On Modeling ICME Cross-Sections as Static MHD Columns

Solar coronal mass ejections are well-known to expand as they propagate through the heliosphere. Despite this, their cross-sections usually modeled static plasma columns within magnetohydrodynamics (MHD) framework. We test validity of this approach using in-situ data from 151 magnetic clouds (MCs) observed by WIND spacecraft and 45 Helios spacecraft. find that most probable cross-section expansion speeds for events only $\approx 0.06$ times Alfvén speed inside MCs, while is 0.03$ . MC can thus be considered nearly over an crossing timescale. Using estimates electrical conductivity arising Coulomb collisions, we Lundquist number MCs high ( 10^{13}$ ), suggesting MHD description well justified. The Joule heating rates our several orders magnitude lower than requirement near Earth. While (low) compute consistent with description, discrepancy points possible departures need a better understanding in MCs.