Magneto-Atmospheric Waves in a Horizontally Stratified Conducting Medium

The propagation of magneto-atmospheric waves in an inviscid, compressible and perfectly electrically conducting medium in the presence of a vertical gravitational field and a horizontal magnetic field is investigated. If the medium is isothermal and its magnetic pressure is proportional to its mechanical pressure, it is found that the convective instability arises when the ratio of the Alfvén speed to the sound speed exceeds [2(1 — 1/γ)]½ where γ is the adiabatic index. When the medium is found to be stable, a disturbance in the medium will travel as waves in three different modes. For small wavenumbers, two of these modes are similar to the acoustic and gravity modes found in the nonmagnetic case. For very large wavenumbers, the acoustic mode becomes the modified sound wave found in the case of a uniform medium, while the gravity wave behaves as the corresponding modified hydromagnetic wave unless the component of the wavenumber in the direction of the magnetic lines is sufficiently small. The third mode which does not appear in the nonmagnetic case behaves nearly as a pure hydromagnetic wave; it is influenced only to a minor extent by compressibility and gravity. The damping of each mode due to viscosity, finite electrical and thermal conductivity is discussed.