Global mode ULF pulsations in a magnetosphere with a nonmonotonic Alfvén velocity profile

For a nonmonotonic Alfvén velocity profile, the global mode of the magnetosphere can be shown to couple to multiple resonant field lines. The mode structure itself is greatly altered by introduction of nonmonotonicity. Because of the rapid decrease of density at the plasmapause, the variation of the Alfvén velocity naturally separates the magnetosphere into inner and outer parts. ULF wave perturbations are found to be trapped in the Alfvén velocity valleys and, under certain circumstances, ULF wave amplitudes in the inner magnetosphere are greatly enhanced. The results obtained may explain some ULF perturbations observed in the low‐latitude regions of the magnetosphere.