Cluster satellite observations of mHz pulsations in the dayside magnetosphere

On 17 August 2002 the Cluster spacecraft moved through the dayside magnetosphere. Between 16:00 and 18:30 UT clear monochromatic oscillations are seen in both electric field and magnetometer data. The frequency is 4.2 mHz in the spacecraft frame of reference. The oscillations have a clear spatial localisation. The magnetic field oscillations are radially polarised in the plane perpendicular to the background magnetic field, indicating that the wave is in the poloidal mode. From the difference in phase between the satellites we estimate the azimuthal wave number, m, to be about 130, consistent with the magnetic field polarisation. The frequency is stable for different L-values as well as over time. From the value of m, the Doppler shift due to satellite motion is estimated to 0.5 mHz. By looking at the phase of the electric and the magnetic field close to the equator we conclude that the oscillations are in a mode with an odd number of half wavelengths between the two ionospheres.