Kelvin-Helmholtz instability induced field line resonance during a geomagnetic storm event

We report on conjunction observations by the THEMIS A, E, and D spacecraft of Kelvin-Helmholtz (KH) surface waves at the dusk magnetopause and Pc5 pulsations in the magnetosphere during the main phase of the intense storm on March 17, 2015. These Pc5 pulsations were likely driven by field line resonance associated with the Kelvin-Helmholtz instability (KHI). Notably, the compressional component of the magnetic perturbation exhibits a frequency-doubling feature. We demonstrate that this frequency doubling arises from plasma mixing during the nonlinear stage of KH waves and its coupling to antisymmetric standing modes within the magnetosphere. Although previous studies often attribute modulated plasma fluxes that show in-phase or antiphase relationships with magnetic perturbations to drift mirror instability, our event is likely driven by KHI, as the low plasma beta environment precludes mirror instability. This work provides an alternative mechanism for explaining harmonic compressional waves that does not rely on internal instabilities.