Modelling CME rotation during propagation in the heliosphere with EUHFORIA

Coronal mass ejections (CMEs) are large scale magnetized plasma eruptions from the Sun that propagate to Earth and cause disruptions in space and ground-based technologies. While propagating through the heliosphere, they undergo interaction with other CMEs, as well as structures in the solar wind like high-speed streams, and co-rotating/stream interaction regions. We present a case-study of two Earth-directed interacting CMEs that erupted from the Sun on September 8, 2014, and September 10, 2014, respectively. While the first CME was a side hit, it is the second CME which is the focus of this study. With remote observations of the CME helicity and tilt, the second CME was predicted to be geoeffective. However, a mismatch in the tilt of the second CME was observed close to Earth, pointing to CME rotation during its propagation. Unexpectedly, the ejecta resulted in positive Bz but a geoeffective sheath was developed during the evolution and the interaction in the heliosphere that resulted in a minimum Dst ~ -100nT at Earth. Hence, the geoeffectiveness of the various sub-structures involved in this event was mispredicted. 

It is challenging to capture the complete picture of the CME and solar wind dynamics with in-situ observations taken at sparse localized points in the heliosphere. Therefore, we perform 3D MHD simulations that provide a global picture, making it convenient to probe into the interesting phenomena of this event. With the EUropean Heliosphere FORecasting Information Asset (EUHFORIA), we model the background solar wind in 3D, launch the flux rope CMEs in it and let the CME evolve till Earth. In this work, we aim to reproduce the observed plasma and magnetic field properties, especially the negative Bz of the sheath and the positive Bz of the ejecta at Earth. We address the possible factors and processes responsible for the development of geoeffectiveness from the CME rotation, the interplay of the two CMEs and the heliosphere. 

This research has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870405 (EUHFORIA 2.0)