We investigate the cyclic evolutionary behavior of CME accelerations for accelerating and decelerating CME events in cycle 23 from 1997 January to 2007 December. It is found that the absolute values of semiannual mean accelerations of both accelerating and decelerating CME events roughly wax and wane in a cycle,delaying the sunspot cycle in time phase.We also investigate the semiannual number of CMEs with positive and negative acceleration and find that there are more decelerating CME events than accelerating CME events during the maximum period of a cycle(about three years),but there are more accelerating CME events than decelerating CME events during the rest of the time interval of the cycle.Our results seem to suggest that the different driving mechanisms may be acting accelerate and decelerate CME events:for accelerating CME events,the propelling force(Fp)statistically seems to play a significant role in pushing CMEs outward; for decelerating CME events,the drag(Fd)statistically seems to play a more effective role in determining CME kinematic evolution in the outer corona.During the maximum period of a cycle,because of the V 2 dependence,F dis generally stronger;because of the magnetic field dependence,Fp is also generally stronger.Thus,the absolute values of both the negative and positive accelerations are generally larger during that time.Because of the V 2 dependence,F d may be more effective during the maximum period of a cycle. Hence,there are more decelerating CME events than accelerating CME events during that time.During the minimum time interval of a cycle,CMEs have relatively small speeds,and Fp may be more effective.Therefore,there are more accelerating CME events than decelerating CME events during that time.
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