Mathematical aspects of the cold plasma model
  A simple model for electromagnetic wave propagation through zero-temperature plasma is analyzed. Many of the complexities of the plasma state are present even under these idealized conditions, and a number of mathematical difficulties emerge. In particular, boundary value problems formulated on the basis of conventional electromagnetic theory turn out to be ill-posed in this context. However, conditions may be prescribed under which solutions to the Dirichlet problem exist in an appropriately weak sense. In addition to its physical interest, analysis of the cold plasma model illuminates generic difficulties in formulating and solving boundary value problems for mixed elliptic-hyperbolic partial differential equations. 
Thomas H. Otway