Four-field model for dispersive field-line resonances: Effects of coupling between shear-Alfvén and slow modes

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