Abstract. We use Van Allen Probes and GOES-13 and -15 multipoint magnetic field, electric field, plasma, and energetic particle observations to study the spatial, temporal, and spectral characteristics of compressional Pc5 pulsations observed during the recovery phase of a strong geomagnetic storm on January 1, 2016. From ~ 19:00 UT to 23:02 UT, successive magnetospheric compressions enhanced the peak-to-peak amplitudes of Pc5 waves with 4.5–6.0 mHz frequencies from 0–2 to 10–15 nT at both Van Allen Probes A and B, particularly in the prenoon magnetosphere. Poloidal Pc4 pulsations with frequencies of ~ 22–29 mHz were present in the radial Bx component. The frequencies of these Pc4 pulsations diminished with increasing radial distance, as expected for resonant Alfvén waves standing along field lines. The GOES spacecraft observed Pc5 pulsations with similar frequencies to those seen by the Van Allen Probes, but Pc4 pulsations with lower frequencies. Both Van Allen Probes A and B observed frequency doubling in the compressional component of the magnetic field during the Pc5 waves, indicating a meridional sloshing of the equatorial node over a combined range in ZSM from 0.25 to −0.08 Re, suggesting that the amplitude of this meridional oscillation was ~ 0.16 Re about an equatorial node whose mean position was near ZSM = ~ 0.08 Re. Van Allen Probes A and B HOPE and MagEIS observations provide the first evidence for a corresponding frequency doubling in the plasma density and the flux of energetic electron, respectively. Energetic electron fluxes oscillated out of phase with the magnetic field strength with no phase shift at any energy. In the absence of any solar wind trigger or phase shift with energy, we interpret the compressional Pc5 pulsations in terms of the mirror mode instability.
