Magnetic Field Oscillations Observed by Swarm Satellites in the Nightside Upper Ionosphere During Low‐Latitude Pi2 Pulsations

Abstract

AbstractPi2 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.