Abstract:
We analyse the characteristics of interplanetary coronal mass ejections (ICMEs) during Solar Cycles 23 and 24. The present analysis is primarily based on the near-Earth ICME catalogue (Richardson and Cane, 2010). An important aspect of this study is to understand the near-Earth and geoeffective aspects of ICMEs in terms of their association (type II ICMEs) versus absence (non-type II ICMEs) of decameter-hectometer (DH) type II radio bursts, detected by Wind/WAVES and STEREOS/WAVES. Notably, DH type II radio bursts driven by a CME indicate powerful MHD shocks leaving the inner corona and entering the interplanetary medium. We find a drastic reduction in the occurrence of ICMEs by 56% in Solar Cycle 24 compared to the previous cycle (64 versus 147 events). Interestingly, despite a significant decrease in ICME/CME counts, both cycles contain almost the same fraction of type II ICMEs (~47%). Our analysis reveals that, even at a large distance of 1 AU, type II CMEs maintain significantly higher speeds compared to non-type II events (523 km/s versus 440 km/s). While there is an obvious trend of decrease in ICME transit times with increase in the CME initial speed, there also exists a noticeable wide range of transit times for a given CME speed. Contextually, Cycle 23 exhibits 10 events with shorter transit times ranging between 20-40 hours of predominantly type II categories while, interestingly, Cycle 24 almost completely lacks such "fast" events. We find a significant reduction in the parameter VICME×Bz, the dawn to dusk electric field, by 39% during Solar Cycle 24 in comparison with the previous cycle. Further, VICME×Bz shows a strong correlation with Dst index, which even surpasses the consideration of Bz and VICME alone. The above results imply the crucial role of VICME×Bz toward effectively modulating the geoeffectiveness of ICMEs.