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

Abstract

Coronal mass ejections (CMEs) are the primary sources of intense disturbances at Earth, where their geo-effectiveness 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 analyse three successive CMEs that erupted from the Sun during September 4-6, 2017, investigating the role of CME-CME interactions as source of the associated intense geomagnetic storm (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 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 September 4-6, 2017 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 space-time due to interaction processes can be characterised 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 (helio-effectiveness).

Estimating the Magnetic Structure of an Erupting CME Flux Rope From AR12158 Using Data-Driven Modeling

“…The intrinsic flux rope properties can give early warning of the potential space weather consequences, but most importantly it provides critical information for constraining flux ropes in a variety of semiempirical and first-principle models describing the propagation and evolution of CMEs in the corona and heliosphere, such as ForeCAT and FIDO (Kay et al, 2013(Kay et al, , 2017, 3DCORE (Möstl et al, 2018), INFROS (Sarkar et al, 2020), Enlil (Odstrcil et al, 2004), EUropean Heliospheric FORecasting Information Asset (EUHFORIA; Pomoell and Poedts, 2018), and SUSANOO-CME (Shiota and Kataoka, 2016). Although first-principle models are so-far routinely run with only cone-model CMEs for space weather forecasting purposes, for example EUHFORIA is now actively tested with magnetized CMEs to give improved predictions and more realistic information on the effect of CME interactions (Scolini et al, 2019(Scolini et al, , 2020Verbeke et al, 2019). The intrinsic magnetic field structure of a CME flux rope can be estimated using indirect observational proxies that combine characteristics of structures in the solar atmosphere related to the erupting CME, such as filament details, flare ribbons, and sigmoids (e.g., Palmerio et al, 2017Palmerio et al, , 2018Gopalswamy et al, 2018, and references therein).…”

Over-expansion of a coronal mass ejection generates sub-Alfvénic plasma conditions in the solar wind at Earth

“…On the other hand, it is entirely possible that ICME2 had a shock before reaching ACE, since the upstream conditions were different. In that context, this shock may have propagated through ICME1 and may even have merged with the shock in front of ICME1 (see Lugaz et al 2005;Scolini et al 2020b, for more details on shock magnetic cloud interactions).…”

Geoeffectiveness Prediction of CMEs

“…It was shown that interacting CMEs in the heliosphere amplify the geomagnetic response (Scolini et al, 2020). This amplification could be due to shock compression inside interplanetary CMEs (ICMEs) (see e.g., Shen et al, 2018;Xu et al, 2019), due to the generation of high energy protons (see e.g., Joshi et al, 2013) or due to the heights above the photosphere at which the shocks are formed (see e.g., Gopalswamy et al, 2013).…”