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Abstract
Context. An injection of energy towards a magnetic null point can drive reversals of currentsheet polarity leading to timedependent, oscillatory reconnection (OR), which may explain periodic phenomena generated when reconnection occurs in the solar atmosphere. However, the details of what controls the period of these currentsheet oscillations in realistic systems is poorly understood, despite being of crucial importance in assessing whether a specific model of OR can account for observed periodic behaviour.
Aims. This paper aims to highlight that different types of reconnection reversal are supported about null points, and that these can be distinct from the oscillation in the closedboundary, linear systems considered by a number of authors in the 1990s. In particular, we explore the features of a nonlinear oscillation local to the null point, and examine the effect of resistivity and perturbation energy on the period, contrasting it to the linear, closedboundary case.
Methods. Numerical simulations of the singlefluid, resistive MHD equations are used to investigate the effects of plasma resistivity and perturbation energy upon the resulting OR.
Results. It is found that for small perturbations that behave linearly, the inverse Lundquist number dictates the period, provided the perturbation energy (i.e. the free energy) is small relative to the inverse Lundquist number defined on the boundary, regardless of the broadband structure of the initial perturbation. However, when the perturbation energy exceeds the threshold required for ‘nonlinear’ null collapse to occur, a complex oscillation of the magnetic field is produced which is, at most, only weaklydependent on the resistivity. The resultant periodicity is instead strongly influenced by the amount of free energy, with more energetic perturbations producing higherfrequency oscillations.
Conclusions. Crucially, with regards to typical solarbased and astrophysicalbased input energies, we demonstrate that the majority far exceed the threshold for nonlinearity to develop. This substantially alters the properties and periodicity of both null collapse and subsequent OR. Therefore, nonlinear regimes of OR should be considered in solar and astrophysical contexts.
Aims. This paper aims to highlight that different types of reconnection reversal are supported about null points, and that these can be distinct from the oscillation in the closedboundary, linear systems considered by a number of authors in the 1990s. In particular, we explore the features of a nonlinear oscillation local to the null point, and examine the effect of resistivity and perturbation energy on the period, contrasting it to the linear, closedboundary case.
Methods. Numerical simulations of the singlefluid, resistive MHD equations are used to investigate the effects of plasma resistivity and perturbation energy upon the resulting OR.
Results. It is found that for small perturbations that behave linearly, the inverse Lundquist number dictates the period, provided the perturbation energy (i.e. the free energy) is small relative to the inverse Lundquist number defined on the boundary, regardless of the broadband structure of the initial perturbation. However, when the perturbation energy exceeds the threshold required for ‘nonlinear’ null collapse to occur, a complex oscillation of the magnetic field is produced which is, at most, only weaklydependent on the resistivity. The resultant periodicity is instead strongly influenced by the amount of free energy, with more energetic perturbations producing higherfrequency oscillations.
Conclusions. Crucially, with regards to typical solarbased and astrophysicalbased input energies, we demonstrate that the majority far exceed the threshold for nonlinearity to develop. This substantially alters the properties and periodicity of both null collapse and subsequent OR. Therefore, nonlinear regimes of OR should be considered in solar and astrophysical contexts.
Original language  English 

Article number  A106 
Pages (fromto)  112 
Number of pages  12 
Journal  Astronomy and Astrophysics 
Volume  621 
Early online date  15 Jan 2019 
DOIs  
Publication status  Published  Jan 2019 
Keywords
 Magnetic reconnection
 Magnetohydrodynamics (MHD)
 Sun: Magnetic fields
 Sun: Oscillations
 Waves
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Projects
 1 Finished

Dynamics of Complex Magnetic Fields: From the Corona to the Solar Wind (Joint with University of Durham)
1/04/16 → 30/09/19
Project: Research