Observations of a Pc5 global (cavity/waveguide) mode outside the plasmasphere by THEMIS

Standing fast mode waves known as global modes, or cavity/waveguide modes, have been extensively studied as a potential driver of monochromatic shear Alfvén waves in the Earth's magnetosphere via the field line resonance (FLR) mechanism. However, their existence outside of the plasmasphere remains controversial. We present here a global mode observation outside the plasmasphere, using simultaneous multispacecraft 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 (2–7 mHz) global mode. The global mode transfers energy toward the plasmapause, where it drives localized shear Alfvén wave activity in the Pc5 frequency range. Global modes are thus a viable mechanism for converting broadband energy sources to monochromatic, radially localized shear Alfvén waves in the Pc5 frequency range. Key Points We observe a cavity/waveguide mode outside the plasmasphere The most likely drivers are broadband dynamic pressure fluctuations The Pc5 frequency is appropriate for driving localized Alfven waves