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Implosive collapse about magnetic null points: A quantitative comparison between 2D and 3D nulls  

Jonathan Thurgood   Submitted: 2018-03-08 05:58

Null collapse is an implosive process whereby MHD waves focus their energy in the vicinity of a null point, forming a current sheet and initiating magnetic reconnection. We consider, for the first time, the case of collapsing 3D magnetic null points in nonlinear, resistive MHD using numerical simulation, exploring key physical aspects of the system as well as performing a detailed parameter study. We find that within a particular plane containing the 3D null, the plasma and current density enhancements resulting from the collapse are quantitatively and qualitatively as per the 2D case in both the linear and nonlinear collapse regimes. However, the scaling with resistivity of the 3D reconnection rate - which is a global quantity - is found to be less favourable when the magnetic null point is more rotationally symmetric, due to the action of increased magnetic back-pressure. Furthermore, we find that with increasing ambient plasma pressure the collapse can be throttled, as is the case for 2D nulls. We discuss this pressure-limiting in the context of fast reconnection in the solar atmosphere and suggest mechanisms by which it may be overcome. We also discuss the implications of the results in the context of null collapse as a trigger mechanism of Oscillatory Reconnection, a time-dependent reconnection mechanism, and also within the wider subject of wave-null point interactions. We conclude that, in general, increasingly rotationally-asymmetric nulls will be more favourable in terms of magnetic energy release via null collapse than their more symmetric counterparts. https://doi.org/10.3847/1538-4357/aab0a0

Authors: J.O. Thurgood, D.I. Pontin, J.A. McLaughlin
Projects: None

Publication Status: Published in ApJ, Open Access (https://doi.org/10.3847/1538-4357/aab0a0)
Last Modified: 2018-03-08 09:44
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Three-dimensional oscillatory magnetic reconnection  

Jonathan Thurgood   Submitted: 2017-07-05 13:31

Here we detail the dynamic evolution of localised reconnection regions about three-dimensional (3D) magnetic null points by using numerical simulation. We demonstrate for the first time that reconnection triggered by the localised collapse of a 3D null point due to an external MHD wave involves a self-generated oscillation, whereby the current sheet and outflow jets undergo a reconnection reversal process during which back-pressure formation at the jet heads acts to prise open the collapsed field before overshooting the equilibrium into an opposite-polarity configuration. The discovery that reconnection at fully 3D nulls can proceed naturally in a time-dependent and periodic fashion is suggestive that oscillatory reconnection mechanisms may play a role in explaining periodicity in astrophysical phenomena associated with magnetic reconnection, such as the observed quasi-periodicity of solar and stellar flare emission. Furthermore, we find a consequence of oscillatory reconnection is the generation of a plethora of freely-propagating MHD waves which escape the vicinity of the reconnection region

Authors: J.O. Thurgood, D.I. Pontin, J.A. McLaughlin
Projects: None

Publication Status: Accepted for publication in ApJ.
Last Modified: 2017-07-06 10:45
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Implosive collapse about magnetic null points: A quantitative comparison between 2D and 3D nulls
Three-dimensional oscillatory magnetic reconnection

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