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

Pontin, D. I.; Bhattacharjee, A.; Galsgaard, K.

doi:10.1063/1.2722300

Cited by 88 publications

(117 citation statements)

References 30 publications

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“…For short loops, in contrast, the orientation is far from the vertical, the null point is positioned well over on the side of the dome nearest the far loop footpoint, and the dome is strongly asymmetric. In the latter case, it is much easier to displace the inner and outer spine lines along the fan surface, thereby distorting the potential null point into a current patch where reconnection can occur (Antiochos 1996;Pontin et al 2007). In the former case, the reconnection is impeded very effectively until the onset of an ideal kink-like instability strongly breaks the nearaxisymmetry of the dome.…”

Section: Results I: Long and Short Of Loop Jetsmentioning

confidence: 99%

Simulations of Solar Jets Confined by Coronal Loops

Wyper

DeVore

2016

ApJ

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Coronal jets are collimated, dynamic events that occur over a broad range of spatial scales in the solar corona. In the open magnetic field of coronal holes, jets form quasi-radial spires that can extend far out into the heliosphere, while in closed-field regions the jet outflows are confined to the corona. We explore the application of the embedded-bipole model to jets occurring in closed coronal loops. In this model, magnetic free energy is injected slowly by footpoint motions that introduce twist within the closed dome of the jet source region, and is released rapidly by the onset of an ideal kink-like instability. Two length scales characterize the system: the width (N) of the jet source region and the footpoint separation (L) of the coronal loop that envelops the jet source. We find that both the conditions for initiation and the subsequent dynamics are highly sensitive to the ratio L/N. The longest-lasting and most energetic jets occur along long coronal loops with large L/N ratios, and share many of the features of open-field jets, while smaller L/N ratios produce shorter-duration, less energetic jets that are affected by reflections from the far-loop footpoint. We quantify the transition between these behaviors and show that our model replicates key qualitative and quantitative aspects of both quiet Sun and active-region loop jets. We also find that the reconnection between the closed dome and surrounding coronal loop is very extensive: the cumulative reconnected flux at least matches the total flux beneath the dome for small L/N, and is more than double that value for large L/N.

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Section: Results I: Long and Short Of Loop Jetsmentioning

confidence: 99%

Simulations of Solar Jets Confined by Coronal Loops

Wyper

DeVore

2016

ApJ

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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: 95%

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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“…temporally localised) perturbation of the null point, described by Pontin et al (2007a) and Priest & Pontin (2009). In these investigations, the boundary driving was applied for a fixed period of time and then reduced to zero.…”

Section: Discussionmentioning

confidence: 99%

“…In previous simulations (e.g. Pontin et al 2007a) this was a relatively simple process, as the diffusion region was limited to a finite size. However, the situation here is different.…”

Section: Reconnection Ratementioning

confidence: 99%

Steady state reconnection at a single 3D magnetic null point

Galsgaard¹,

Pontin

2011

A&A

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Aims. We systematically stress a rotationally symmetric 3D magnetic null point by advecting the opposite footpoints of the spine axis in opposite directions. This stress eventually concentrates in the vicinity of the null point, thereby forming a local current sheet through which magnetic reconnection takes place. The aim is to look for a steady state evolution of the current sheet dynamics, which may provide scaling relations for various characteristic parameters of the system. Methods. The evolution is followed by solving numerically the non-ideal MHD equations in a Cartesian domain. The null point is embedded in an initially constant density and temperature plasma. Results. It is shown that a quasi-steady reconnection process can be set up at a 3D null by continuous shear driving. It appears that a true steady state is unlikely to be realised because the current layer tends to grow until it is restricted by the geometry of the computational domain and the imposed driving profile. However, ratios between characteristic quantities clearly settle after some time to stable values, so that the evolution is quasi-steady. The experiments show a number of scaling relations, but they do not provide a clear consensus for extending to lower magnetic resistivity or faster driving velocities. More investigations are needed to fully clarify the properties of current sheets at magnetic null points.

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

Cited by 88 publications

References 30 publications

Simulations of Solar Jets Confined by Coronal Loops

Simulations of Solar Jets Confined by Coronal Loops

Braginskii magnetohydrodynamics for arbitrary magnetic topologies: coronal applications

Steady state reconnection at a single 3D magnetic null point

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