157 research outputs found
On the nature of reconnection at a solar coronal null point above a separatrix dome
No full textThree-dimensional magnetic null points are ubiquitous in the solar corona and in any generic mixed-polarity magnetic field. We consider magnetic reconnection at an isolated coronal null point whose fan field lines form a dome structure. Using analytical and computational models, we demonstrate several features of spine-fan reconnection at such a null, including the fact that substantial magnetic flux transfer from one region of field line connectivity to another can occur. The flux transfer occurs across the current sheet that forms around the null point during spine-fan reconnection, and there is no separator present. Also, flipping of magnetic field lines takes place in a manner similar to that observed in the quasi-separatrix layer or slip-running reconnection.Peer reviewe
Current sheets at three-dimensional magnetic nulls:effect of compressibility
No full textThe nature of current sheet formation in the vicinity of three-dimensional (3D) magnetic null points is investigated. The particular focus is upon the effect of the compressibility of the plasma on the qualitative and quantitative properties of the current sheet. An initially potential 3D null is subjected to shearing perturbations, as in a previous paper [Pontin et al., Phys. Plasmas 14, 052106 (2007)] . It is found that as the incompressible limit is approached, the collapse of the null point is suppressed and an approximately planar current sheet aligned to the fan plane is present instead. This is the case regardless of whether the spine or fan of the null is sheared. Both the peak current and peak reconnection rate are reduced. The results have a bearing on previous analytical solutions for steady-state reconnection in incompressible plasmas, implying that fan current sheet solutions are dynamically accessible, while spine current sheet solutions are not
Current sheet formation and nonideal behavior at three-dimensional magnetic null points
No full textThe nature of the evolution of the magnetic field, and of current sheet formation, at three-dimensional (3D) magnetic null points is investigated. A kinematic example is presented that 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 that renders the evolution of the field at the null ideal. Resistive magnetohydrodynamics simulations are described that 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 localized 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 localized 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
Resistive magnetohydrodynamic reconnection : resolving long-term, chaotic dynamics
No full textWe acknowledge financial support from the EC FP7/2007-2013 Grant Agreement SWIFF (No. 263340) and from project GOA/2009/009 (KU Leuven). This research has been funded by the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (IAP P7/08 CHARM). Part of the simulations used the infrastructure of the VSC-Flemish Supercomputer Center, funded by the Hercules Foundation and the Flemish Government-Department EWI. Another part of the simulations was done at the former Danish Center for Scientific Computing at Copenhagen University which is now part of DeIC Danish e-Infrastructure Cooperation.In this paper, we address the long-term evolution of an idealised double current system entering reconnection regimes where chaotic behavior plays a prominent role. Our aim is to quantify the energetics in high magnetic Reynolds number evolutions, enriched by secondary tearing events, multiple magnetic island coalescence, and compressive versus resistive heating scenarios. Our study will pay particular attention to the required numerical resolutions achievable by modern (grid-adaptive) computations, and comment on the challenge associated with resolving chaotic island formation and interaction. We will use shock-capturing, conservative, grid-adaptive simulations for investigating trends dominated by both physical (resistivity) and numerical (resolution) parameters, and confront them with (visco-)resistive magnetohydrodynamic simulations performed with very different, but equally widely used discretization schemes. This will allow us to comment on the obtained evolutions in a manner irrespective of the adopted discretization strategy. Our findings demonstrate that all schemes used (finite volume based shock-capturing, high order finite differences, and particle in cell-like methods) qualitatively agree on the various evolutionary stages, and that resistivity values of order 0.001 already can lead to chaotic island appearance. However, none of the methods exploited demonstrates convergence in the strong sense in these chaotic regimes. At the same time, nonperturbed tests for showing convergence over long time scales in ideal to resistive regimes are provided as well, where all methods are shown to agree. Both the advantages and disadvantages of specific discretizations as applied to this challenging problem are discussed.Peer reviewe
Dynamical investigation of three-dimensional reconnection in quasi-separatrix layers in a boundary-driven magnetic field
No full textQuasi-separatrix layers are regions in space where the mapping of field line connectivity changes especially rapidly. These layers have been suggested as special locations in three-dimensional magnetic fields that may host magnetic reconnection. Previous investigations have been analytical and have taken different simplifying: assumptions to investigate the problem. This paper takes a numerical approach to investigate the dynamical properties of quasi-separatrix layers. The magnetic topology is stressed using: drivers suggested by the analytical investigations but modified to fit the adopted boundary conditions. The experiments show that current; does accumulate at specific locations in the numerical domain. The current magnitude and location depend strongly on the profile of the imposed driver, and they are found to be generated by the changes in field line parts imposed by the driving. They are therefore the manifestation of free magnetic energy in the perturbed magnetic field. After the stressing of the field has stopped, it is found that the plasma pressure is able to balance the Lorentz force of the stressed magnetic field and prevent a. continued growth of the current amplitude in the current layers. Field-line changes are produced in the experiments that include magnetic resistivity. The reconnection takes place at locations where the electric field component along the magnetic field is large. The changes in field-line connectivity initiate flow velocities across the magnetic field lines at only a small fraction of the local Alfven velocity.</p
