tokens: two or more consecutive CMEs originated in the same source region. It is worth to note that the maximum fraction of free energy exhausted by one CME may be 25% ( Gopalswamy et al. 2005 ; Emslie et al. 2012 ; Feng et al. 2013 ). Finally, the speed distributions of all leading/trailing CMEs show a small but systematic tendency to have higher mean and median velocities for shorter WTs. The mean and median speed of the trailing CME are a few km s −1 higher than that of the leading CMEs, for WTs shorter than 7 h, after a WT of 10 h the differences remain constant and negligible. Nevertheless, these differences are higher (tens of km s −1 ) when we only take into account spatially-related CMEs (see Fig. 8 ). As the mean is not the best location indicator for highly skewed distributions, we also analyzed the quartiles (which are better suited to characterizing these distributions), the results are plotted in Fig. 9 . To assess the accuracy of the analysis, we applied the sign test to show that the differences between the medians of trailing and leading CMEs are statistically significant. The phases of the solar cycle where the sign-test probability (with the null hypothesis H 0 : ) is less than 0.1 are shown in Figs. 10 and 11 . These observations may be explained by the drag force models. It is clear that the magnetic field provides the necessary energy to launch and first acceleration of CMEs. Although this energy does not play an important role in the CME dynamics after few solar radii, at least for fast CMEs (such as those that form the tail of the distributions). Recently, Sachdeva et al. (2017) investigated CME dynamics using coronographic observations, and they concluded that for fast CMEs, the Lorentz force has its maximum effect at heights lower than 2.5 R ⊙ , and then, at heights of 3.5–4 R ⊙ becomes negligible compared with the drag force. In the case of slow CMEs, the Lorentz force may be significant at larger distances (up to 50 R ⊙ , but slow CMEs do not contribute to the tail of the distributions). The drag force is proportional to the drag coefficient ( C D ), the transverse area of the CME ( A ), the speed difference between the CME ( V CME ) and the solar wind ( V SW ), and the ambient density ( ρ ) ( Cargill 2004 ; Borgazzi et al. 2009 ; Vršnak et al. 2010 ). In a statistical analysis of a large number of events, the main values of A and V SW are similar for both leading and trailing CMEs. On the other hand, the leading CME may sweep the ambient medium, and the trailing CME encounters a less dense medium and therefore the drag force decreases, resulting in a faster CME. In this case, the shorter the WT, the higher the CME speed. This effect is clearly seen in Figs. 10 and 11 . 10. Conclusions We performed a statistical study of 27 761 CMEs observed by SOHO/LASCO from the ascending phase of cycle 23 to the declining phase of cycle 24, looking for long-range or memory signatures in the genesis and the first stages of evolution of
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