Focusing in the stationary phase approximation


  • L. J. Nickisch


A method for calculating focusing in the stationary phase approximation is presented. By considering variations of Hamilton's equations a set of equations is obtained which, by integration along a single ray, provides information on the convergence or divergence of infinitesimally separated rays. The method applies to three-dimensional ionospheric structure and is shown to be much more accurate than conventional finite flux tube approaches.