Observations of a cavity mode outside the plasmasphere by THEMIS

Standing fast mode waves known as cavity modes have been extensively studied as a potential driver of monochromatic shear Alfvén waves in Earth's magnetosphere via the field line resonance (FLR) mechanism, but their existence outside of the plasmasphere remains controversial. We present here the first definitive identification of a cavity mode outside the plasmasphere near the dawn flank, using simultaneous multi-spacecraft observations in the solar wind, the magnetosheath and the outer magnetosphere, as well as ground magnetometer data. Broadband solar wind dynamic pressure fluctuations are the most likely drivers of the Pc5 frequency range cavity mode.