Abstract:

Data from the four Cluster spacecraft during the mid-altitude cusp crossings on September 10 of 2002 are presented.The encounters occur under conditions of weak and steady southward Interplanetary Magnetic Field(IMF).The data show that there are the field-aligned currents(FACs)in the cusp causing large magnetic field disturbances.The boundary structures of the cusps are characterized by the FACs interfaces.A method for calculating the velocities of the interface based on multi-point measurements of spacecraft are introduced.Results show that the interfaces are roughly parallel to magnetic field lines,and their velocities are almost dawnward for the southern crossing and almost duskward in the northern pass.The velocities have small values(0.36—0.80km/s)with respect to satellite's velocity in the cusp crossings.