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 Alfvén-speed profile .
In a \beta = 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 fan-plane ( 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 , mode-coupling , quasi-periodic pulsations and phase-mixing .