Relationship Between the Magnetic Flux Ropes in Magnetotail and Interplanetary Magnetic Field

The magnetic flux ropes observed by Cluster from 2001 to 2005 in the magnetotail are surveyed in this paper. The statistic study have been performed on the relationship between the formation of the magnetic flux ropes and Interplanetary Magnetic Field (IMF). Considering the IMF condition when the flux ropes are observed, for the 80 % of the total 73 flux rope cases, there are dominant By in IMF accordingly, while in the 78 % of all cases there are core fields with the same direction as the IMF By. It is possible that IMF can change the relative direction of the magnetic fields in the northern and southern plasma sheet to form the magnetic topology favoring the formation of the magnetic flux rope. IMF also has a decisive effect on the directions of the core field in the flux ropes. As indicated in these statistic results, the formation of the flux ropes in the magnetotail is independent of the direction of the IMF Bz.