Coronal mass ejection propagation in the dynamically coupled space weather tool: COCONUT + EUHFORIA

Context. Numerical magnetohydrodynamics (MHD) models such as the European heliospheric forecasting information asset (EUHFORIA) have been developed to predict the arrival time of coronal mass ejections (CMEs) and accelerated high-energy particles. However, in EUHFORIA, transient magnetic structures are injected at 0.1 AU into a background solar wind created from a static solar wind model. This means the inserted CME model is completely independent of the coronal magnetic field and thus is missing all potential interactions between the CME and the solar wind in the corona. Aims. This paper aims to present the time-dependent coupling between the coronal model COolfluid COroNal UnsTructured (COCONUT) and the heliospheric forecasting tool EUHFORIA. This first attempt to couple these two simulations should allow us to follow directly the propagation of a flux rope from the Sun to Earth. Methods. We performed six COCONUT simulations where a flux rope is implemented at the solar surface using either the Titov-Démoulin CME model or the regularised Biot-Savart law (RBSL) CME model. At regular intervals, the magnetic field, velocity, temperature, and density of the 2D surface R b = 21.5 R ⊙ were saved in boundary files. These series of coupling files were read in a modified version of EUHFORIA in order to progressively update its inner boundary. After presenting the early stage of the propagation in COCONUT, we examined how the disturbance of the solar corona created by the propagation of flux ropes is transmitted into EUHFORIA. In particular, we considered the thermodynamic and magnetic profiles at L1 and compared them with those obtained at the interface between the two models. Results. We demonstrate that the properties of the heliospheric solar wind in EUHFORIA are consistent with those in COCONUT, acting as a direct extension of the coronal domain. Moreover, the disturbances initially created from the propagation of flux ropes in COCONUT continue to evolve from the corona in the heliosphere to Earth, with a smooth transition at the interface between the two simulations. Looking at the profile of magnetic field components at Earth and different distances from the Sun, we also find that the transient magnetic structures have a self-similar expansion in COCONUT and EUHFORIA. However, the amplitude of the profiles depends on the flux rope model used and its properties, thus emphasising the important role of the initial properties in solar source regions for accurately predicting the impact of CMEs. Conclusions. The dynamically coupled COCONUT plus EUHFORIA model chain constitutes a new space weather forecasting tool that can predict the characteristics of the flux-rope CMEs upon their arrival at L1.