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

Tomoya Iju   Submitted: 2013-04-01 04:14

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. 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 the SOHO/LASCO, IPS, and in situ, we then identify 39 ICMEs that could be analyzed carefully. Here, we define two speeds VSOHO and Vbg, which are initial speed of ICME and the speed of the background solar wind, respectively. Examinations of these speeds yield the following results; i) Fast ICMEs (with VSOHO - Vbg > 500 km s-1) rapidly decelerate, moderate ICMEs (with 0 km s-1 < VSOHO - Vbg < 500 km s-1) show either gradually decelerating or uniform motion, and slow ICMEs (with VSOHO - Vbg < 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 deceleration are nearly complete by 0.79 (+- 0.04) AU, and those are ended when the ICME speed reaches a 480 (+- 21) km s-1. iii) For the fast and moderate ICMEs, a linear equation with a constant coefficient is more appropriate than a quadratic equation to describe their kinematics, because the chi-square 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.

Authors: Iju, T., M. Tokumaru, and K. Fujiki
Projects: Other

Publication Status: Solar Physics (in press)
Last Modified: 2013-04-01 10:29
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Radial Speed Evolution of Interplanetary Coronal Mass Ejections during Solar Cycle 23

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