Abstract
The propagation of magnetoacoustic waves in the neighbourhood of a 2D null point is investigated for both β=0 and β ≠ 0 plasmas. Previous work has shown that the Alfvén speed, here v A ∝r, plays a vital role in such systems and so a natural choice is to switch to polar coordinates. For β=0 plasma, we derive an analytical solution for the behaviour of the fast magnetoacoustic wave in terms of the Klein–Gordon equation. We also solve the system with a semi-analytical WKB approximation which shows that the β=0 wave focuses on the null and contracts around it but, due to exponential decay, never reaches the null in a finite time. For the β ≠ 0 plasma, we solve the system numerically and find the behaviour to be similar to that of the β=0 system at large radii, but completely different close to the null. We show that for an initially cylindrically-symmetric fast magnetoacoustic wave perturbation, there is a decrease in wave speed along the separatrices and so the perturbation starts to take on a quasi-diamond shape; with the corners located along the separatrices. This is due to the growth in pressure gradients that reach a maximum along the separatrices, which in turn reduces the acceleration of the fast wave along the separatrices leading to a deformation of the wave morphology.
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Acknowledgements
The author acknowledges IDL support provided by STFC. He also wishes to thank Alan Hood for insightful discussions and constant encouragement.
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McLaughlin, J.A. Behaviour of Magnetoacoustic Waves in the Neighbourhood of a Two-Dimensional Null Point: Initially Cylindrically Symmetric Perturbations. J Astrophys Astron 37, 2 (2016). https://doi.org/10.1007/s12036-016-9376-y
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DOI: https://doi.org/10.1007/s12036-016-9376-y