IMAGE EUV observation of radially bifurcated plasmaspheric features: First observations of a possible standing ULF waveform in the inner magnetosphere

We present EUV observations of the plasmasphere from 1938 to 2211 UT on 28 June 2000 characterized by the presence of bifurcated radial enhancements of the He⁺ plasma distribution in the nightside sector. These features remain stable throughout the period of observation and are found to move about the Earth at 67% of the corotation rate. Twodimensional simulation of the plasmasphere assuming the presence of an azimuthal standing wave patterns at L = 1.8 and 2.5 suggests that the organization of the outer plasmasphere is the result of convective motion driven by a ULF standing wave. Preliminary analysis of ground-based magnetometer data provided by the IMAGE magnetometer network during the period of EUV observation indicates the presence of several narrow-frequency oscillations extending down to 0.68 mHz, consistent with simulation results. Similar narrow-frequency features are found in preliminary analysis of ACE Solar Wind Electron Proton Alpha Monitor (SWEPAM) bulk solar wind velocity data. We speculate that the observed radial He⁺ enhancements of the nightside plasmasphere may be the result of a nonresonant oscillation driven by the solar wind.