Long-Lasting Ground-Satellite High Coherence of Compressional Dayside Pc3–Pc4 Pulsations

We present observations of broadband (∼10–50 mHz) Pc3–Pc4 waves on 4 January 2014. The waves were detected on the dayside simultaneously in a compressional component (δB_z) at the Radiation Belt Storm Probes A (RBSP-A) in the inner magnetosphere and in the north-south component (δH) on the ground at a low-latitude Bohyun (BOH) station (L = 1.3) during an interval of small interplanetary magnetic field cone angle, suggesting that upstream ultralow frequency (ULF) waves were the source of the magnetospheric Pc3–Pc4 waves. We observed the ground-satellite high coherence between RBSP-A δB_z and BOH δH for a prolonged time interval lasting 6.5 hr, which has not been reported previously, during which RBSP-A was on the inbound and outbound legs moving from L = ∼3.3 to ∼6.3. In order to understand the spatial mode structure of the compressional waves, we examined the ground-satellite cross phase for the high-coherence interval and found that the waves observed in and out of the plasmasphere propagated earthward at the average fast-mode speed of ∼700–1,000 km/s. We also observed cross-phase values smoothly changing with the radial distance of the spacecraft across the plasmapause. This indicates that the presence of the plasmapause has little effect on our fast-mode waves propagating into the inner magnetosphere and to the ground low-latitude station.