Journal of astrophysics and astronomy | |
Behaviour of Magnetoacoustic Waves in the Neighbourhood of a Two-Dimensional Null Point: Initially Cylindrically Symmetric Perturbations | |
J. A. McLaughlin11  | |
[1] Department of Mathematics and Information Sciences, Northumbria University, Newcastle-upon-Tyne, NE1 8ST, United Kingdom.$$ | |
关键词: Magnetohydrodynamics (MHD); waves; magnetic fields; Sun: atmosphere; corona.; | |
DOI : | |
学科分类:天文学(综合) | |
来源: Indian Academy of Sciences | |
【 摘 要 】
The propagation of magnetoacoustic waves in the neighbourhood of a 2D null point is investigated for both $eta =$ 0 and $eta$ $eq$ 0 plasmas. Previous work has shown that the Alfvén speed, here $u_A propto r$, plays a vital role in such systems and so a natural choice is to switch to polar coordinates. For $eta =$ 0 plasma, we derive an analytical solution for the behaviour of the fast magnetoacoustic wave in terms of the Klein–Gordone equation. We also solve the system with a semi-analytical WKB approximation which shows that the $eta =$ 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 $eta$ $eq$ 0 plasma, we solve the system numerically and find the behaviour to be similar to that of the $eta =$ 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.
【 授权许可】
Unknown
【 预 览 】
Files | Size | Format | View |
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RO201912040493922ZK.pdf | 5203KB | download |