Theories of Pc 5 pulsations with high azimuthal wavelength generally work in artificial geometries. After discussion of a simple tutorial example in a straight field geometry a preliminary theory is developed which shows how to evaluate the effect of resonant particles on standing hydromagnetic waves which have been calculated in a realistic cylindrically symmetrical geomagnetic field. Marginal stability is assumed. A reanalysis of data from the event of October 27, 1978, is made. The theory allows estimates of a number of plasma properties to be made. These show internal consistency which is encouraging for the theory. ¿ American Geophysical Union 1994