Van Allen Probe observations of periodic rising frequencies of the fast magnetosonic mode

Near simultaneous periodic dispersive features of fast magnetosonic mode emissions are observed by both Van Allen Probes spacecraft while separated in magnetic local time by similar to 5h: Probe A at 15 and Probe B at 9-11 h. Both spacecraft see similar frequency features, characterized by a periodic repetition at similar to 180s. Each repetition is characterized by a rising frequency. Since no modulation is observed in the proton shell distribution, the plasma density, or in the background magnetic field at either spacecraft we conclude that these waves are not generated near the spacecraft but external to both spacecraft locations. Probe A while outside the plasmapause sees the start of each repetition similar to 40s before probe B while deep inside the plasmasphere. We can qualitatively reproduce the dispersive features but not the quantitative details. The cause for this phenomena remains to be identified.