Magnetic field reconnection theory and the solar wind — Magnetosphere interaction: A review

This review considers the theory of the magnetic field line reconnection and its application to the problem of the interaction between the solar wind and the Earth's magnetosphere. In particular, we discuss the reconnection models by Sonnerup and by Petschek (for both incompressible and compressible plasmas, for the asymmetric and nonsteady-state cases), the magnetic field annihilation model by Parker; Syrovatsky's model of the current sheet; and Birn's and Schindler's solution for the plasma sheet structure. A review of laboratory and numerical modelling experiments is given.Results concerning the field line reconnection, combined with the peculiarities of the MHD flow, were used in investigating the solar wind flow around the magnetosphere. We found that in the presence of a frozen-in magnetic field, the flow differs significantly from that in a pure gas dynamic case; in particular, at the subsolar. part of the magnetopause a stagnation line appears (i.e., a line along which the stream lines are branching) instead of a stagnation point. The length and location of the stagnation line determine the character of the interaction of the solar wind with the Earth's magnetosphere. We have developed the theory of that interaction for a steady-state case, and compare the results of the calculations with the experimental data.In the last section of the review, we propose a qualitative model of the solar wind — the Earth's magnetosphere interaction in the nonsteady-state case on the basis of the solution of the problem of the spontaneous magnetic field line reconnection.     

Structure of the magnetopause: Observations and implications for reconnection

The Earth's magnetopause is the boundary between a hot tenuous plasma in the magnetosphere and a cooler denser plasma in the magnetosheath. Both of these plasmas contain magnetic fields whose directions are usually different but whose magnitudes are often comparable. Efforts to understand the structure of the magnetosphere have been hampered by the variability and complexity of this boundary. Waves on the magnetopause surface propagate toward the magnetotail and produce the multiple boundary crossings frequently seen by spacecraft. Boundary velocities are poorly known and range anywhere within an order of magnitude of 10 km s^–1. Typical thicknesses are probably on the order of a few hundred km which is a few times the gyroradius of a thermal proton. Although conclusive direct evidence for a field component, B _n , across the magnetopause has not been found, this lack of evidence may reflect the difficulty in determining B _n in the presence of magnetopause waves rather than the real absence of this component. Considerable indirect evidence exists for an open magnetosphere, but the importance of the reconnection process thought to produce open field lines has recently been questioned.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Magnetopause stability threshold for patchy reconnection

This review is devoted to the problem of the internal fine structure of the Earth's magnetopause. A number of theoretical and experimental papers dealing with this subject is discussed from a unified viewpoint. The Vlasov kinetic approach is used to study the stability of magnetopause magnetic surfaces that can be destructed by the growth and overlapping of magnetic islands. The stochastic wandering of magnetic field lines between the destructed surfaces can result in magnetic percolation, i.e. the appearance of a topological connection of interplanetary and geomagnetic field lines. Such a process may be considered as a mechanism of the macroscopic (but spatially localized) reconnection. We discuss this in relation with the phenomena of spontaneous patchy reconnection, recently observed at ISEE satellites and now known as flux transfer events.Drift tearing mode, which is responsible for the growth of magnetic islands can be stabilized due to its coupling with ion sound waves, and the process of percolation will be interrupted if even a thin region with smooth stable magnetic surfaces exists within the magnetopause. Accordingly, we obtain a magnetopause stability threshold for localized reconnection. It is represented in the form of dependence of marginal dimensionless thickness of the magnetopause on the angle of magnetic field rotation within it.Further, we discuss the possible role of lower hybrid turbulence permanently observed within the. magnetopause and speeding up the process of reconnection. Nonlinear calculations supporting the developed model are given in the appendices. We consider briefly the motion of reconnecting flux tubes and evaluate the time necessary for the accomplishment of percolation. The calculations show that the appearance of reconnection patchies at the dayside magnetopause cannot occur too far from the stagnation region. The latter agrees with experimental indications on the most probable site of the formation of flux transfer events. In the concluding part of the review we discuss the necessary limitations on the theory, possible lines of its future advance and comparison with the experimental data.     

