Dependence of successive GMSs with Dst <-100 nT on solar and interplanetary parameters

The intense Geomagnetic Storms (GMSs) with Dst < -100 nT have been investigated for the period from Jan 1996 to Dec 2006. Seven GMSs of doublet and four of triplet nature are observed. Firstly, each GMS has been studied separately as if they are associated with independent Coronal Mass Ejections (CMEs). Secondly, for each doublet and triplet, the accumulated effect on GMS has been investigated and correlated with Dst index so as to understand the geoeffectiveness of Successive Intense GMSs. Majority of the successive intense GMSs have occurred during maximum phase of Solar cycle. During the occurrence of overlapping successive storms Dst falls abruptly. For non-overlapping successive storms, the Dst value falls gradually to minimum, showing a trend of recovery before the geosphere is hit by another storm. It is observed that the combined effect on GMSs is due to the Solar Wind (SW) being complex, having a very high value of SW velocity (Vsw) continuously for a very long period of 2 to 6 days. Further, Bz falls to a much lower value and B rises to a pretty high value for accumulated effect than for isolated GMS. When the GMSs are considered as separate entity, the correlation coefficient of Interplanetary Magnetic Field,(IMF) parameters B and Bz and further, their products Vsw.B and Vsw.Bz correlated with Dst index are found to be -0.65, 0.72, -0.66 and 0.77 respectively; whereas, the coefficients are much better with the respective values of -0.7, 0.87, -0.78 and 0.90, for the accumulated effect of GMSs. Thus, it is preferable to investigate the accumulated effect of CMEs causing successive GMSs as compared to their isolated effects.