Multiple flux rope events at the high-latitude magnetopause: Cluster/rapid observations on January 26, 2001

From 11:10 to 11:40UT on January 26, 2001 four Cluster-II spacecrafts were located in the dusk-side high latitude regions of the magnetosheath and the magnetosheath boundary layer. During this time interval the interplanetary magnetic field (IMF) had a negative B_z component. A detailed study on the multiple flux ropes (MFRs) observed in this time period is conducted in this paper. It is found that the MFRs in the high latitude magnetosheath boundary appeared quasi- periodically with a repeated time period of about 78s, which is much shorter than the averaged occurrence period (about 8-11 minutes) of the flux transfer events (FTEs) at the dayside magnetopause. All the flux ropes observed in this event had a strong core magnetic field. The principal axis of the most flux ropes are found to lie in the direction of the minimum magnetic field variation. A few flux ropes had their axes lying in the direction of the middle magnetic field variation; while for the remainders their principle axes could not be determined by the method of Principal Axis Analysis (PAA) . The reason that causes this complexity relays on the different trajectories of the spacecraft passing through the flux ropes. Each flux rope had a good corresponding HT frame of reference in which it was in a quasi-steady state. All flux ropes moved along the surface of the magnetopause in a similar direction indicating that these flux ropes were coming from the dawnside low latitude. Their radial scale is found to be 1-2R_E, comparable to the normal diameter of FTEs. The energetic ions originated from the magnetosphere were flowing out to the magnetosheath on the whole, while the solar wind plasma was seen to flow into the magnetosphere along the axis of the flux ropes. The flux ropes offered channels for the transport of the solar wind plasma into the magnetosphere and the escape of the magnetospheric plasma into the interplanetary space. Each event was accompanied by an enhanced reversal of the dusk-dawn electric field, which could be identified to be the convective electric field in nature.