LXXVI. Conditions for the occurrence of electrical discharges in astrophysical systems

Dungey, J. W.

doi:10.1080/14786440708521050

Cited by 364 publications

(168 citation statements)

References 9 publications

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“…Sui et al (2004) suggested that this motion is due to the fact that the current sheet develops as a result of the collapse of the X-type neutral point. This scenario was first introduced by Dungey (1953) such that the X-type neutral point is locally unstable if the source region of the relevant configuration could move freely, and the X-point would consequently collapse and develop to a current sheet with a Y-type neutral point (or a reversed current and singularity) at each end (see also Figs. 2.1 and 2.2 of Priest and Forbes 2000).…”

Section: Downward Reconnection Outflowsmentioning

confidence: 99%

Review on Current Sheets in CME Development: Theories and Observations

et al. 2015

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We introduce how the catastrophe model for solar eruptions predicted the formation and development of the long current sheet (CS) and how the observations were used to recognize the CS at the place where the CS is presumably located. Then, we discuss the direct measurement of the CS region thickness by studying the brightness distribution of the CS region at different wavelengths. The thickness ranges from 10 4 km to about 10 5 km at heights between 0.27 and 1.16 R from the solar surface. But the traditional theory indicates that the CS is as thin as the proton Larmor radius, which is of order tens of meters in the corona. We look into the huge difference in the thickness between observations and theoretical expectations. The possible impacts that affect measurements and results are studied, and physical causes leading to a thick CS region in which reconnection can still occur at a reasonably fast rate are analyzed. Studies in both theories and observations suggest that the difference between the true value and the apparent value of the CS thickness is not significant as long as the CS could be recognised in observations. We review observations that show complex structures and flows inside the CS region and present recent numerical modelling results on some aspects of these structures. Both observations and numerical experiments indicate that the downward reconnection outflows are usually slower than the upward ones in the same eruptive event. Numerical simulations show that the complex structure inside CS and its temporal behavior as a result of turbulence and the Petschek-type slow-mode shock could probably account for the thick CS and fast reconnection. But whether the CS itself is that thick still remains unknown since, for the time being, we cannot measure the electric current directly in that region. We also review the most recent laboratory experiments of reconnection driven by energetic laser beams, and discuss some important topics for future works.

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Section: Downward Reconnection Outflowsmentioning

confidence: 99%

Review on Current Sheets in CME Development: Theories and Observations

et al. 2015

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“…Dungey 1953;1958). Following §1.3.1 of Priest & Forbes (2000), we assume a magnetic field of the form:…”

Section: Mathematical Description Of Null Pointsmentioning

confidence: 99%

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

McLaughlin¹,

Hood²,

Moortel³

2010

Space Sci Rev

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We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfvén speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfvén wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfvén-speed profile. In a β = 0 plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfvén wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfvén wave accumulates along the separatrices (in 2D) or along the spine or fan-plane (in 3D). Hence, Alfvén wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, mode-coupling, quasi-periodic pulsations and phase-mixing.

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“…In this paper we present a generalization of exact solutions for a 2D curvilinear geometry 8 , the so-called two and a half dimensions (2 1 2 D) case, where a translational invariant component of velocity and magnetic field along the third direction exists. These solutions describe steady incompressible resistive magnetohydrodynamics (MHD) flows in a configuration with non-vanishing magnetic field.…”

Section: Introductionmentioning

confidence: 99%

Exact solutions for reconnective annihilation in magnetic configurations with three sources

2003

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Exact solutions of the steady resistive three dimensional (3D) magnetohydrodynamics (MHD) equations in cylindrical coordinates for an incompressible plasma are presented. The solutions are translationally invariant along one direction and in general they describe a process of reconnective annihilation in a curved current layer with non vanishing magnetic field. In the derivation of the solutions the ideal case with vanishing resistivity and electric field is considered first and then generalized to include the effects of finite electric field and resistivity. Particular attention is devoted to the analysis how the latter ones influence the presence of singularities in the solutions. In this respect comparisons with the purely two-dimensional case are made and the resulting important differences are highlighted. Finally, applications of the solutions for modeling an important class of solar flares are discussed.PACS numbers: 52.30, 96.60.R

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LXXVI. Conditions for the occurrence of electrical discharges in astrophysical systems

Cited by 364 publications

References 9 publications

Review on Current Sheets in CME Development: Theories and Observations

Review on Current Sheets in CME Development: Theories and Observations

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

Exact solutions for reconnective annihilation in magnetic configurations with three sources

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