Spatial structure of beamlets according to Cluster observations

Beamlets are sporadic bursty ion beams with energies about 10–30 keV observed in the lobes—plasma sheet interface. To investigate the spatial structure of this phenomenon we used Cluster data for the fall and summer of 2001 time interval. Both case study and statistical study have been performed. At a later stage of statistical study Interball data also have been used. Detailed investigation shows that beamlet may be considered as “kink-like” magnetic field line distortion propagating earthward in the X – Y GSM plane with a high-velocity. The observed features of such a structure are the following: (1) besides pronounced velocity along the non-disturbed magnetic field both in the southern and in the northern lobes, the beamlet has a convection velocity V y ∼ 150 km / s ; (2) variations of the magnetic field strongly correlate with variations of the ion velocity; (3) variations of B y component associated with beamlets have the same sign in both the hemispheres; (4) cold (background) lobeʹs ion population drapes around the propagating beamlet core with an ambient magnetic field.