Cusp Location Dependence On IMF:Cluster Statistical Study

The issue,that the interaction among the solar wind,magnetopause and ionosphere,is one of the most important field.The cusp is the region which has open magnetic field lines.In order to maintain the balance between the solar wind pressure and magnetic pressure,the region with many high density particles must be existed,then the cusp has been found.And many studies has been observed.This paper shows the cusp location has dependence on IMF with statistic study.Firstly,we attain data from Cluster spacecraft.Secondly,we rule the criteria of cusp crossings,including:(1)a sudden increase in the low-energy ion and electron density(greater than 5cm-3);(2)an electron thermal energy less then 100 eV;(3)the presence of significant He++(usuallygreater than 0.5 cm-3).Tsyganenko 96 model is combined with the criteria to identify the boundary of the cusp.Finally,the 616 mid and high altitude cusp crossings have been identified.The 616 mid and high altitude cusp crossings,collected from Cluster observation for the time interval from 1st February 2001 to 31th December 2010,are used to study IMF e?ect on cusp locations.Although the statistical errors in our study are quite significant,our results are shown as follows:(1)With the increasing of the negative Bx,the invariant latitude of the cusp moves to poleward for northward IMF,while it slightly shifts to equatorward for southward IMF.For the increasing positive Bx,the invariant latitude of the cusp doesn't change too much.This phenomenon is observed obviously for the southern hemisphere,but no apparent phenomenon merges on the northern hemisphere.When cone angle is greater than 90°,the southern cusp has a polarward movement with increasing cone angle.(2)When the negative By is enhanced,the magnetic local time of cusp turns to dawnward(duskward)for northern(southern)hemisphere under southward IMF.(3)Consistent with previous study,we also find that with the increasing of southward Bz,the invariant latitude of the cusp moves equatorward for both northern and southern hemisphere,while it keeps almost unchanged for northward Bz.