Kinematic Properties of Slow ICMEs and an Interpretation of a Modified Drag Equation for Fast and Moderate ICMEs

Abstract

We report kinematic properties of slow interplanetary coronal mass ejections (ICMEs) identified by SOHO/LASCO, interplanetary scintillation, and in situ observations and propose a modified equation for the ICME motion. We identified seven ICMEs between 2010 and 2011 and compared them with 39 events reported in our previous work. We examined 15 fast (V _SOHO−V _bg>500 km s⁻¹), 25 moderate (0 km s⁻¹≤V _SOHO−V _bg≤500 km s⁻¹), and 6 slow (V _SOHO−V _bg<0 km s⁻¹) ICMEs, where V _SOHO and V _bg are the initial speed of ICMEs and the speed of the background solar wind. For slow ICMEs, we found the following results: i) They accelerate toward the speed of the background solar wind during their propagation and reach their final speed by 0.34±0.03 AU. ii) The acceleration ends when they reach 479±126 km s⁻¹; this is close to the typical speed of the solar wind during the period of this study. iii) When γ ₁ and γ ₂ are assumed to be constants, a quadratic equation for the acceleration a=−γ ₂(V−V _bg)|V−V _bg| is more appropriate than a linear one a=−γ ₁(V−V _bg), where V is the propagation speed of ICMEs, while the latter gives a smaller χ ² value than the former. For the motion of the fast and moderate ICMEs, we found a modified drag equation a=−2.07×10⁻¹²(V−V _bg)|V−V _bg|−4.84×10⁻⁶(V−V _bg). From the viewpoint of fluid dynamics, we interpret this equation as indicating that ICMEs with 0 km s⁻¹≤V−V _bg≤2300 km s⁻¹ are controlled mainly by the hydrodynamic Stokes drag force, while the aerodynamic drag force is a predominant factor for the propagation of ICME with V−V _bg>2300 km s⁻¹.