Analysis on the interplanetary causes of intense geomagnetic storms during solar cycle 23

The interplanetary causes of 90 intense geomagnetic storms (Dst≤-100nT) and their solar dependence occurring during solar cycle 23 (1996-2006) are identified. During this solar cycle, magnetic reconnection between the southward IMF and magnetopause fields are happened when the interplanetary magnetic field was southwardly directed for long durations of time, which induces the cause of the geomagnetic storms and ionospheric storms. In addition, the relationships between ionospheric storms and the solar activity are investigated. It is found that: 1) Interplanetary coronal mass ejection (ICME) is the main cause of intense geomagnetic storms. Moreover, in the associated ICMEs, magnetic cloud ICME is more important than non magnetic cloud ICME, which is the main cause of intense geomagnetic storms. 2) We have found more storms in the maximum and declining phase than in the rising phase. There is a three peak distribution of intense geomagnetic storms, 1998, 2001-2002 and 2004-2005. The peaks at 2001-2002 and 2004-2005 correspond to the solar maximum and declining phase peaks. One possibility is that 1999could be an unusual year, with a low rate of intense geomagnetic storms. 3) ionospheric storms caused by ICME are seven times more than that caused by CIR, which shows that CME is also the main cause of ionospheric storms.