Electron Distributions in Kinetic Scale Field Line Resonances: A Comparison of Simulations and Observations

Observations in kinetic scale field line resonances, or eigenmodes of the geomagnetic field, reveal highly field-aligned plateaued electron distributions. By combining observations from the Van Allen Probes and Cluster spacecraft with a hybrid kinetic gyrofluid simulation we show how these distributions arise from the nonlocal self-consistent interaction of electrons with the wavefield. This interaction is manifested as electron trapping in the standing wave potential. The process operates along most of the field line and qualitatively accounts for electron observations near the equatorial plane and at higher latitudes. In conjunction with the highly field-aligned plateaus, loss cone features are also evident, which result from the action of the upward-directed wave parallel electric field on the untrapped electron populations.