CME-CME Interactions as Sources of CME Geo-effectiveness: The Formation of the Complex Ejecta and Intense Geomagnetic Storm in Early September 2017 (1911.10817v1)

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 Bz 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 Bz 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 Bz 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).

• The paper demonstrates that interactions between successive CMEs amplify magnetic fields, enhancing their geo-effectiveness.
• It employs advanced 3D MHD simulations using the EUHFORIA model to replicate and predict the merging dynamics of CMEs.
• The study shows that the relative timing and speed between CMEs are critical factors in intensifying geomagnetic storms, evidenced by a peak Dst index of -142 nT.

Overview of CME--CME Interactions as Sources of CME Geo-effectiveness

The paper by Scolini et al. investigates the interactions between coronal mass ejections (CMEs) and their impact on geo-effectiveness, focusing on events from early September 2017. The paper aims to understand how interactions between successive CMEs can enhance their geo-effectiveness, contributing to intense geomagnetic storms on Earth. By analyzing three CMEs that occurred from September 4–6, 2017, the paper presents insights into how CME interactions impact their magnetic field structures and dynamics using both observational data and simulations.

Key Findings

1. CME Interactions in Interplanetary Space:
   • The paper explores the interactions between a set of three CMEs that merged and propagated through interplanetary space as a complex ejecta. Using data from remote solar observations and in-situ spacecraft measurements, it illustrates that CME interactions can amplify the magnetic field components, significantly altering their geo-effectiveness.
   • The interaction of CMEs can lead to an enhanced southward magnetic field component, which is crucial in driving geomagnetic storms when such fields encounter the Earth's magnetosphere.
2. Simulation of CME Propagation:
   • Employing the EUHFORIA model, an advanced 3D MHD heliospheric simulation, the paper conducts simulations to replicate the observed events. The simulations incorporate observation-based initial CME parameters to evaluate the predictive capabilities of the model.
   • The model successfully captures the merging of CMEs and predicts their subsequent arrival at Earth, providing insights into the amplifications of the magnetic field observed in spacecraft data.
3. Geo-effectiveness and Helio-effectiveness:
   • The interaction stages among CMEs are characterized by distinct phases: growth, peak, and decay of the amplified magnetic field strength. Notably, the phase of maximum amplification coinciding with the CME's arrival at Earth played a crucial role in the intensity of the geomagnetic storm on September 7, 2017, exemplified by a Dst index reaching -142 nT.
4. Impact of Relative Speed and Timing:
   • The paper emphasizes the importance of the timing between successive CME eruptions and their relative speeds in determining the evolution and eventual geo-effectiveness of the merged CME structures. Earlier studies suggested that variations in these parameters significantly influence interplanetary interactions and resultant storm strengths.

Implications and Future Directions

The research by Scolini et al. advances the understanding of how CME interactions enhance geo-effectiveness, suggesting that predictions for space weather events require consideration of these interaction phenomena. The results have both theoretical significance and practical applications, particularly in space weather forecasting and protecting Earth-based technologies from geomagnetic disturbances.

The authors propose that future studies explore the parameter space of CME interactions more thoroughly, potentially leading to a predictive model identifying regions in space with increased likelihood for helio-effective CME interactions. Future work is also poised to explore the angular dependencies of CME interactions, considering the CME path with respect to observing spacecraft or planetary bodies.

In conclusion, the paper provides a significant advancement in understanding CME interactions, offering a mesoscale comprehension of helio-effectiveness amplification processes and their profound effects on space weather phenomena. These findings will inform both future research directions and operational forecasting efforts, serving as a foundation for enhanced predictive models in solar-terrestrial relations.