Modeling the Dawn/Dusk Asymmetry of Field Line Resonances

Field line resonances (FLRs) are observed to occur preferentially and have larger amplitudes at dawn compared to dusk. We present simulations of FLR excitation in a magnetospheric waveguide that can reproduce this behavior. Crucially, our equilibrium is asymmetric about noon. Even when this system is driven in a symmetric fashion about noon, the fast waves that are established in the magnetosphere develop asymmetries—as do the FLRs they excite. Fast mode ray trajectories are employed to show that the asymmetry evolves due to refraction. Preferential FLR excitation at dawn is further reinforced by calculating the Resonance Map. This shows that the Resonant Zone at dawn coincides with a large-amplitude coherent fast mode driver, which is not the case at dusk. These factors result in FLRs having a larger amplitude at dawn compared to dusk.