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

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 (B_z) component of ionospheric magnetic fields when Swarm spacecraft were at |magnetic latitude| < 30° with or without an accompanying ionospheric field perturbation in the radial (B_x) component, depending on the latitude of the spacecraft. It is found that the phase and amplitude relationship between B_x and B_z 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 B_x and B_z components is the consequence of the spatial mode structure in the north-south direction of trapped fast mode waves inside the plasmasphere.