Using an MHD simulation to interpret the global context of a coronal mass ejection observed by two spacecraft

In late February 1999 the ACE spacecraft observed a coronal mass ejection (CME) at 1 AU, in the ecliptic plane. Thirteen days later, Ulysses observed a CME at 5 AU and 22°S. We present a detailed analysis of the plasma, magnetic field, and composition signatures of these two events. On the basis of this comparison alone, it is not clear that the two spacecraft observed the same solar event. However, using a generic MHD simulation of a fast CME initiated at the Sun by magnetic flux cancellation and propagated out into the solar wind, together with additional evidence, we argue that indeed the same CME was observed by both spacecraft. Although force‐free models appear to fit the observed events well, our simulation results suggest that the ejecta underwent significant distortion during its passage through the solar wind, indicating that care should be taken when interpreting the results of force‐free models. Comparison of composition measurements at the two spacecraft suggests that significant spatial inhomogeneities can exist within a single CME.