Observational support of reconnection in solar flares

We present a detailed analysis of the magnetic topology of flaring active region. TheH kernels are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibrils or/and transverse magnetic field direction, is taken into account. We show that the kernels are magnetically connected by field lines passing close to the separator. We confirm, for other flares, previous studies which show that photospheric current concentrations are located at the borders of flare ribbons. Moreover we found two photospheric current concentrations of opposite sign, linked in the corona by field lines which follow separatrices. These give evidence that magnetic energy is released by reconnection processes in solar flares.     

Energetics of terrestrial and Jovian magnetospheres

Energy supply rates to magnetospheres of Earth and Jupiter are estimated on the basis of most recent information on controlling parameters, and these are compared with energy consumption rates inside respective magnetospheres. Dominant role played by the reconnection process is suggested, but the high ejection rate of energetic electrons from the Jovian magnetosphere reported by Conlon (1978) might suggest the importance of the planetary rotation as the energy source.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

About ‘reconnection’ in a collisionless plasma

Two kinds of mechanisms have been advertised for magnetic field line reconnection in plasmas: a slow diffusive process, proposed by Parker and Sweet, related to the Tearing mode and field line stochasticity; an Alfénic flow, with a fast merging rate, the so-called Petschek theory. We consider both mechanisms successively and emphasize the yet unsolved theoretical difficulties.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Manifestation of magnetic reconnection in coronal streamer current sheets

We investigate the possibility of observing the effects of magnetic reconnection inside a current sheet forming in a coronal streamer in the extended corona. In particular we study the possibility to observe with the UVCS of SOHO the excitation of the tearing instability in the current sheet.     

Magnetic reconnection,merging, and viscous interaction in the magnetosphere

Two ideas were advanced for the process of solar wind-magnetospheric interaction in the same year 1961. Dungey suggested that the interplanetary magnetic field (IMF), although weak, might determine the nature of this process by magnetic reconnection as the solar wind plasma flows across the separatrix surface which divides the IMF from the geomagnetic field. Axford and Hines pointed out that the flow inside the magnetopause is in the same sense as the magnetosheath flow and appears to be viscously coupled. Within a few years the dependence of geomagnetic activity on the IMF predicted by Dungey's mechanism was observed, and reconnection began to dominate current theories. One difficulty, that of the implied dissipation at the magnetopause, was troublesome; however, the ISEE-1/2 observations of the predicted high speed flows on several occasions was enough to convince many persons that reconnection ideas were basically correct. Several investigators found some evidence in the ISEE-3 data in the distant magnetotail for the steady-state reconnection line, as demanded by the Dungey model, in the form of a southward sense of the magnetic field through the current sheet. Here, again, there is some hard contrary evidence when the data are analyzed exactly at the cross-tail current sheet: the instantaneous values show a northward sense, even at high values of auroral activity. Coupled with the anti-Sunward plasma flow, this repudiates the steady-state Dungey model. On the other hand, it lends strong support to some kind of viscous effect through the medium of the magnetospheric boundary layer. This is not a semantic problem, as the sense of the electric field (as well as the magnetic field) is opposite for the two cases. The downfall of the reconnection model is its implicit use of frozen-field convection; this problem is obvious when the problem is viewed in three dimensions. Instead, the view is taken that the relevant process must be essentially time-dependent, three-dimensional, and localized. It is proposed that the term merging be used for this generalized timedependent form of reconnection. The merging process (whatever it is) must permit solar wind plasma to cross the magnetopause onto closed field lines of the boundary layer. Once it is there, it provides the viscous-like effect that Axford and Hines had envisaged.     

