A long-duration narrowband Pc5 pulsation

We report on a rare event of a narrowband ULF magnetospheric pulsation that is observed at geosynchronous orbit over five consecutive days in the recovery period of a magnetospheric storm, with frequencies ranging from 5 to 9 mHz. The pulsation displays distinctive localization, being observed over several hours of local time in the noon and postnoon sectors during successive passes of GOES-8 and GOES-9. Geotail, which is traversing the outer magnetosphere a couple of RE farther out from geosynchronous orbit at the time the pulsations are observed, shows no distinct narrowband fluctuation in the magnetic field, excluding oscillations in the solar wind as a possible source of the pulsations. On the ground, several of the IMAGE magnetometers observe pulsations with similar spectral characteristics. By performing a phase cross-spectrogram analysis on measurements from adjacent and latitudinally aligned ground stations we estimate the azimuthal mode number m of the pulsation, which we find to range between 20 and 55. Phase difference calculations indicate an eastward phase propagation of these pulsations. Density measurements from LANL spacecraft at geosynchronous orbit show evidence of a plasmasphere refilling process that is slower than usual, possibly associated with the observed slow frequency decrease and the unusually long lasting of the pulsations.