Over-expansion of coronal mass ejections modelled using 3D MHD EUHFORIA simulations

Abstract

Context: Coronal mass ejections (CMEs) are large-scale eruptions observed close to the Sun. They travel through the heliosphere and possibly interact with the Earth environment, creating interruptions or even damaging new-technology instruments. Most of the time their physical conditions (velocity, density and pressure) are measured in situ at only one point in space, with no possibility of having information on the variation of these parameters during their journey from the Sun to Earth. Aim: Our aim is to understand the evolution of the internal physical parameters of a set of three particular fast halo CMEs. These CMEs were launched between 15 and 18 July 2002. Surprisingly, the related interplanetary CMEs (ICMEs), observed near Earth, have a low, and in one case a very low, plasma density. Method: We use the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) model to simulate the propagation of the CMEs in the background solar wind by placing virtual spacecraft along the Sun–Earth line. We set up the initial conditions at 0.1 au, first with a cone model and then with a linear force-free spheromak model. Results: Relatively good agreement between the simulation results and observations concerning the speed, density and arrival times of the ICMEs is obtained by adjustment of the initial CME parameters. In particular, this is achieved by increasing the initial magnetic pressure so that a fast expansion is induced in the inner heliosphere. This resulted in the development of fast expansion for two of the three ICMEs. In contrast, the intermediate ICME is strongly overtaken by the last ICME, so its expansion is strongly limited. Conclusions: First, we show that a magnetic configuration with an out-of-force balance close to the Sun mitigates the EUHFORIA assumptions related to an initial uniform velocity. Second, the overexpansion of the ejected magnetic configuration in the inner heliosphere is one plausible origin for the low density observed in some ICMEs at 1 au. Furthermore, we conclude for one ICME, surrounded by two other ICMEs, that the in situ observed very low density has a possible coronal origin.