Global structures of Alfvén‐ballooning modes in magnetospheric plasmas

We show that a steep plasma pressure gradient can lead to radially localized Alfvén modes, which are damped through coupling to field line resonances. These have been called drift Alfvén ballooning modes (DABM) by [Chen and Hasegawa, 1991] and are the prime candidates to explain Pc4–Pc5 geomagnetic pulsations observed during the recovery phase of geomagnetic storms. A strong dependence of the damping rate on the azimuthal wave number m is established, as well as on the equilibrium profile. A minimum azimuthal mode number can be found for the DABM to be radially trapped. We find that higher m DABMs are better localized, which is consistent with high‐m observations.