CME-CME Interactions as Sources of CME Geoeffectiveness: The Formation of the Complex Ejecta and Intense Geomagnetic Storm in 2017 Early September

Coronal mass ejections (CMEs) are the primary sources of intense disturbances at Earth, where their geoeffectiveness is largely determined by their dynamic pressure and internal magnetic field, which can be significantly altered during interactions with other CMEs in interplanetary space. We analyze three successive CMEs that erupted from the Sun during 2017 September 4–6, investigating the role of CME–CME interactions as a source of the associated intense geomagnetic storm (${\mathrm{Dst}}_{\min }=-142$ nT on September 7). To quantify the impact of interactions on the (geo)effectiveness of individual CMEs, we perform global heliospheric simulations with the European Heliospheric Forecasting Information Asset (EUHFORIA) model, using observation-based initial parameters with the additional purpose of validating the predictive capabilities of the model for complex CME events. The simulations show that around 0.45 au, the shock driven by the September 6 CME started compressing a preceding magnetic ejecta formed by the merging of two CMEs launched on September 4, significantly amplifying its B z until a maximum factor of 2.8 around 0.9 au. The following gradual conversion of magnetic energy into kinetic and thermal components reduced the B z amplification until its almost complete disappearance around 1.8 au. We conclude that a key factor at the origin of the intense storm triggered by the 2017 September 4–6 CMEs was their arrival at Earth during the phase of maximum B z amplification. Our analysis highlights how the amplification of the magnetic field of individual CMEs in spacetime due to interaction processes can be characterized by a growth, a maximum, and a decay phase, suggesting that the time interval between the CME eruptions and their relative speeds are critical factors in determining the resulting impact of complex CMEs at various heliocentric distances (helioeffectiveness).status: publishe