Current accumulation at an asymmetric 3D null point caused by generic shearing motions
No full textContext. Here we investigate the dynamical evolution of the reconnection process at an initially linear 3D null point that is stressed by a localised shear motion across the spine axis. The difference to previous investigations is that the fan plane is not rotationally symmetric and this allows for different behaviours depending on the alignment of the fan plane relative to the imposed driver direction.Aims. The aim is to show how the current accumulation and the associated reconnection process at the non-axisymmetric null depends on the relative orientation between the driver imposed stress across the spine axis of the null and the main eigenvector direction in the fan plane.Methods. The time evolution of the 3D null point is investigated solving the 3D non-ideal MHD equations numerically in a Cartesian box. The magnetic field is frozen to the boundaries and the boundary velocity is only non-zero where the imposed driving for stressing the system is applied.Results. The current accumulation is found to be along the direction of the fan eigenvector associated with the smallest eigenvalue until the direction of the driver is almost parallel to this eigenvector. When the driving velocity is parallel to the weak eigenvector and has an impulsive temporal profile the null only has a weak collapse forming only a weak current layer. However, when the null point is stressed continuously boundary effects dominates the current accumulation.Conclusions. There is a clear relation between the orientation of the current concentration and the direction of the fan eigenvector corresponding to the small eigenvalue. This shows that the structure of the magnetic field is the most important in determining where current is going to accumulate when a single 3D null point is perturbed by a simple shear motion across the spine axis. As the angle between the driving direction and the strong eigenvector direction increases, the current that accumulates at the null becomes progressively weaker.</p
Numerical modelling of 3D reconnection due to rotational footpoint motions
No full textThe rapid dynamical evolution of the photospheric magnetic carpet provides
a large energy source for the solar corona. In this context, the role of
3D magnetic reconnection is crucial in releasing the free magnetic energy,
build up due to the continuous footpoint motions. To understand the processes
by which this can take place, we have to obtain a better understanding of the
basic reconnection process that can take place in 3D magnetic field
configurations. In this paper, we investigate magnetic reconnection, driven
by rotational footpoint motions, using 3D numerical MHD simulations. The
model consists of two positive and two negative sources, which are placed
symmetrically on opposite boundaries of the cubic domain. The initially
potential fluxtubes are forced to interact by the rotational driving of the
flux concentrations on the boundaries. We consider two variations of this
setup, namely with and without an additional, constant, background magnetic
field. In the no-background case, the magnetic connectivity is divided into
independent regions by separatrix surfaces, while the case with a background
field is represented by one global connectivity region. The dynamical
evolution is followed and found to differ significantly from the comparable
potential evolution. Strong currents are concentrated along separatrix
surfaces or rapidly developing quasi-separatrix layers (QSLs). Investigating
the reconnection rates of the systems shows that the stronger the
background field is, the more efficient the reconnection process of the
flux in the respective fluxtubes
Fragment driven magnetic reconnection
No full textThe heating of the million degree, diffuse coronal plasma may be caused by a large number of events that are too small to be identified by present days observations. One explanation for these events could be the local interaction between magnetic flux systems that divide space into numerous flux regions. When such regions are independently advected by photospheric motions the expected outcome is the formation of enhanced current concentration at specific locations in space. Due to magnetic resistivity, these currents dissipate and heat the plasma. In this paper, we investigate a simple model where two, initially unconnected, flux systems are forced to interact in response to the imposed boundary driving by solving the non-ideal 3D MHD equations numerically. The reconnection rate of the dynamical process is determined and compared with the corresponding rate for the potential evolution of the magnetic field. This shows that the dynamic reconnection rate is about a factor of two smaller than the potential (perfect, instantaneous) rate for realistic solar driving velocities demonstrating that this three-dimensional magnetic reconnection process is fast. The energy input for a fixed advection distance is found to be independent of the driving velocity. The Joule dissipation associated with the reconnection process is also found to be basically dependent on the advection distance rather than driving velocity. This implies that the timescale for the event determines the effect the heating has on the temperature increase. Finally, the numerical experiments indicate that the observational structure of the reconnection site changes dramatically depending on the phase of the evolution of the passage of the two flux sources. In the initial phase, where the sources become connected, the heating is confined to a compact region. For the disconnecting phase the energy gets distributed over a larger area due to the reconnected field line connectivity.</p
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