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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.

Plain Language Summary Kinetic scale field line resonances (KFLRs) are standing waves along closed magnetic field lines that are prominent in the inner magnetosphere at times of strong geomagnetic activity. These waves are important for magnetosphere-ionosphere coupling and for facilitating the diffusion of electrons and ions across magnetic field lines, which is fundamental to understanding radiation belt dynamics. Satellite observations reveal that electron distributions within KFLRs are highly stretched in the direction parallel to the background magnetic field. In this work, by comparing computer simulations of electrons within the KFLR wavefields with observations from both the Van Allen Probes and Cluster satellites, we for the first time illustrate that the spatial and temporal structure of these distributions naturally result from the trapping of electrons by the standing wave electric potential. This work is also the first multisatellite comparison of KFLRs with simulations and additionally illustrates that loss cone features seen in observed distribution functions (particularly closer to the ionosphere) are also naturally reproduced in the simulations. These features are related to the precipitation to the ionosphere of untrapped electrons within the wavefield. KFLRs are thus important for energy transfer between the magnetosphere and ionosphere.