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We study the clustering properties of fast Coronal Mass Ejections (CMEs) thatoccurred during solar cycles 23 and 24. We apply two methods: the Max spectrummethod can detect the predominant clusters and the de-clustering threshold timemethod provides details on the typical clustering properties and time scales.Our analysis shows that during the different phases of solar cycles 23 and 24,CMEs with speed $\geq 1000\ km/s$ preferentially occur as isolated events andin clusters with on average two members. However, clusters with more membersappear particularly during the maximum phases of the solar cycles. Over thetotal period and in the maximum phases of solar cycles 23 and 24, about 50% areisolated events, 18% (12%) occur in clusters with 2 (3) members, and another20% in larger clusters $\geq 4$, whereas in solar minimum fast CMEs tend tooccur more frequently as isolated events (62%). During different solar cyclephases, the typical de-clustering time scales of fast CMEs are $\tau_c=28-32\hrs$, irrespective of the very different occurrence frequencies of CMEs duringsolar minimum and maximum. These findings suggest that $\tau_c$ for extremeevents may reflect the characteristic energy build-up time for large flare andCME-prolific active ARs. Associating statistically the clustering properties offast CMEs with the Disturbance storm index Dst at Earth suggests that fast CMEsoccuring in clusters tend to produce larger geomagnetic storms than isolatedfast CMEs. This may be related to CME-CME interaction producing a more complexand stronger interaction with the Earth magnetosphere.

Clustering of fast Coronal Mass Ejections during the solar cycles 23 and 24 and implications for CME-CME interactions