Modelling CME–High-Speed Stream Interactions Using a Novel Flux-Rope Model

One of the major challenges in space weather forecasting is the reliable prediction of the magnetic structure of interplanetary coronal mass ejections (ICMEs) in near-Earth space. This challenge becomes even more pronounced when a CME interacts with high-speed streams (HSSs) or other CMEs during its interplanetary evolution. Within the framework of global MHD modeling, several efforts have been made to simulate the CME magnetic field from the Sun to Earth. However, it remains difficult to deduce a flux-rope solution that can robustly reproduce the magnetic structure of CMEs. Moreover, a comprehensive understanding of how CME–HSS interactions lead to enhanced space weather impacts of CMEs and their associated sheath regions is still lacking.

In this work, we implement a new flux-rope model in the European Heliospheric Forecast Information Asset (EUHFORIA), featuring an initially force-free toroidal flux rope embedded in the low-coronal magnetic field. The novel embedding technique self-consistently generates a draping field around the flux rope, preserving the normal component of the magnetic field at the flux-rope boundary. The flux-rope dynamics in the low and middle corona are solved using a non-uniform advection constrained by the observed kinematics of the CME. This produces a global, non-toroidal, stretched loop-like magnetic structure, in which the lower half of the torus remains below the inner boundary of the heliospheric model. At heliospheric distances, the subsequent evolution is modeled as an MHD process using EUHFORIA, yielding a classical flux-rope geometry consistent with observations of bi-directional electrons.

We further investigate CME–HSS interactions using this modeling framework by constructing synthetic high-speed streams and studying their interaction with CMEs of varying kinematics. Our results show that CME–HSS interactions lead to significant deformation of the CME magnetic structure. We find that the relative speed between the CME and the HSS plays a decisive role in determining the degree of ICME compression and the resulting enhancement of its space weather impact.