds that do not permit the assumption of static equilibrium anymore. In astrophysics, observations in the full range of electromagnetic radiation has revealed the primary importance of plasma flows in such diverse situations as coronal flux tubes, stellar winds, rotating accretion disks, and jets emitted from radio galaxies. These flows have speeds which substantially influence the background stationary equilibrium state, if such a state exists at all. Consequently, it is important to study both the stationary states of magnetized plasmas with flow and the waves and instabilities they exhibit. We will present new results along these lines, extending from the discovery of gaps in the continuous spectrum and low-frequency Alfvén waves driven by rotation to the nonlinear flow patterns that occur when the background speed traverses the full range from sub-slow to super-fast. This revised version was published online in August 2006 with corrections to the Cover Date.     

Relation between laser flux,temperature and ionisation equilibrium in laser produced plasmas

Laser produced plasmas are a useful source of highly stripped ions for XUV spectroscopy and it is therefore important to understand the relation between the incident laser flux, plasma temperature and ionisation equilibrium.     

Waves in space plasmas: Highlights of a Conference held in Hawaii, 7–11 February 1983

The Conference was called to bring together investigators of magnetospheric plasma waves having frequencies from VLF whistlers and emissions down through ELF and ULF to Pc5 long period pulsations. The emphasis was on the physics and techniques underlying the entire frequency range. Topics included wave electron interactions and electron precipitation, ray tracing and other methods to track down sources of VLF and ULF waves, VLF-ULF relationships, heavy ion effects in ULF propagation, and long period ULF waves.     

Wing stressed state in the region of torque discontinuities

A problem of determining the stressed state of the thin-walled beam in the zone of deplanation constraint caused by a torque discontinuity is considered using Odinokov’s formulation of mutually-balanced end loading.     

Fields and plasmas in the outer solar system

The most significant information about fields and plasmas in the outer solar system, based on observations by Pioneer 10 and 11 investigations, is reviewed. The characteristic evolution of solar wind streams beyond 1 AU has been observed. The region within which the velocity increases continuously near 1 AU is replaced at larger distances by a thick interaction region with abrupt jumps in the solar wind speed at the leading and trailing edges. These abrupt increases, accompanied by corresponding jumps in the field magnitude and in the solar wind density and temperature, consist typically of a forward and a reverse shock. The existence of two distinct corotating regions, separated by sharp boundaries, is a characteristic feature of the interplanetary medium in the outer solar system. Within the interaction regions, compression effects are dominant and the field strength, plasma density, plasma temperature and the level of fluctuations are enhanced. Within the intervening quiet regions, rarefaction effects dominate and the field magnitude, solar wind density and fluctuation level are very low. These changes in the structure of interplanetary space have significant consequences for the many energetic particles propagating through the medium. The interaction regions control the access to the inner solar system of relativistic electrons from Jupiter's magnetosphere. The interaction regions and shocks appear to be associated with an acceleration of solar protons to MeV energies. Flare-generated shocks are observed to be propagating through the outer solar system with constant speed, implying that the previously recognized deceleration of flare shocks takes place principally near the Sun. Radial gradients in the solar wind and interplanetary field parameters have been determined. The solar wind speed is nearly constant between 1 and 5 AU with only a slight deceleration of 30 km s⁺¹ on the average. The proton flux follows an r ⁺² dependence reasonably well, however, the proton density shows a larger departure from this dependence. The proton temperature decreases steadily from 1 to 5 AU and the solar wind protons are slightly hotter than anticipated for an adiabatic expansion. The radial component of the interplanetary field falls off like r ⁺² and, on the average, the magnitude and spiral angle also agree reasonably well with theory. However, there is evidence, principally within quiet regions, of a significant departure of the azimuthal field component and the field magnitude from simple theoretical models. Pioneer 11 has obtained information up to heliographic latitudes of 16°. Observations of the interplanetary sector structure show that the polarity of the field becomes gradually more positive, corresponding to outward-directed fields at the Sun, and at the highest latitudes the sector structure disappears. These results confirm a prior suspicion that magnetic sectors are associated with an interplanetary current sheet surrounding the Sun which is inclined slightly to the solar equator.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Control of a large space structure using MMAE/MMAC techniques

The application of moving-bank multiple model adaptive estimation and control (MMAE/MMAC) algorithms to an actual spade structure (Space Integrated Controls Experiment (SPICE)) being examined at Phillips Laboratory at Kirtland AFB, NM, is presented. The structure consists of a large platform and a smaller platform connected by three legs in a tripod fashion. Kalman filtering and LQG (linear system, quadratic cost, Gaussian noise) control techniques are utilized as the primary design tools for the components of the MMAE/MMAC. Implementing a bank of filters or controllers increases the robustness of the algorithms when uncertainties exist in the system model, whereas the moving bank is utilized to reduce the computational load. Several reduced-order models are developed from the truth model using modal analysis and modal cost analysis. The MMAE/MMAC design with a substantially reduced-order filter model provides an excellent method to estimate a wide range of parameter variations and to quell oscillations in the structure.     

