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

Abstract

We report radial-speed evolution of interplanetary coronal mass ejections (ICMEs) detected by the Large Angle and Spectrometric Coronagraph onboard the Solar and Heliospheric Observatory (SOHO/LASCO), interplanetary scintillation (IPS) at 327 MHz, and in-situ observations. We analyze solar-wind disturbance factor (g-value) data derived from IPS observations during 1997 – 2009 covering nearly the whole period of Solar Cycle 23. By comparing observations from SOHO/LASCO, IPS, and in situ, we identify 39 ICMEs that could be analyzed carefully. Here, we define two speeds [V _SOHO and V _bg], which are the initial speed of the ICME and the speed of the background solar wind, respectively. Examinations of these speeds yield the following results: i) Fast ICMEs (with V _SOHO−V _bg>500 km s⁻¹) rapidly decelerate, moderate ICMEs (with 0 km s⁻¹≤V _SOHO−V _bg≤500 km s⁻¹) show either gradually decelerating or uniform motion, and slow ICMEs (with V _SOHO−V _bg<0 km s⁻¹) accelerate. The radial speeds converge on the speed of the background solar wind during their outward propagation. We subsequently find; ii) both the acceleration and the deceleration are nearly complete by 0.79±0.04 AU, and those are ended when the ICMEs reach a 480±21 km s⁻¹. iii) For ICMEs with (V _SOHO−V _bg)≥0 km s⁻¹, i.e. fast and moderate ICMEs, a linear equation a=−γ ₁(V−V _bg) with γ ₁=6.58±0.23×10⁻⁶ s⁻¹ is more appropriate than a quadratic equation a=−γ ₂(V−V _bg)|V−V _bg| to describe their kinematics, where γ ₁ and γ ₂ are coefficients, and a and V are the acceleration and speed of ICMEs, respectively, because the χ ² for the linear equation satisfies the statistical significance level of 0.05, while the quadratic one does not. These results support the assumption that the radial motion of ICMEs is governed by a drag force due to interaction with the background solar wind. These findings also suggest that ICMEs propagating faster than the background solar wind are controlled mainly by the hydrodynamic Stokes drag.