Abstract Details


Name: Marcel Goossens
Affiliation: CmPA KU Leuven
Conference ID: TVS202510109
Title: Fundamental properties of MHD waves in non-uniform flowing solar plasmas
Authors and Co-Authors: Arregui I, Soler R, Terradas J, Van Doorsselaere T
Abstract Type: Contributory Presentation
Abstract: The concept of MHD waves with mixed properties does not seem to be well known in the Solar Plasma Physics community. It is not widely realized that a single global solution may and in most cases does show different character- istics in different parts of the magnetic equilibrium. For a uniform plasma of infinite extent the MHD waves can be put in separate boxes as magneto-sonic waves and Alfv´en waves. This clear division relies on the presence or absence of pressure variations and a zero or non-zero component of vorticity parallel to the equilibrium magnetic field. The Alfv´en waves are the only waves that propagate parallel vorticity and have no pressure variations, while the magneto-acoustic waves do not propagate parallel vorticity but are accompanied by pressure vari- ations. Stratification transverse to the magnetic field causes mixed properties in the MHD waves. The clear separation, well known for an infinite uniform plasma, is no longer present. In general, the simultaneous presence of pressure per- turbations and parallel vorticity is hard to avoid. This behaviour with mixed properties is caused by non-uniformity transverse to the magnetic field. When an MHD wave propagates through a non-uniform plasma it encounters a spa- tially changing environment. It changes its appearance and can evolve from a mainly fast/slow magneto-sonic wave into a wave that is primarily Alfv´enic and vice versa. The adjective Alfv´enic refers to the fact that the total pressure perturbation is non-zero everywhere. Non-uniformity generates non-zero paral- lel vorticity and hence Alfv´enic behaviour in the MHD waves. The transverse stratification also provides damping by resonant absorption. Equilibrium flows transform any given MHD wave present in a static plasma into two waves, respectively backward and forward propagating. As in the static case transverse stratification causes resonant damping of the propagating waves. Spatial damping is far stronger for upstream propagating (counter-streaming) waves than for downstream propagating waves. Even for slow sub-Alfv´enic flows the damping length of the counter-streaming propagating wave is substantially shorter than that of the forward propagating wave. This provides a natural ex- planation for the scarcity of observations on counter-streaming Alfv´enic waves. The effect of flow on the damping lengths is remarkably similar to that on parallel vorticity. Parallel vorticity is reduced for upstream propagating waves while it is increased for downstream propagating waves. This means that the Alfv´enic nature is increased/decreased for downstream/upstream MHD waves.