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Magnetopause surface waves: THEMIS observations compared to MHD theory

On 5 and 6 October 2011, the three inner THEMIS spacecraft observed oscillatory magnetopause (MP) motion in the early afternoon sector over extended periods of time, caused by the passage of surface waves. The close configuration of the spacecraft tangential to the MP allowed for determination of wave propagation velocities via cross‐correlation analysis. This enabled us to compare observations with solutions of the surface wave dispersion relation derived in the framework of ideal MHD theory. We find that observations and theoretical predictions are in quantitative agreement if a high plasma flow velocity (of around 340 km∕s) can be assumed in the plasma depletion layer on the magnetosheath side of the MP. Although the surface waves propagated more slowly than the plasma on the magnetosheath side, they were not subject to the Kelvin‐Helmholtz instability. We suggest that waves generated in the quasi‐parallel bow shock or upstream foreshock regions may have impinged on the MP generating the surface waves. Additionally, the spacecraft separation in the boundary normal direction enabled us to infer directly the waveform and MP boundary layer structure. Interestingly, the trailing edges of the waves were more inclined than the leading edges, contrary to theory and prior observations of Kelvin‐Helmholtz‐driven surface waves.