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Why are flare ribbons associated with the spines of magnetic null points generically elongated?  

David Pontin   Submitted: 2016-05-20 02:43

Coronal magnetic null points exist in abundance as demonstrated by extrapolations of the coronal field, and have been inferred to be important for a broad range of energetic events. These null points and their associated separatrix and spine field lines represent discontinuities of the field line mapping, making them preferential locations for reconnection. This field line mapping also exhibits strong gradients adjacent to the separatrix (fan) and spine field lines, that can be analysed using the `squashing factor', Q. In this paper we make a detailed analysis of the distribution of Q in the presence of magnetic nulls. While Q is formally infinite on both the spine and fan of the null, the decay of Q away from these structures is shown in general to depend strongly on the null-point structure. For the generic case of a non-radially-symmetric null, Q decays most slowly away from the spine/fan in the direction in which |B| increases most slowly. In particular, this demonstrates that the extended, elliptical high-Q halo around the spine footpoints observed by Masson et al. (Astrophys. J., 700, 559, 2009) is a generic feature. This extension of the Q halos around the spine/fan footpoints is important for diagnosing the regions of the photosphere that are magnetically connected to any current layer that forms at the null. In light of this, we discuss how our results can be used to interpret the geometry of observed flare ribbons in `circular ribbon flares', in which typically a coronal null is implicated. We conclude that both the physics in the vicinity of the null and how this is related to the extension of Q away from the spine/fan can be used in tandem to understand observational signatures of reconnection at coronal null points.

Authors: D. I. Pontin, K. Galsgaard, P. Demoulin
Projects: None

Publication Status: In press - Solar Physics
Last Modified: 2016-05-20 22:55
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Generalised models for torsional spine and fan magnetic reconnection  

David Pontin   Submitted: 2011-05-16 02:02

Three-dimensional null points are present in abundance in the solar corona,and the same is likely to be true in other astrophysical environments. Recentstudies suggest that reconnection at such 3D nulls may play an important rolein the coronal dynamics. In this paper the properties of the torsional spineand torsional fan modes of magnetic reconnection at 3D nulls are investigated.New analytical models are developed, which for the first time include a currentlayer that is spatially localised around the null, extending along either thespine or the fan of the null. These are complemented with numericalsimulations. The principal aim is to investigate the effect of varying thedegree of asymmetry of the null point magnetic field on the resultingreconnection process - where previous studies always considered a non-genericradially symmetric null. The geometry of the current layers within whichtorsional spine and torsional fan reconnection occur is found to be stronglydependent on the symmetry of the magnetic field. Torsional spine reconnectionstill occurs in a narrow tube around the spine, but with ellipticalcross-section when the fan eigenvalues are different, and with the short axisof the ellipse being along the strong field direction. The spatiotemporal peakcurrent, and the peak reconnection rate attained, are found not to dependstrongly on the degree of asymmetry. For torsional fan reconnection, thereconnection occurs in a planar disk in the fan surface, which is againelliptical when the symmetry of the magnetic field is broken. The short axis ofthe ellipse is along the weak field direction, with the current being peaked inthese weak field regions. The peak current and peak reconnection rate in thiscase are clearly dependent on the asymmetry, with the peak current increasingbut the reconnection rate decreasing as the degree of asymmetry is increased.

Authors: D. I. Pontin, A. K. Al-Hachami and K. Galsgaard
Projects: None

Publication Status: submitted to Astronomy & Astrophysics
Last Modified: 2011-05-16 08:41
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3D null point reconnection regimes  

David Pontin   Submitted: 2009-10-16 07:44

Recent advances in theory and computational experiments have shown the need to refine the previous categorisation of magnetic reconnection at three-dimensional null points - points at which the magnetic field vanishes. We propose here a division into three different types, depending on the nature of the flow near the spine and fan of the null. The spine is an isolated field line which approaches the null (or recedes from it), while the fan is a surface of field lines which recede from it (or approach it). So-called ''torsional spine reconnection'' occurs when field lines in the vicinity of the fan rotate, with current becoming concentrated along the spine, so that nearby field lines undergo rotational slippage. In ''torsional fan reconnection'' field lines near the spine rotate and create a current that is concentrated in the fan with a rotational flux mismatch and rotational slippage. In both of these regimes, the spine and fan are perpendicular and there is no flux transfer across spine or fan. The third regime, called ''spine-fan reconnection'', is the most common in practice and combines elements of the previous spine and fan models. In this case, in response to a generic shearing motion, the null point collapses to form a current sheet that is focused at the null itself, in a sheet that locally spans both the spine and fan. In this regime the spine and fan are no longer perpendicular and there is flux transfer across both of them.

Authors: E. R. Priest and D. I. Pontin
Projects: None

Publication Status: Physics of Plasmas, in press
Last Modified: 2009-10-16 08:54
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Why are flare ribbons associated with the spines of magnetic null points generically elongated?
Generalised models for torsional spine and fan magnetic reconnection
3D null point reconnection regimes

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