Simultaneous Pi2 pulsation detected by CSES-01, Swarm, RBSP and Arase satellites

In this study, we present simultaneous multi-point observations of Pi2 magnetic pulsations studied, for the first time, through joined measurements of multiple missions: the CSES-01 and Swarm (A and C), in the topside ionosphere, Van Allen Probe (or Radiation Belt Storm Probes, RBSP) (A and B) and Arase in the magnetosphere. We focused on the compressional component of the satellites and the horizontal component of magnetic field from Kakioka (KAK) ground station in Japan. The Pi2 event occurred from 12:40 to 12:56 UT on January 12, 2019, where CSES-01, RBSP-A, and KAK were on the night side; Swarm-A/C, were on the day side while RBSP-B and Arase were in the dusk sector. We observed 90-degree phase delay between RBSP-A-Bz and RBSP-A-Ey which can be interpreted as a radially trapped fast mode for the compressional oscillation. Both the wavelet transforms and the Hilbert-Huang transform (HHT) were applied for signal analysis, revealing wave-like structures and strong coherence among all data sets and confirming the Pi2 pulsation nature. The compressional component in the topside ionosphere and in the magnetosphere seem very similar with the horizontal component of KAK station. During 12:43-12:45 UT, CSES-01 and Swarm-A/C exhibited an in-phase variation while both were in the Southern Hemisphere. However, as CSES-01 transitioned to the Northern Hemisphere between 12:47 and 12:56 UT, the corresponding signals became out of phase. During the selected Pi2 event, RBSP-B was located very close to Arase in the dusk sector and detected compressional oscillations with a waveform nearly identical to that observed by RBSP-A, suggesting that the observed Pi2 exhibited cavity resonance characteristics. To understand their propagation mechanism, we conduct further analysis of the duskside Pi2 pulsations in this event. We found that the penetration/propagation speed of the low-frequency Pi2 pulsations is high (|m|~ 0.3) and much larger than the average Alfven speed in the plasmasphere, while the high-frequency Pi2 pulsations have a finite m number (m ~ -1.7) and their phase speed is comparable to the average Alfven speed. We suggest that the nightside Pi2 pulsations propagate sunward through a waveguide-like mode, consistent with the high-frequency Pi2 signatures detected on the duskside in the magnetosphere.