Pi2 oscillations (40–150 s) in the nightside upper ionosphere are studied using magnetic field data acquired by multiple Swarm spacecraft in low-Earth orbit and at the low-latitude Bohyun (BOH, L = 1.3) station on 22 October 2014. Four Pi2 events were identified from the BOH data near midnight (magnetic local time = 1.5 hr), while Swarm-A, Swarm-B, and Swarm-C spacecraft were orbiting in the premidnight (magnetic local time = 21–22 hr) meridian from 70° to −60° in magnetic latitude at ∼450- to 500-km altitudes. Unlike previous low-Earth orbit studies, which used a single point observation, the latitudinal structure of the amplitude and phase of ionospheric magnetic field perturbations can be determined by simultaneous multipoint observations along the latitude at a constant radial distance. We observed that the horizontal H component of BOH data is well correlated with the compressional (Bz) component of ionospheric magnetic fields when Swarm spacecraft were at |magnetic latitude| < 30° with or without an accompanying ionospheric field perturbation in the radial (Bx) component, depending on the latitude of the spacecraft. It is found that the phase and amplitude relationship between Bx and Bz along the latitude is consistent with the model ionospheric field perturbations at 500-km altitude, which are associated with a plasmaspheric resonance excited in a dipole numerical simulation. This indicates that the latitudinal variation of the ionospheric Pi2 pulsations in both Bx and Bz components is the consequence of the spatial mode structure in the north-south direction of trapped fast mode waves inside the plasmasphere.
Bibliographical noteFunding Information:
K.-H. Kim is grateful to J. Park in KASI for valuable comments on the Swarm magnetic field data usage. Swarm data used in this study are available at the https://earth.esa.int/web/guest/swarm/data-access website. The Bohyun magnetic field data were provided by the Solar and Space Weather Research Group in Korea Astronomy and Space Science Institute (KASI). This work was supported by BK21+ through the National Research Foundation (NRF) funded by the Ministry of Education of Korea. The work of K.-H. Kim was supported by the Basic Science Research Program through NRF funded by NRF-2019R1F1A1055444 and also supported by Project PE19020 of the Korea Polar Research Institute. The work of J. Hwang was supported by the basic research funding from KASI. R. Lysak's contributions were supported by NSF Grant AGS-1558134.
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- Pi2 pulsations
- plasmaspheric cavity mode