Steady state reconnection at a single 3D magnetic null point

Galsgaard, K.; Pontin, D. I.

doi:10.1051/0004-6361/201014359

Cited by 39 publications

(32 citation statements)

References 26 publications

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“…Magnetic null points are known to be a location that can host magnetic reconnection processes, and through this process change the connectivity of the magnetic field globally while affecting all of the dome region due to the flipping of magnetic field lines through the diffusion region. The basic evolution of an initially potential magnetic null point has been investigated in detail by Pontin et al (2007), Galsgaard & Pontin (2011b), and Galsgaard & Pontin (2011a), which shows how the null point structure is perturbed strongly and generates systematic outflow regions that push the reconnected flux out of the current sheet region. A more realistic configuration is investigated by Pontin et al (2013).…”

Section: Discussionmentioning

confidence: 99%

Magnetic topological analysis of coronal bright points

Galsgaard

Madjarska

Moreno‐Insertis

et al. 2017

A&A

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Context. We report on the first of a series of studies on coronal bright points investigating the physical mechanism that generates these phenomena. Aims. The aim of this paper is to understand the magnetic-field structure that hosts the bright points. Methods. We use longitudinal magnetograms taken by the Solar Optical Telescope with the Narrowband Filter Imager. For a single case, magnetograms from the Helioseismic and Magnetic Imager were added to the analysis. The longitudinal magnetic field component is used to derive the potential magnetic fields of the large regions around the bright points. A magneto-static field extrapolation method is tested to verify the accuracy of the potential field modelling. The three dimensional magnetic fields are investigated for the presence of magnetic null points and their influence on the local magnetic domain. Results. In 9 out of 10 cases the bright point resides in areas where the coronal magnetic field contains an opposite polarity intrusion defining a magnetic null point above it. It is found that X-ray bright points reside, in these 9 cases, in a limited part of the projected fan dome area, either fully inside the dome or expanding over a limited area below which typically a dominant flux concentration resides. The 10th bright point is located in a bipolar loop system without an overlying null point. Conclusions. All bright points in coronal holes and two out of tree bright points in quiet Sun regions are seen to reside in regions containing a magnetic null point. An yet unidentified process(es) generates the BPs in specific regions of the fan-dome structure.

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Section: Discussionmentioning

confidence: 99%

Magnetic topological analysis of coronal bright points

Galsgaard

Madjarska

Moreno‐Insertis

et al. 2017

A&A

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“…Three-dimensional (3D) null points are thought to be one such location (Klapper et al 1996). Various forms of reconnection can occur at 3D nulls (for a review see Priest & Pontin 2009) and the nature of the reconnection has been analyzed in some detail (e.g., Pontin et al 2004Pontin et al , 2005Galsgaard & Pontin 2011).…”

Section: Introductionmentioning

confidence: 99%

A Time-Dependent Model for Magnetic Reconnection in the Presence of a Separator

Wilmot-Smith

Hornig

2011

ApJ

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We present a model for separator reconnection due to an isolated reconnection process. Separator reconnection is a process which occurs in the neighborhood of a distinguished field line (the separator) connecting two null points of a magnetic field. It is, for example, important for the dynamics of magnetic flux at the dayside magnetopause and in the solar corona. We find that, above a certain threshold, such a reconnection process generates new separators, which leads to a complex system of magnetic flux tubes connecting regions of previously separated flux. Our findings are consistent with the findings of large numbers of separators in numerical simulations. We discuss how to measure and interpret the reconnection rate in a configuration with multiple separators.

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“…The boundaries are closed in all three directions, as in other null point studies (e.g. Pontin, Bhattacharjee & Galsgaard 2007;Galsgaard & Pontin 2011). On the upper and lower boundaries we impose twisting motions.…”

Section: Illustrative Applicationmentioning

confidence: 99%

Braginskii magnetohydrodynamics for arbitrary magnetic topologies: coronal applications

2017

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We investigate single-fluid magnetohydrodynamics (MHD) with anisotropic viscosity, often referred to as Braginskii MHD, with a particular eye to solar coronal applications. First, we examine the full Braginskii viscous tensor in the single-fluid limit. We pay particular attention to how the Braginskii tensor behaves as the magnetic field strength vanishes. The solar corona contains a magnetic field with a complex and evolving topology, so the viscosity must revert to its isotropic form when the field strength is zero, e.g. at null points. We highlight that the standard form in which the Braginskii tensor is written is not suitable for inclusion in simulations as singularities in the individual terms can develop. Instead, an altered form, where the parallel and perpendicular tensors are combined, provides the required asymptotic behaviour in the weak-field limit. We implement this combined form of the tensor into the Lare3D code, which is widely used for coronal simulations. Since our main focus is the viscous heating of the solar corona, we drop the drift terms of the Braginskii tensor. In a stressed null point simulation, we discover that small-scale structures, which develop very close to the null, lead to anisotropic viscous heating at the null itself (that is, heating due to the anisotropic terms in the viscosity tensor). The null point simulation we present has a much higher resolution than many other simulations containing null points so this excess heating is a practical concern in coronal simulations. To remedy this unwanted heating at the null point, we develop a model for the viscosity tensor that captures the most important physics of viscosity in the corona: parallel viscosity for strong field and isotropic viscosity at null points. We derive a continuum model of viscosity where momentum transport, described by this viscosity model, has the magnetic field as its preferred orientation. When the field strength is zero, there is no preferred direction for momentum transport and viscosity reverts to the standard isotropic form. The most general viscous stress tensor of a (single-fluid) plasma satisfying these conditions is found. It is shown that the Braginskii model, without the drift terms, is a specialization of the general model. Performing the stressed null point simulation with this simplified model of viscosity reveals very similar heating profiles compared to the full Braginskii model. The new model, however, does not produce anisotropic heating at the null point, as required. Since the vast majority of coronal simulations use only isotropic viscosity, we perform the stressed null point simulation with isotropic viscosity and compare the heating profiles to those of the anisotropic models. It is shown than the fully isotropic viscosity can over-estimate the viscous heating by an order of magnitude.

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Steady state reconnection at a single 3D magnetic null point

Cited by 39 publications

References 26 publications

Magnetic topological analysis of coronal bright points

Magnetic topological analysis of coronal bright points

A Time-Dependent Model for Magnetic Reconnection in the Presence of a Separator

Braginskii magnetohydrodynamics for arbitrary magnetic topologies: coronal applications

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