Four-field model for dispersive field-line resonances: Effects of couplingbetween shear-Alfven and slow modes

A new theoretical model is proposed far dispersive field-line resonances in collisionless magnetospheric plasmas on the basis of reduced four-field eq uations. The model improves upon the predictive capabilities of earlier two -field models. In particular, due to the coupling of the shear-Alfven mode to the slow mode in the four-field system, it is now possible to account fo r the observed low frequencies of field-line resonances. Furthermore, paral lel electric fields can be large without requiring the field-aligned curren t density to be unrealistically large. Qualitative implications for recent FAST and ground-based observations are discussed.