Abstract
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.
Highlights
Ultralow frequency (ULF) pulsations – with periods of 100 s or greater, with high azimuthal wave numbers (m), with magnetic field perturbations in the radial direction, and with electric field perturbations in the azimuthal direction – within the Earth’s magnetosphere are typically poloidal waves (Sugiura and Wilson, 1964)
We demonstrate that the energetic particles respond directly to the compressional Pc5 pulsations and exhibit a double-frequency oscillation
When RBSP-A and RBSP-B were in the vicinity of the geomagnetic equator, the compressional Pc5 pulsations displayed peculiar features indicating frequency doubling
Summary
Ultralow frequency (ULF) pulsations – with periods of 100 s or greater, with high azimuthal wave numbers (m), with magnetic field perturbations in the radial direction, and with electric field perturbations in the azimuthal direction – within the Earth’s magnetosphere are typically poloidal waves (Sugiura and Wilson, 1964). Periodic solar wind dynamic pressure variations directly drive some compressional magnetospheric magnetic field oscillations (e.g., Kepko and Spence, 2003; Motoba et al, 2003). Takahashi and Ukhorskiy (2008) considered solar wind pressure variations as the main external driver of Pc5 pulsations observed at geosynchronous orbit in the dayside magnetosphere. One possible generation mechanism for compressional Pc5 pulsations at geosynchronous orbit is a drift-mirror instability of ring current particles (e.g., Lanzerotti et al, 1969). While the observed anticorrelated magnetic field strength and ion flux oscillations are expected for a drift-mirror wave (Kremser et al, 1981), the instability criterion is generally not satisfied (Pokhotelov et al, 1986).
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