Modeling the Propagation of Fast Magnetosonic Waves and Their Conversion to Electromagnetic Ion Cyclotron Waves at Low L Shells.

The propagation of fast magnetosonic (MS) waves from high to extremely low L shells and their conversion into electromagnetic ion cyclotron (EMIC) waves is investigated with a ray tracing model and a full‐wave model. The ray tracing simulations show that MS waves in the vicinity of the local H+−He+ ${\mathrm{H}}^{+}{-\text{He}}^{+}$ crossover frequency at low L shells have a latitudinal range from −20° ${-20}^{{}^{\circ}}$ to 20° ${20}^{{}^{\circ}}$ with wave normal angles from 0° ${0}^{{}^{\circ}}$ to 180° ${180}^{{}^{\circ}}$, both of which provide the opportunity for their mode conversion to EMIC waves. Results from ray tracing are fed into the full‐wave model as initial conditions. The full‐wave simulations show that the incoming MS waves with small (20° ${20}^{{}^{\circ}}$) and intermediate (40° ${40}^{{}^{\circ}}$) wave normal angles may be efficiently converted to right‐handedly polarized H+ ${\mathrm{H}}^{+}$ band EMIC waves, which may be further converted to right‐handedly polarized He+ ${\text{He}}^{+}$ band EMIC waves or to guided left‐handedly polarized H+ ${\mathrm{H}}^{+}$ band EMIC waves through bi‐ion resonance reflection and polarization reversal. With intermediate (40° ${40}^{{}^{\circ}}$) or larger wave normal angles, via polarization reversal and left‐handed polarization cut‐off reflection, the incoming MS waves may be efficiently converted to guided left‐handedly polarized H+ ${\mathrm{H}}^{+}$ band EMIC waves and the optimal wave normal angles corresponding to the maximum conversion efficiencies decrease with magnetic latitudes. Our study verifies the mode conversion mechanism of MS waves to EMIC waves proposed with a previous ray tracing study and examines the dependence of the efficiency of the wave energy transfer on the magnetic latitudes and wave normal angles, which may help to understand the wave properties and distribution of MS and EMIC waves observed at extremely low L shells. Key Points: Propagation and mode conversion of fast magnetosonic (MS) to electromagnetic ion cyclotron (EMIC) waves at extremely low L shells is studied with ray tracing and full‐wave simulationsRight‐handedly polarized H+ ${\mathrm{H}}^{+}$ band EMIC converted from MS with small (20° ${20}^{{}^{\circ}}$) and intermediate (40° ${40}^{{}^{\circ}}$) wave normal angles may be efficiently converted to right‐handedly polarized He+ ${\text{He}}^{+}$ band EMIC or guided left‐handedly polarized H+ ${\mathrm{H}}^{+}$ band EMICMS with intermediate (40° ${40}^{{}^{\circ}}$) or larger wave normal angles may be efficiently converted to guided left‐handedly polarized H+ ${\mathrm{H}}^{+}$ band EMIC and the optimal wave normal angles corresponding to the maximum conversion efficiencies decrease with magnetic latitudes [ABSTRACT FROM AUTHOR]