Current amplification and magnetic reconnection at a three‐dimensional null point: Physical characteristics

Pontin, D. I.; Galsgaard, K.

doi:10.1029/2006ja011848

Cited by 91 publications

(110 citation statements)

References 30 publications

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“…Fig. 4 (t = 1, 2, 3, 5) Thus, for γ > 1 they have current parallel to the spine, which we expect to be resultant from rotational motions, and have very different current sheet and flow structures 25,29 .…”

Section: A Current Evolution and Plasma Flowmentioning

confidence: 94%

Current sheet formation and nonideal behavior at three-dimensional magnetic null points

2007

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The nature of the evolution of the magnetic field, and of current sheet formation, at threedimensional (3D) magnetic null points is investigated. A kinematic example is presented which demonstrates that for certain evolutions of a 3D null (specifically those for which the ratios of the null point eigenvalues are time-dependent) there is no possible choice of boundary conditions which renders the evolution of the field at the null ideal. Resistive MHD simulations are described which demonstrate that such evolutions are generic. A 3D null is subjected to boundary driving by shearing motions, and it is shown that a current sheet localised at the null is formed. The qualitative and quantitative properties of the current sheet are discussed. Accompanying the sheet development is the growth of a localised parallel electric field, one of the signatures of magnetic reconnection. Finally, the relevance of the results to a recent theory of turbulent reconnection is discussed.

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Section: A Current Evolution and Plasma Flowmentioning

confidence: 94%

Current sheet formation and nonideal behavior at three-dimensional magnetic null points

2007

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“…The eigenvalues of the magnetic field gradient B  define whether a null is radial (degenerates into an Xpoint in 2D) or spiral (degenerates into an O-point; Lau & Finn 1990;Parnell et al 1996). In the magnetohydrodynamic (MHD) approximation the theory of magnetic reconnection at null points has been derived by Pontin et al (2004) andPontin & Galsgaard (2007), and reconnection regimes were classified and generalized by Priest & Pontin (2009. Isolated magnetic nulls have been studied by means of MHD ) and kinetic particle-in-cell (PIC; Baumann & Nordlund 2012) simulations.…”

Section: Introductionmentioning

confidence: 99%

Magnetic Null Points in Kinetic Simulations of Space Plasmas

Olshevsky

Deca

Divin

et al. 2016

ApJ

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We present a systematic attempt to study magnetic null points and the associated magnetic energy conversion in kinetic particle-in-cell simulations of various plasma configurations. We address three-dimensional simulations performed with the semi-implicit kinetic electromagnetic code iPic3D in different setups: variations of a Harris current sheet, dipolar and quadrupolar magnetospheres interacting with the solar wind,and a relaxing turbulent configuration with multiple null points. Spiral nulls are more likely created in space plasmas: in all our simulations except lunar magnetic anomaly (LMA) and quadrupolar mini-magnetosphere the number of spiral nulls prevails over the number of radial nulls by a factor of 3-9. We show that often magnetic nulls do not indicate the regions of intensive energy dissipation. Energy dissipation events caused by topological bifurcations at radial nulls are rather rare and short-lived. The so-called X-lines formed by the radial nulls in the Harris current sheet and LMA simulations are rather stable and do not exhibit any energy dissipation. Energy dissipation is more powerful in the vicinity of spiral nulls enclosed by magnetic flux ropes with strong currents at their axes (their crosssections resemble 2D magnetic islands). These null lines reminiscent of Z-pinches efficiently dissipate magnetic energy due to secondary instabilities such as the two-stream or kinking instability, accompanied by changes in magnetic topology. Current enhancements accompanied by spiral nulls may signal magnetic energy conversion sites in the observational data.

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“…These perturbations may be caused by a local driver (Rickard & Titov 1996;Bulanov et al 2002;Pontin et al 2004Pontin et al , 2007aPontin et al ,b, 2011Pontin & Galsgaard 2007;Priest & Pontin 2009;Masson et al 2009;Wyper & Jain 2010;Al-Hachami & Pontin 2010) or by an indirect perturbation (Pariat et al , 2010Edmondson et al 2009;Masson et al 2009Masson et al , 2012 far away from the null. In particular, flow patterns that twist the field about the spine, known as torsional reconnection regimes, dissipate currents that lie parallel to the spine and result in the slippage of the field lines about the spine.…”

Section: Introductionmentioning

confidence: 99%

Magnetohydrodynamics dynamical relaxation of coronal magnetic fields

Fuentes-Fernández

Parnell

2013

A&A

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Context. There are various types of reconnection that may take place at 3D magnetic null points. Each different reconnection scenario must be associated with a particular type of current layer. Aims. A range of current layers may form because the topology of 3D nulls permits currents to form by either twisting the field about the spine of the null or by folding the fan and spine into each other. Additionally, the initial geometry of the field can lead to variations in the currents that are accumulated. Here, we study current accumulations in so-called 3D "tilted" nulls formed by a folding of the spine and fan. A non-zero component of current parallel to the fan is required such that the null's fan plane and spine are not perpendicular. Our aims are to provide valid magnetohydrostatic equilibria and to describe the current accumulations in various cases involving finite plasma pressure. Methods. To create our equilibrium current structures we use a full, non-resistive, magnetohydrodynamic (MHD) code so that no reconnection is allowed. A series of experiments are performed in which a perturbed 3D tilted null relaxes towards an equilibrium via real, viscous damping forces. Changes to the initial plasma pressure and to magnetic parameters are investigated systematically.Results. An initially tilted fan is associated with a non-zero Lorentz force that drives the fan and spine to collapse towards each other, in a similar manner to the collapse of a 2D X-point. In the final equilibrium state for an initially radial null with only the current perpendicular to the spine, the current concentrates along the tilt axis of the fan and in a layer about the null point with a sharp peak at the null itself. The continued growth of this peak indicates that the system is in an asymptotic regime involving an infinite time singularity at the null. When the initial tilt disturbance (current perpendicular to the spine) is combined with a spiral-type disturbance (current parallel to the spine), the final current density concentrates in three regions: one on the fan along its tilt axis and two around the spine, above and below the fan. The increased area of current accumulation leads to a weakening of the singularity formed at the null. The 3D spine-fan collapse with generic current studied here provides the ideal setup for non-steady reconnection studies.

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Current amplification and magnetic reconnection at a three‐dimensional null point: Physical characteristics

Cited by 91 publications

References 30 publications

Current sheet formation and nonideal behavior at three-dimensional magnetic null points

Current sheet formation and nonideal behavior at three-dimensional magnetic null points

Magnetic Null Points in Kinetic Simulations of Space Plasmas

Magnetohydrodynamics dynamical relaxation of coronal magnetic fields

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