Ground-Space Observations of Pc5 Poleward Moving Auroral Arc Pulsations and Field-Line Resonances in the Post-Midnight Sector: THEMIS Observations

Abstract We investigate the Pc5 poleward moving auroral arc (PMAA) pulsations (~ 4–5 min period) using the ground-based all-sky imager network and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) A, D, and E satellites, whose footprints were located near the PMAA in the post-midnight sector. The Pc5 PMAA pulsations considered herein occurred in conjunction with the enhancement of the magnetic and electric field oscillations observed near the equatorial plane of the magnetosphere. The magnetospheric oscillation signal displayed three-cycle oscillations, which correspond primarily to the PMAA pulsations. The value of coherence between the magnetospheric oscillations and the luminosity pulsations was higher than 0.9. Based on these observations, it is suggested that the PMAA pulsations and the magnetospheric field oscillations are initiated by the same physical mechanism and thus oscillate concurrently by the magnetosphere-ionosphere (M-I) coupling. The satellite data indicated a longer period of magnetospheric oscillations at the higher latitude site. On the other hand, the measured period of the PMAA pulsation was almost constant in the lower latitude region (~ 68.5°-70.0° MLAT), whereas in the higher latitude region (~ 70.0°-70.5° MLAT) it increased with increasing latitude. This signature demonstrates that the oscillations on the lower latitudinal side of the PMAA conformed with the monochromatic frequency field-line resonance (FLR) where the oscillation period was constant and independent of latitude, whereas the higher latitude side of the PMAA presented a multi-frequency FLR region where the period lengthened with increasing latitude. The Pc5 magnetic pulsations observed on the ground neither exhibited a clear coincidence with the PMAA pulsations nor with the magnetospheric magnetic oscillations. On the other hand, the H component of magnetic pulsations demonstrated a rather similar behavior to that of the ion pressure variation within the magnetosphere. The solar wind speed was significantly high, approximately 650 km/s, during this event. The magnetospheric magnetic and electric field oscillations could be triggered simultaneously in a wide region by an impulse such as rapid convection changes caused by the sudden variations of the interplanetary magnetic field (IMF) Bz, which was observed by the SuperDARN radar and the Geotail satellite.