Comments with reply, on `A new look at nonseparable syntheticaperture radar processing' by A. Di Cenzo

The commenters point out that the idea using a two-dimensional digital correlation technique to perform synthetic-aperture-radar (SAR) processing, presented as new in the above-titled paper (see ibid., vol.24, p.218-23, May 1988), was described by them as early as 1978 and has since been described by other authors. They discuss some of these earlier studies. The author replies that he was unaware of the earlier work, and that he did not intend to convey the impression that the nonseparable transform domain processor that he presented was the first     

Comments, with reply, on `Optimal detection and performance ofdistributed sensor systems'

The commenter observes that the general solution proposed in the above paper (see ibid., vol.AES-23, no.1, p.24-30, Jan. 1988) for the global optimization of a distributed sensor detection system with fusion leads to coupled equations whose solution is a formidable computational task. This necessitated several simplifying assumptions that he comments on here. In response, the authors review the extent of the equal local threshold assumption throughout the work and make comments on the numerical performance comparison they gave there     

试论高等职业教育如何应对知识经济的挑战

知识经济时代给我国高等职业教育带来的诸多挑战，只有深化高等职业教育改革才能应对挑战。     

Further comments on “Optimal sensor selection strategy fordiscrete-time estimators” [and reply]

The fundamental TPBVP usually underlying true “optimal sensor selection strategy” is revisited to obtain practical real-time mechanizations as a solution to an exclusively initial value problem     

Comments, with reply, on `Status of CR-like lower bounds fornonlinear filtering' by T.H. Kerr

The commenters object to criticisms of their work that appeared in the above-titled paper (ibid., vol.25, no.5, p.590-601, Sept. 1989) and provide a defense of their results. The author presents further arguments to buttress his claims     

建筑师的麦加哥伦布小镇

<正>城市,在二十一世纪早已变成了一言难尽的人工风景。游览现代城市,美国无疑是首选之地。如果因循专业人士的建议,看完了芝加哥、纽约、旧金山、波士顿和华盛顿,那么,美国建筑师学会(AlA)将指引你去哥伦布(Columbus)。没错,要跑到中西部的印第安纳州,从首府印第安纳波利斯市还要驱车向南四十英里。然而,就是这个只有四万人口的小镇,在美国城市建设排行榜上,却赫然在列,居第六位。     

Tools of Kinematics: A Crital Review

A review of the basic tools of kinematics widely used by aerospace engineers is presented. Two different approaches are discussed. One is the familiar "vectorial mechanics," while the other is called the "variational method." It is indicated that either approach should produce the same set of motion equations for the dynamical system considered.     

Checkout: A Changing Technology

On-board/in-flight checkout of future aerospace systems will necessitate a fundamental departure from today's design and checkout activities. The interrelationship of checkout, incorporated into prime vehicular functions, man as a subsystem, and standardization Of hardware/software must be considered as a functional and integral entity if the efforts of today's long-range planning are to become tomorrow's reality. This paper will describe future systems checkout, maintenance and support considerations, and actions and tentative measures necessary for implementation. The realization of the implementation of these concepts into functional elements will depend, accurately and economically, upon the degree of government perceptiveness and the extent of industrial support.     

Stretch: A Time-Transformation Technique

Stretch is a passive, linear, time-variant technique for performing temporal operations on many classes of signals. The technique employs three dispersive networks and a mixer. Signal slowdown, speedup, or time reversal can be attained by choice of network slopes. These temporal operations are performed within a signal "window," and the duration of the window is determined by the network time-bandwidth products. Both heuristic argumentation and rigorous analysis are presented, as are the results of a simple laboratory experiment.     

The geologic development of Mars: A review

The planet Mars has been the subject of a continuing program of exploration with the flyby missions of 1964 and 1969, the orbiter of 1971, and the present Viking Project with both orbiters and landers. The overall view of Mars has changed from Earthlike in the prespacecraft era to Moonlike following the flyby missions and finally to a planet with intermediate characteristics. There are many impact craters as on the Moon, but tectonic and volcanic features resembling structures on Earth are also present. However, there is a lack of evidence for the compressional deformation associated with terrestrial plate tectonics and continental drift.The current analyses indicate that Mars has a differentiated interior with a crust and mantle and perhaps a core. Whatever the nature of interior processes, whether overall mantle expansion, plumes, or full scale convection, the effects at the surface have been predominantly vertical with formation of broad regions of uplift and depression. One of the results is hemispheric asymmetry with cratered terrain in the south and younger uncratered plains in the north.