Abstract. We use Van Allen Probes (Radiation Belt Storm Probes A and B, henceforth
RBSP-A and RBSP-B) and GOES-13 and GOES-15 (henceforth G-13 and G-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 1 January 2016. From ∼ 19:00 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 RBSP-A and RBSP-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 RBSP but Pc4 pulsations with lower
frequencies. Both RBSP-A and RBSP-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. RBSP-A and RBSP-B HOPE (Helium Oxygen Proton Electron)
and MagEIS (Magnetic Electron Ion Spectrometer) observations provide the first evidence for a corresponding
frequency doubling in the plasma density and the flux of energetic electrons,
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 significant solar wind trigger or phase shift with energy, we interpret
the compressional Pc5 pulsations in terms of the mirror-mode instability.
