Bibcode
Goossens, M.; Andries, J.; Soler, R.; Van Doorsselaere, T.; Arregui, I.; Terradas, J.
Bibliographical reference
The Astrophysical Journal, Volume 753, Issue 2, article id. 111 (2012).
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7
2012
Journal
Citations
120
Refereed citations
115
Description
Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere.
Alfvén waves and magneto-sonic waves are particular classes of
MHD waves. These wave modes are clearly different and have pure
properties in uniform plasmas of infinite extent only. Due to plasma
non-uniformity, MHD waves have mixed properties and cannot be classified
as pure Alfvén or magneto-sonic waves. However, vorticity is a
quantity unequivocally related to Alfvén waves as compression is
for magneto-sonic waves. Here, we investigate MHD waves superimposed on
a one-dimensional non-uniform straight cylinder with constant magnetic
field. For a piecewise constant density profile, we find that the
fundamental radial modes of the non-axisymmetric waves have the same
properties as surface Alfvén waves at a true discontinuity in
density. Contrary to the classic Alfvén waves in a uniform plasma
of infinite extent, vorticity is zero everywhere except at the cylinder
boundary. If the discontinuity in density is replaced with a continuous
variation of density, vorticity is spread out over the whole interval
with non-uniform density. The fundamental radial modes of the
non-axisymmetric waves do not need compression to exist unlike the
radial overtones. In thin magnetic cylinders, the fundamental radial
modes of the non-axisymmetric waves with phase velocities between the
internal and the external Alfvén velocities can be considered as
surface Alfvén waves. On the contrary, the radial overtones can
be related to fast-like magneto-sonic modes.