Comparison of I-ICME and M-ICME Fittings and In Situ Observation Parameters for Solar Cycles 23 and 24 and Their Influence on Geoeffectiveness

To understand the weaker geomagnetic activity in Solar Cycle 24, we present comparisons of interplanetary coronal mass ejections (ICMEs) fittings and in situ observation parameters in Solar Cycles 23 and 24. According to their in situ features, ICMEs are separated into two categories: isolated ICMEs (I-ICMEs) and multiple ICMEs (M-ICMEs). The number of I-ICMEs in Solar Cycles 23 and 24 does not show a strong difference, while the number of M-ICMEs, which have a high probability of causing intense geomagnetic storms, declines proportionally to the sunspot number in Solar Cycle 24. Despite no obvious variation in their distribution, the geoeffective ICMEs in Solar Cycle 23 have a larger average total magnetic field strength and a larger southern magnetic field than those of Solar Cycle 24. Since the average solar wind velocities of the two solar cycles differ, the geoeffective ICMEs in Solar Cycle 23 have a higher velocity and distinct speed distributions from those in Solar Cycle 24. The total magnetic flux and radius of I-ICMEs in Solar Cycle 23 are larger than those in Solar Cycle 24, while the axial magnetic field intensity is basically the same. We propose that geomagnetic activity in Solar Cycle 24 is lower than that of Solar Cycle 23, due to the smaller M-ICME number, the slower ICME speed, and absence of ICME events with significant southward magnetic field.