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Schrijver, Carolus J.; Title, A. M.

doi:10.1023/a:1016295516408

Cited by 78 publications

(51 citation statements)

References 16 publications

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“…Null points are common features in the solar atmosphere (Schrijver & Title 2002) and many flares have been associated (An mpeg animation of this figure is available in the online journal.) with such topology (e.g., Démoulin et al 1994a;Mandrini et al 1995;).…”

Section: Introductionmentioning

confidence: 99%

The Nature of Flare Ribbons in Coronal Null-Point Topology

Masson¹,

Pariat²,

Aulanier³

et al. 2009

ApJ

336

432

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Flare ribbons are commonly attributed to the low-altitude impact, along the footprints of separatrices or quasiseparatrix layers (QSLs), of particle beams accelerated through magnetic reconnection. If reconnection occurs at a three-dimensional coronal magnetic null point, the footprint of the dome-shaped fan surface would map a closed circular ribbon. This paper addresses the following issues: does the entire circular ribbon brighten simultaneously, as expected because all fan field lines pass through the null point? And since the spine separatrices are singular field lines, do spine-related ribbons look like compact kernels? What can we learn from these observations about current sheet formation and magnetic reconnection in a null-point topology? The present study addresses these questions by analyzing Transition Region and Coronal Explorer and Solar and Heliospheric Observatory/Michelson Doppler Imager observations of a confined flare presenting a circular ribbon. Using a potential field extrapolation, we linked the circular shape of the ribbon with the photospheric mapping of the fan field lines originating from a coronal null point. Observations show that the flare ribbon outlining the fan lines brightens sequentially along the counterclockwise direction and that the spine-related ribbons are elongated. Using the potential field extrapolation as initial condition, we conduct a low-β resistive magnetohydrodynamics simulation of this observed event. We drive the coronal evolution by line-tied diverging boundary motions, so as to emulate the observed photospheric flow pattern associated with some magnetic flux emergence. The numerical analysis allows us to explain several observed features of the confined flare. The vorticity induced in the fan by the prescribed motions causes the spines to tear apart along the fan. This leads to formation of a thin current sheet and induces null-point reconnection. We also find that the null point and its associated topological structure is embedded within QSLs, already present in the asymmetric potential field configuration. We find that the QSL footprints correspond to the observed elongated spine ribbons. Finally, we observe that before and after reconnecting at the null point, all field lines undergo slipping and slip-running reconnection within the QSLs. Field lines, and therefore particle impacts, slip or slip-run according to their distance from the spine, in directions and over distances that are compatible with the observed dynamics of the ribbons.

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Section: Introductionmentioning

confidence: 99%

The Nature of Flare Ribbons in Coronal Null-Point Topology

Masson¹,

Pariat²,

Aulanier³

et al. 2009

ApJ

336

432

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show abstract

“…Null points are useful for studying different topological features. 4,5 The null points of a magnetic field vector are important locations where strong currents can be found. 6 Strong currents play an important role in energy release processes such as magnetic reconnection where a change in the magnetic field topology could release energy to the surrounding plasma.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of charged particle motion in the vicinity of three dimensional magnetic null points: Energization and chaos

Gascoyne

2015

Physics of Plasmas

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Using a full orbit test particle approach, we analyse the motion of a single proton in the vicinity of magnetic null point configurations which are solutions to the kinematic, steady state, resistive magnetohydrodynamics equations. We consider two magnetic configurations, namely, the sheared and torsional spine reconnection regimes [E. R. Priest and D. I. Pontin, Phys. Plasmas 16, 122101 (2009); P. Wyper and R. Jain, Phys. Plasmas 17, 092902 (2010)]; each produce an associated electric field and thus the possibility of accelerating charged particles to high energy levels, i.e., > MeV, as observed in solar flares [R. P. Lin, Space Sci. Rev. 124, 233 (2006)]. The particle's energy gain is strongly dependent on the location of injection and is characterised by the angle of approach b, with optimum angle of approach b opt as the value of b which produces the maximum energy gain. We examine the topological features of each regime and analyse the effect on the energy gain of the proton. We also calculate the complete Lyapunov spectrum for the considered dynamical systems in order to correctly quantify the chaotic nature of the particle orbits. We find that the sheared model is a good candidate for the acceleration of particles, and for increased shear, we expect a larger population to be accelerated to higher energy levels. In the strong electric field regime (E 0 ¼ 1500 V/m), the torsional model produces chaotic particle orbits quantified by the calculation of multiple positive Lyapunov exponents in the spectrum, whereas the sheared model produces chaotic orbits only in the neighbourhood of the null point. V C 2015 AIP Publishing LLC.

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“…Gorbachev & Somov 1989;Longcope & Klapper 2002;Schrijver & Title 2002;Close et al 2005;Démoulin et al 1994a;Priest et al 2005;Maclean et al 2009). Such models can therefore result in a complex distribution of distinct flux domains, whose boundaries are defined by separatrix fan surfaces.…”

Section: Introductionmentioning

confidence: 99%

How skeletons turn into quasi-separatrix layers in source models

Restante

Aulanier²,

Parnell

2009

A&A

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Context. In situations where there are no magnetic null points located above a reference photospheric plane, and when the photospheric magnetic field is modeled by discrete flux concentrations, the magnetic connectivity is defined by the magnetic skeleton of the configuration. For a continuous distribution of non-zero photospheric flux, the connectivity is defined by quasi-separatrix layers (QSLs). Both the magnetic skeleton and QSLs can account for current sheet formation and dissipation. Observationally, though, only some portions of the skeleton are found to be related to flare ribbons, which are generally associated with QSL footpoints. Aims. In potential magnetic source models, a transition from the skeleton to QSLs has been shown to occur when the sources are displaced below the photospheric plane. The objective of this paper is to understand the topological and geometrical nature of this transition, and to derive rules to predict which parts of a given skeleton will give rise to QSLs. Methods. We consider magnetic configurations, derived from potential magnetic sources, which possess no coronal null points. We have calculated their skeletons, composed of null points, spine field lines and separatrix (fan) surfaces. Choosing a reference photospheric plane above the sources, we have calculated their QSL footprints. Results. As already known, the latter mostly match with subphotospheric spine field lines since, above these lines, field lines tend to diverge as a result of approaching a null and lying either side of the separatrix surface extending out of from this null. However, many non-spine related QSL footprints are also found, which we call branches. They correspond to the intersection with the photosphere of portions of fan field lines which "branch" away from the sources and result in QSLs due to the inclination of the coronal field lines. Conclusions. Our findings allow a better geometrical understanding of the relations between QSLs and skeletons. We show that in the absence of coronal null points, spines, as well as specific portions of fans as calculated in standard potential source models, are good predictors for the location of QSL footprints and of flare ribbons.

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Cited by 78 publications

References 16 publications

The Nature of Flare Ribbons in Coronal Null-Point Topology

The Nature of Flare Ribbons in Coronal Null-Point Topology

Dynamics of charged particle motion in the vicinity of three dimensional magnetic null points: Energization and chaos

How skeletons turn into quasi-separatrix layers in source models

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