Compressional Alfv¿en waves in the terrestrial magnetospheric cavity constitute a discrete spectrum of global modes which can resonate with specific components of the continuum spectrum of shear Alfv¿en wave (SAW) field line resonant frequencies. We investigate the effect of the ponderomotive force (PF) of excited standing SAWs on the nonlinear saturation of field line resonances (FLRs). In low &bgr; plasmas FLR saturation occurs due to a nonlinear phase slip between the fast mode driver and SAW field. Ponderomotive forces also lead to density depletions at the ionospheres, nonlinear narrowing of the FLR and meridional scales comparable to those embedded within temporally modulated discrete auroral arcs. Observational features relating to these effects are discussed. ¿ American Geophysical Union 1995