Observation and Numerical Simulation of Cavity Mode Oscillations Excited by an Interplanetary Shock

Abstract

Cavity mode oscillations (CMOs) are basic magnetohydrodynamic eigenmodes in the magnetosphere predicted by theory and are expected to occur following the arrival of an interplanetary shock. However, observational studies of shock‐induced CMOs have been sparse. We present a case study of a dayside ultralow‐frequency wave event that exhibited CMO properties. The event occurred immediately following the arrival of an interplanetary shock at 0829 UT on 15 August 2015. The shock was observed in the solar wind by the Time History of Events and Macroscale Interactions during Substorms‐B and ‐C spacecraft, and magnetospheric ultralow‐frequency waves were observed by multiple spacecraft including the Van Allen Probe‐A and Van Allen Probe‐B spacecraft, which were located in the dayside plasmasphere at L ∼1.4 and L ∼ 2.4, respectively. Both Van Allen Probes spacecraft detected compressional poloidal mode oscillations at ∼13 mHz (fundamental) and ∼26 mHz (second harmonic). At both frequencies, the azimuthal component of the electric field (Eϕ) lagged behind the compressional component of the magnetic field (Bμ) by ∼90°. The frequencies and the Eϕ‐Bμ relative phase are in good agreement with the CMOs generated in a dipole magnetohydrodynamic simulation that incorporates a realistic plasma mass density distribution and ionospheric boundary condition. The oscillations were also detected on the ground by the European quasi‐Meridional Magnetometer Array, which was located near the magnetic field footprints of the Van Allen Probes spacecraft.

L Versus Time Structures of Dayside Magnetic Pulsations Detected by the European Quasi‐Meridional Magnetometer Array

“…This is consistent with the spacecraft potential data from THEMIS-D that indicate the spacecraft, which was in the posmidnight sector, crossed the plasmapause at L ∼ 5.4 at ∼0650 UT, as shown in Figure 3d of Takahashi et al (2018). The frequency (f T1 , Figure 2d) of the fundamental toroidal (T1) wave that is determined by applying the cross-phase technique (Waters et al, 1991a) to the EMMA data for 0900-1000 UT (Takahashi et al, 2018) decreases monotonically with L, consistent with the n e profile. The frequency (f T1 , Figure 2d) of the fundamental toroidal (T1) wave that is determined by applying the cross-phase technique (Waters et al, 1991a) to the EMMA data for 0900-1000 UT (Takahashi et al, 2018) decreases monotonically with L, consistent with the n e profile.…”

“…These initial variations are followed by damped E and B oscillations, which have been attributed to multiharmonic cavity mode oscillations (Takahashi et al, 2018). These initial variations are followed by damped E and B oscillations, which have been attributed to multiharmonic cavity mode oscillations (Takahashi et al, 2018).…”

“…The nature of the oscillations becomes clearer in Figure 3b, which displays magnetic field perturbations in what we call the "EMMAgram" format. Figure 4 highlights the cavity mode oscillations reported by Takahashi et al (2018). We use running boxcar averages with boxcar length T boxcar to define the slow variations.…”

“…To specify the equatorial V f (∼ V A ), we adopted the model equatorial mass density eq (amu/cm 3 ) = −81 + 18,000L −2.7 , which was obtained by least squares fitting to the magnetoseismic eq estimates reported by Takahashi et al (2018, Figure 12b) using EMMA data for 0900-1000 UT. To specify the equatorial V f (∼ V A ), we adopted the model equatorial mass density eq (amu/cm 3 ) = −81 + 18,000L −2.7 , which was obtained by least squares fitting to the magnetoseismic eq estimates reported by Takahashi et al (2018, Figure 12b) using EMMA data for 0900-1000 UT.…”

“…The selected frequency differs from 26 mHz reported byTakahashi et al (2018) because the spectra were computed using a shorter data window (5 min vs. 10 min), resulting in a different Fourier spectral resolution. The spectral parameters in this figure are for 0830-0835 UT and are evaluated at 25 mHz in reference to E , B , and B .…”