Analysis on the interplanetary causes of the great magnetic storms in solar maximum (2000-2001)

In this paper, we analyze the interplanetary causes of eight great geomagnetic storms (Dst⩽-200nT) during the solar maximum (2000-2001). The result shows that the interplanetary causes were the intense southward magnetic field and the notable characteristic among the causal mechanism is compression. Six of eight great geomagnetic storms were associated with the compression of southward magnetic field, which can be classified into (1) the compression between ICMEs (2) the compression between ICMEs and interplanetary medium. It suggests that the compressed magnetic field would be more geoeffective. At the same time, we also find that half of all great storms were related to successive halo CMEs, most of which originated from the same active region. The interactions between successive halo CMEs usually can lead to greater geoeffectiveness by enhancing their southward field Bs interval either in the sheath region of the ejecta or within magnetic clouds (MCs). The types of them included: the compression between the fast speed transient flow and the slow speed background flow, the multiple MCs, besides shock compression. Further, the linear fit of the Dst versus (-VB¯z)α(Δt)β gives the weights of -VB¯z and Δt as α=2.51 and β=0.75, respectively. This may suggest that the compression mechanism, with associated intense Bs, rather than duration, is the main factor in causing a great geomagnetic storm.