Surface waves and field line resonances: A THEMIScase study

摘要

1 Using magnetic field and plasma observations from four of the five Time History ofEvents and Macroscale Interactions during Substorms (THEMIS) spacecraft, a surfacewave at the dawn flank of the magnetopause was identified on 25 April 2007.The wave had an amplitude of about 1 RE and propagated tailward with a velocity of about190-240 km/s. Its azimuthal wavelength was in the range of 9-11 RE. Magnetosheathvelocity values support the hypothesis that this surface wave was generated by theKelvin-Helmholtz instability. Simultaneously, an ultralow-frequency (ULF) pulsationevent was detected by the fifth THEMIS spacecraft deeper in the magnetosphere, at adistance of about 5-7 RE from the magnetopause. This ULF event showed all thesignatures predicted for waves generated by the classical field line resonance process.Frequency and phases of the detected ULF oscillations were found to be in closeagreement with the magnetopause surface periodic disturbances. We conclude that theobserved ULF wave event was most likely directly generated by the magnetopause surfacewave and thus represents one of the few known manifestations of the classical field lineresonance process in space directly observed, a conclusion made possible due to thespecial configuration of the THEMIS mission and its five spacecraft.