Magneto-acoustic waves in the stratified solar atmosphere : single to multi-fluid approach

• Main Author: Mather, James
• Other Authors: Fay-Siebenburgen, Robertus
• Published: University of Sheffield 2017
• Subjects: 510
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.736535

Mechanical waves are ubiquitous within solar plasmas, coming in the form of Magnetohydrodynamic (MHD) waves. The Sun and its atmosphere are not a single homogeneous medium and, rather, have complex structures stratified by gravity, temperature gradients, complex magnetic field structures, subject to bulk flows and even partial ionisation, with all of these affecting how oscillations propagate within the plasma. Magneto-acoustic gravity (MAG) waves have been investigated extensively within solar physics, with three popular choices of analytical modelling for a Cartesian coordinate system: a magnetic field parallel, perpendicular, or at an angle to the gravitational field. Firstly we study the eigen-modes of bounded solar plasmas embedded in a magnetic field perpendicular to the gravitational field and their energy distribution in both single and two-layer models. We show that, indeed, modes can still be split into fast and slow MAG with stratification decreasing the magnetic energy with height and, thus, only waves with predominantly internal energy are more evenly distributed in the atmosphere. A discontinuity in temperature between layers also reflects waves but we show there is an inherent coupling between waves in both layers. Secondly we investigate the effect of a bulk flow on MAG surface waves, with the magnetic field parallel to the surface and a gravitational field perpendicular to this. We find that waves along the penumbra, where Evershed flows are present can change their direction of propagation and even the Kelvin-Helmholtz instability can occur. Waves within solar plasmas have generally been studied in fully ionised or completely neutral media extensively and, as such, partial ionisation has not been covered much until fairly recently, meaning there is great scope for exciting studies in this sub-set of solar physics. Using single and multi-fluid methods we investigate the stability of partially ionised plasma slabs with bulk flows. We find that in the single fluid approximation that dissipative instabilities can occur for flow speeds of the internal tube speed, with a neutrals providing stability. In the two-fluid approximation our analysis confirms the existence of a mode that arises due to the shear in flow in the neutral fluid and that, in a highly collisional plasma, has a semi-resonant interaction between itself and the classical modes of incompressible or compressible magnetic slabs.