Critical-level behaviour of ion cyclotron waves

This paper provides an analysis of the propagation characteristics of ion cyclotron waves in a magnetized plasma whose density varies in a direction perpendicular to the ambient magnetic field. It is shown that the component of the wave energy flux across the density stratification is conserved except at a critical level (which is associated with a resonance singularity in the wave equation) at which the jump in the wave energy flux is balanced by wave absorption by the plasma. The relationship of this behaviour with field line resonance is briefly discussed, as is its possible relevance to ULF waves propagating in the vicinity of the earth's plasmapause.