Analysis on the Interplanetary Causes of Moderate Geomagnetic Storms During the Whole 23th Solar Cycle

The interplanetary causes of 183 moderate geomagnetic storms ( -100 nT < Dst ≤ -50 nT) and the their distributions during the solar cycle 23 (1996--2006) are identified. Moreover, the solar cycle dependence of the interplanetary source of moderate storms was analysed comparing with that of intense geomagnetic storms (Dst ≤ -100 nT). It is found that: (1) Corotating Interaction Region (CIR) and Interplanetary Coronal Mass Ejection (ICME) are equal importance in moderate storms, moreover, in the associated ICMEs, magnetic cloud and non magnetic cloud ICME are also equal importance, while the combined sheath and ICME fields (Sh+MC and Sh+nonMC) are more important in associated moderate storms; (2) the yearly distribution of moderate storms in solar cycle 23 shows that double-peak and double-valley distribution, with the first peak appearing at solar maximum 2001 and the second peak at declining phase 2003, and the double-valley appearing at solar minimum 1996 and 2006, while the moderate storms distributed on average during the other years; (3) during the maximum phase, ICME is the main interplanetary source of moderate storm, and CIR is responsible by a larger number of moderate storms during the rising and declining phase, moreover, the number of moderate storms in the declining phase is almost two times that of the rising phase. However, for intense storms, ICME is almost the most important source during the whole solar cycle 23.