CME-CME Interactions as Sources of CME Helio-Effectiveness: the Early September 2017 Events

Coronal Mass Ejections (CMEs) are the main source of intense space weather disturbances in the heliosphere. It is known that the capability of individual CMEs to drive strong space weather events at Earth (called "geo-effectiveness") and other locations (here referred to as "helio-effectiveness") primarily depends on their speed, density, and magnetic field strength and orientation at the impact location. Moreover, previous studies established that CME--CME interactions can significantly alter the properties of individual CMEs, in such a way that their geo-effectiveness is often dramatically amplified. However, the actual quantification of this amplification has been rarely investigated, and previous studies have mostly focused on the near-Earth region only, i.e. without considering its full space-time evolution as the CMEs propagate to 1 AU and beyond.

Here, we present a study on the role of CME--CME interactions as sources of CME helio-effectiveness by performing simulations of complex CME events with the EUHFORIA heliospheric model. As a case study, we consider a sequence of CMEs observed in early September 2017. As their source region rotated on the solar disk, CMEs were launched over a wide range of longitudes, interacting with each other and paving the way for the propagation of the following ones. At Earth, their interaction resulted in an intense geomagnetic storm. Using initial parameters derived from remote-sensing observations, we perform global simulations of magnetised CMEs with EUHFORIA, investigating how their interactions affected the propagation and internal properties of individual CME structures. Taking advantage of 3D simulation outputs, we quantify the amplification of the helio-effectiveness of the individual CMEs involved, as a function of the interaction phase and of the location within the CME structure. Additionally, we explore the possibility of the existence of a "helio-effectiveness amplification zone", i.e. a characteristic heliocentric distance at which CME--CME interactions have the highest probability to develop into helio-effective events. Results from this study benchmark our current prediction capabilities in the case of complex CME events, and provide new insights on their large-scale evolution and potential impact throughout the heliosphere.