Mode conversion into non‐MHD waves at the Alfvén layer: The case against the field line resonance concept

According to ideal magnetohydrodynamics (MHD), a magnetic field line in an inhomogeneous plasma becomes “resonant” when the condition ω = k‖vA is satisfied on the field line; here ω is the frequency of a perturbation, k| is the component of the wavenumber parallel to the magnetic field, and υA is the local Alfven velocity on the field line. This concept of field line resonance (FLR) has long been used to explain observed magnetospheric pulsations and, in particular, recent models have proposed that magnetospheric FLRs are excited by compressional MHD waves trapped in a surrounding cavity or waveguide. We present here a reexamination of this problem using an analysis based directly on the Maxwell‐Lorentz equations, a more fundamental description of plasma behavior than MHD. Our analysis shows that the ω = k‖υA layer is actually the location where an incident compressional MHD wave mode converts into a non‐MHD inertial electron Alfvén wave. We find that all fields are finite and regular at the ω = k‖υA layer and conclude that field line resonance does not exist in reality and so is an artifact of ideal MHD.