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.
Bibliographical noteFunding Information:
K. Takahashi is grateful to V. Angelopoulos for valuable com ments on THEMIS observations and to K. Koga for providing the ETS-VIII data. K. Takahashi was supported by NASA grants NNX14AB97G, NNX15AI95G, and NNX17AD34G, as well as NSF grant AGS-1106427. R. L. Lysak were supported by NSF grants AGS-1405383 and AGS-1558134. M. D. Hartinger was sup ported by NASA grant NNX17AD35G. Parts of this work were supported by The Johns Hopkins University Applied Physics Laboratory (JHU/APL) contract 921647 under NASA Prime Contract NAS5-01072 and JHU/APL contract 131802 under NASA Prime Contract NNN06AA01C. Data used in this study are available from the following sources: Van Allen Probes Science Operation Centers located at University of Iowa (http://emfisis.physics.uiowa.edu) and University of Minnesota (http://www.space.umn.edu/ missions/rbspefw-home-university- of-minnesota), for RBSP; Space Science Laboratory, University of California, Berkeley (http://themis.ssl.berkeley.edu), for THEMIS; NOAA National Geophysical Data Center (http://satdat.ngdc.noaa.gov), for GOES; Japan Aerospace Exploration Agency, Japan, contact: email@example.com, for ETS-VIII; Geological and Geophysical Institute of Hungary (http://geofizika.canet.hu/plasmon/, contact: firstname.lastname@example.org, for EMMA; and World Data Center for Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp), for Sym-H.
- Van Allen Probes
- cavity mode oscillations
- interplanetary shock