Multiple cusps during an extended northward IMF period with a significant By component

On 21-22 March 2001, four cusp-like regions were observed consecutively in about five hours by all four Cluster spacecraft when the interplanetary magnetic field (IMF) was northward with a significant By component. All four cusp-like encounters were characterized by turbulent magnetic fields, high-density plasma, and plasma flow significantly slower than the magnetosheath level. The cusp-like regions are associated with thermalized, bidirectional distributed plasma electrons. The first encountered cusp is the main cusp; the other three cusp-like regions are temporal effects. The normal velocities v(n) at boundary interfaces (exit from the cusp) are found to be almost three times larger than that at boundary interfaces (entry into the cusp). The boundary normal, velocity, and timing analysis for six clear boundaries of the last three cusps obtained by all four spacecraft indicates that they are most likely one boundary shifting in the dawn-dusk direction between the dayside magnetosphere/trapping region and the cusp region. So the Cluster spacecraft have been observing the same cusp, and it appeared as four cusp-like regions due to possible magnetospheric oscillations. Oscillations with a period of 22 min are observed by the Cluster spacecraft in the high-latitude region, which is in agreement with the cold-dense plasma sheet fluctuations (20 min period) observed by the Geotail satellite. Multiple cusps observed by Cluster and the wavy-like structures in the dusk low-latitude boundary layer observed by Geotail simultaneously suggest the whole magnetosphere is oscillating during northward IMF.