McLaughlin, James A and Thurgood, Jonathan O and Botha, Gert J J and Wiggs, Joshua A (2019) 3D WKB solution for fast magnetoacoustic wave behaviour within a separatrix dome containing a coronal null point. Monthly Notices of the Royal Astronomical Society, 484 (1). pp. 1390-1400. ISSN 0035-8711
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Abstract
The propagation of the fast magnetoacoustic wave is studied within a magnetic topology containing a 3D coronal null point whose fan field lines form a dome. The topology is constructed from a magnetic dipole embedded within a global uniform field. This study aims to improve the understanding of how magnetohydrodynamics (MHD) waves propagate through inhomogeneous media, specifically in a medium containing an isolated 3D magnetic null point. We consider the linearized MHD equations for an inhomogeneous, ideal, cold plasma. The equations are solved utilizing the WKB approximation and Charpit’s Method. We find that for a planar fast wave generated below the null point, the resultant propagation is strongly dependent upon initial location and that there are two main behaviours: the majority of the wave escapes the null (experiencing different severities of refraction depending upon the interplay with the equilibrium Alfvén-speed profile) or, alternatively, part of the wave is captured by the coronal null point (for elements generated within a specific critical radius about the spine and on the z = 0 plane). We also generalize the magnetic topology and find that the height of the null determines the amount of wave that is captured. We conclude that for a wavefront generated below the null point, nulls at a greater height can trap proportionally less of the corresponding wave energy.