Measurements of collisional rate coefficients in laboratory plasmas

Line radiation emitted by highly ionized atoms embedded in hot laboratory plasmas can be utilized to obtain collisional rate coefficients for excitation and ionization. After a discussion of the principles underlying these measurements, the plasma device mostly used is explained briefly as are the various experimental techniques. All experimental results obtained so far are finally discussed and compared with theoretical calculations where possible.     

Reconnective release of magnetic energy in astrophysical plasmas

Two particular examples are considered of astrophysical objects containing a highly conducting tenuous plasma with an excess magnetic energy supplied by an external source. The first example is the solar corona, whose magnetic field is continuously distorted by footpoint shuffling due to photospheric motions. The second case it an extragalactic jet extending from a galactic nucleus with an immersed magnetic field, and which is perturbed by variations in the pressure of the external medium. In both cases it is assumed that the system tends towards its lowest magnetic energy equilibrium via magnetic reconnection, thus providing a fast release of injected magnetic energy. Explicit relations between the characteristics of the external driver and the magnetic energy dissipation rate in these objects have been obtained. The relevance of this mechanism for heating the solar corona and maintaining radio emission from extragalactic jets is then. discussed by comparing these results with observational data.     

飞行器表面电磁缺陷及雷达吸波材料应用

从飞行器表面电磁特性和电磁缺陷分析入手,就如何使用雷达吸波材料进行了探讨,提出了采用雷达吸波材料或特种良导电材料,针对电磁缺陷进行修复的雷达吸波材料应用思路。该方法可减少雷达吸波材料的使用量,从而减轻飞行器重量,提高其性能,降低成本,改善维修性。并进行了试验验证。     

A lower bound estimate of the critical load for helicopter main rotor composite blade according to the limit equilibrium theory. Analysis results

The results of analyzing the stress-strain and limiting states of a main rotor blade of the Ansat helicopter in the hovering mode are given. To estimate the strength, a technique of elastic constants variation is used; it is based on the limit equilibrium theory and enables us to obtain the upper and lower bounds of the critical load simultaneously. Strength margin coefficients of the blade root section are determined depending on the azimuth of a blade turn, when all generalized forces vary in proportion to one loading parameter.     

Auroral plasmas in the evening sector: Satellite observations and theoretical interpretations

Space Science Reviews - Observations and theoretical interpretations of auroral plasma distributions have led to a spectacular advance, in the latter part of the 1970's, in understanding the...     

基于CR理论的大柔性机翼几何非线性结构建模

大柔性机翼在气动载荷的作用下,将产生显著的弹性变形,常规线弹性理论的小变形假设不再成立,需要采用能够考虑几何非线性效应的结构模型进行求解。基于CR(Co-Rotational)共旋转有限元理论,把几何非线性大变形分解为刚体的旋转和平移及局部坐标系下的弹性变形,建立了适用于大柔性机翼几何非线性变形描述的结构模型。以大柔性悬臂梁为例,采用载荷增量法,研究了集中弯矩作用下的非线性变形特征,对静力学方法进行了验证,并讨论了耦合加载几何非线性变形特征;以类"太阳神"太阳能布局无人机(UAV)为例,研究了其几何非线性大变形特性。     

关于热力学空间概念的探讨

在气体力学研究中,不可避免地将引用热力学的基本定律,过去在不少工科著者所用的论述中常将传热量Q和气体所做的功W的微增量写为dQ和dW。但从严格的数学理论而言,这是不正确的表达式。笔者在参与《航空空气动力手册》的编审中曾经提出过此问题,在手册的再版时已经改正为δQ和δW。但此后一些气体力学的著作未能引起注意,仍然习惯地写成dQ和dW。为此,在本文中作一个严格的证明,并提出了热力学空间的概念,以澄清以往的混淆。