Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields
Abstract
We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfvén speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfvén wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfvénspeed profile. In a β=0 plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfvén wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfvén wave accumulates along the separatrices (in 2D) or along the spine or fanplane (in 3D). Hence, Alfvén wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, modecoupling, quasiperiodic pulsations and phasemixing.
 Publication:

Space Science Reviews
 Pub Date:
 July 2011
 DOI:
 10.1007/s112140109654y
 arXiv:
 arXiv:1004.5568
 Bibcode:
 2011SSRv..158..205M
 Keywords:

 Magnetic fields: coronal;
 Magnetic fields: models;
 Waves: magnetohydrodynamic;
 Waves: propagation;
 Magnetohydrodynamics;
 Astrophysics  Solar and Stellar Astrophysics;
 Mathematical Physics
 EPrint:
 34 pages, 5 figures, invited review in Space Science Reviews =>