LAUSR

Storm‐time broadband electromagnetic field variations along the interface between the dipolar field of the Earth's inner‐magnetosphere and the stretched fields of the plasma‐sheet are decomposed as a superposition of fluid‐kinetic modes. Using model eigen‐vectors operating on the full set of Van Allen Probes fields measurements it is shown how these variations are composed of a broad spectrum of dispersive Alfvén waves with significant spectral energy densities in the fast and slow modes over scales extending into the kinetic range. These modes occupy volumes in k $k$ ‐space that define the field variations observed at each spacecraft frame frequency ( fsc ${f}_{sc}$ ). They are in aggregate not necessarily planar and often comprise filamentary structures with no distinct propagation direction in the perpendicular plane. Within these volumes the characteristic parallel phase speeds of the fast and Alfvénic modes coincide over a broad range of fsc ${f}_{sc}$ suggestive of coupling/conversion between modes.