Van Allen Probes Observations of compressional ULF waves and Ion interactions in the magnetosphere dayside during the September 2017 Storm

We present magnetosphere dayside measurements of ion modulations by a ULF wave (Pc 5 range) taken by the RBSPICE and EMFISIS instruments, respectively, onboard NASA's Van Allen Probes during the September 2017 geomagnetic storm. The event was captured by both spacecrafts at 3 - 4 UTC. The RBSPICE measurements indicate that all three ion species: H, He, and O ions in the energy range 7 keV - 300 keV are modulated by the Pc 5 wave. We also show that the plasma pressures oscillate in anti-phase with the Pc 5 wave with the perpendicular pressure being larger that the parallel pressure component. The Pc 5 wave is mostly compressional. Although both spacecrafts are relatively far apart we are able to estimate the m-mode number within the constraints of the measurements. The value of m 3 obtained is consistent with field line resonances driven by the solar wind flow. Furthermore, for this event the plasma beta < 1 indicating an external mechanism of wave excitation.