engleski

CME-CME interactions as sources of CME helio-effectiveness: the early September 2017 events

Coronal Mass Ejections (CMEs) are the primary source of strong space weather disturbances at Earth and other locations in the heliosphere. Understanding the physical processes involved in their formation at the Sun, propagation in the heliosphere, and impact on planetary bodies is therefore critical to improve current space weather predictions throughout the heliosphere. It is known that the capability of individual CMEs to drive strong space weather disturbances at Earth (known as "geo-effectiveness") and other locations in the heliosphere (here referred to as "helio- effectiveness") primarily depends on their dynamic pressure, internal magnetic field strength, and magnetic field orientation at the impact location. At the same time, observational and modelling studies also 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, mostly via observational studies of individual events, or via explorative studies performed using idealized simulations of CME events, for which no truthful comparison with observations is possible. Additionally, the amplification effect of CME-CME interactions has been traditionally quantified only for the near-Earth region of space, without considering its full space-time evolution as the CMEs propagate to the Earth and beyond. In this work, we present a comprehensive 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 the sequence of CMEs observed during the unusually active week of 4-10 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. CME signatures were observed at Mars and Earth, where an intense geomagnetic storm triggered by CME-CME interactions was recorded. Using input parameters derived from remote-sensing multi-spacecraft observations of the CMEs and their source region, we perform global simulations of magnetised CMEs with EUHFORIA. We investigate how their interactions affected the propagation and internal properties of individual CME structures, and their in-situ signatures at Earth and Mars. 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 also 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 highly helio-effective events. Results from this study benchmark our current prediction capabilities in the case of complex CME events, and provide insights on their large-scale evolution and potential impact throughout the heliosphere.

coronal mass ejections, MHD simulations, space weather

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