On the Aerodynamic Drag Force Acting on Interplanetary Coronal Mass Ejections

It is well known that the interaction of an interplanetary coronal mass ejection (ICME) with the solar wind leads to an equalisation of the ICME and solar wind velocities at 1 AU. This can be understood in terms of an aerodynamic drag force per unit mass of the form FD/M=−(ρeACD/M)(Vi−Ve)∣Vi−Ve∣, where A and M are the ICME cross-section and sum of the mass and virtual mass, Vi and Ve the speed of the ICME and solar wind, ρe the solar wind density, CD a dimensionless drag coefficient, and the inverse deceleration length γ=ρeA/M. The optimal radial parameterisation of γ and CD beyond approximately 15 solar radii is calculated. Magnetohydrodynamic simulations show that for dense ICMEs, CD varies slowly between the Sun and 1 AU, and is of order unity. When the ICME and solar wind densities are similar, CD is larger (between 3 and 10), but remains approximately constant with radial distance. For tenuous ICMEs, the ICME and solar wind velocities equalise rapidly due to the very effective drag force. For ICMEs denser that the ambient solar wind, both approaches show that γ is approximately independent of radius, while for tenuous ICMEs, γ falls off linearly with distance. When the ICME density is similar to or less than that in the solar wind, inclusion of virtual mass effects is essential.