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Assume a plasma with no magnetic field (B=0) at rest with constant density, ρ_{0}, undergoes a small perturbation (subscripts 1) resulting in a plasma flow such that,
ρ = ρ_{0} + ρ_{1},
v = v_{1}.
Differentiating Eqn. 23 with respect to time, substituting linearised forms of Eqn. 17 and Eqn. 21, and assuming an isothermal, gravitationally stratified atmosphere as before leads to a generalised wave equation,

(39) 
Once again plane wave solutions for v_{1} exist of of the form,

(40) 
where propagation is along the wavevector k (). Hence, as before, and in Eqn. 39, which becomes,

(41) 
which forms the basis for discussing acoustic and gravity waves.
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James McAteer
20040114