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We report radial speed evolution of interplanetary coronal mass ejections(ICMEs) detected by the SOHO/LASCO coronagraph, interplanetary scintillation(IPS) at 327 MHz, and in-situ observations. In this study, we analyze solarwind disturbance factor (g-value) data derived from IPS observations during1997-2009 covering nearly the whole period of Solar Cycle 23. By comparingobservations from the SOHO/LASCO, IPS, and in situ, we then identify 39 ICMEsthat could be analyzed carefully. Here, we define two speeds V_SOHO and V_bg,which are initial speed of ICME and the speed of the background solar wind,respectively. Examinations of these speeds yield the following results; i) FastICMEs (with V_SOHO - V_bg > 500 km/s) rapidly decelerate, moderate ICMEs (with0 km/s < V_SOHO - V_bg < 500 km/s) show either gradually decelerating oruniform motion, and slow ICMEs (with V_SOHO - V_bg < 0 km/s) accelerate. Theradial speeds converge on the speed of background solar wind during theiroutward propagation. We subsequently find; ii) both the acceleration anddeceleration are nearly complete by 0.79 (+/- 0.04) AU, and those are endedwhen the ICME speed reaches a given speed. We find the value of that to be 480(+/- 21) km/s. iii) For the fast and moderate ICMEs, a linear equation with aconstant coefficient is more appropriate than a quadratic equation to describetheir kinematics, because the chi-square for the linear equation satisfies thestatistical significance level of 0.05, while the quadratic one is not. Theseresults support the hypothesis that the radial motion of ICMEs is governed by adrag force due to interaction with the background solar wind. These findingsalso suggest that ICMEs propagating faster than background solar wind arecontrolled mainly by the hydrodynamic Stokes drag.

Radial Speed Evolution of Interplanetary Coronal Mass Ejections During Solar Cycle 23