The Interaction of Successive Coronal Mass Ejections: A Review (1612.02398v2)

Abstract: We present a review of the different aspects associated with the interaction of successive CMEs in the corona and inner heliosphere, focusing on the initiation of series of CMEs, their interaction in the heliosphere, the particle acceleration associated with successive CMEs, and the effect of compound events on Earth's magnetosphere. The two main mechanisms resulting in the eruption of series of CMEs are sympathetic eruptions and homologous eruptions. The interaction of successive CMEs has been observed remotely in coronagraphs and heliospheric imagers, and inferred from in situ measurements. It is associated with magnetic reconnection, momentum exchange, the propagation of a fast magnetosonic shock through a magnetic ejecta and changes in CME expansion. The presence of a CME a few hours before a fast eruption is connected with higher fluxes of SEPs, while CME-CME interaction occurring in the corona is often associated with unusual radio bursts. Higher suprathermal population, enhanced turbulence and wave activity, stronger shocks, and shock-shock or shock-CME interaction have been proposed as physical mechanisms to explain these SEP events. When measured in situ, CME-CME interaction may be associated with relatively well organized multiple-magnetic cloud events, instances of shocks propagating through a previous magnetic ejecta or more complex ejecta. The compression of a CME by another and the propagation of a shock inside a magnetic ejecta can lead to extreme values of the southward magnetic field, sometimes associated with large values of the dynamic pressure. This can result in intense geomagnetic storms, but also trigger substorms and large earthward motions of the magnetopause, potentially associated with changes in the outer radiation belts. Future measurements by Solar Probe+ and Solar Orbiter may shed light on the evolution of CMEs as they interact.

The Interaction of Successive Coronal Mass Ejections: A Comprehensive Review

The paper undertaken by Lugaz et al. provides a thorough review of the interactions between successive Coronal Mass Ejections (CMEs) and their implications for space weather phenomena. The authors present a detailed survey on the initiation, propagation, and interaction of CMEs in the heliosphere, with significant focus on their impact on Earth's magnetosphere and the acceleration of solar energetic particles (SEPs).

Overview of CME Initiation and Propagation

Central to the understanding of CME interactions are the initiation mechanisms, identified primarily as sympathetic and homologous eruptions. Sympathetic eruptions occur when one solar eruption triggers another, even if spatially separated. Homologous eruptions, however, originate from the same active region and exhibit similar morphological characteristics. The authors highlight the variability in CME initiation rates, noting a substantial increase during solar maximum, providing a basis for frequent interactions of unrelated disturbances in the solar corona.

The CME propagation through the heliosphere is closely examined, with historical observational data from experiments such as LASCO and heliospheric imagers detailing the complex processes involved in CME interactions. Remote solar measurements, supported by in situ data, reveal the nature of interactions which include magnetic reconnection and changes in the CME expansion due to momentum exchange.

Interaction Characteristics and Consequences

The interactions of successive CMEs entail dynamics like fast magnetosonic shock waves propagating through magnetic ejecta, leading to heightened fluxes of SEPs, implicating significant electron acceleration. The interactions also exhibit complex profiling that can lead to phenomena such as unusual radio bursts.

Importantly, Lugaz et al. discuss the possibility of CME-CME interactions leading to heightened geomagnetic storm conditions. The compression of one CME by a following transient event can amplify the intensity of the southward magnetic field component, catalyzing intense geomagnetic storms and modifications to the magnetopause and radiation belts around Earth.

Implications for Particle Acceleration and Radio Phenomena

The paper further explores the implications of CME-CME interactions in particle acceleration. Accelerated particles are crucial for understanding space weather, as the presence of a preceding CME seems to precondition the solar environment, potentially boosting the particles' maximum energy. Interacting CMEs are also linked with intensified radio emissions, especially noting that decreased Alfvén speeds can influence shock strength and reconnection efficiency, which has broader implications for our understanding of space weather conditions.

Geo-effects and Space Weather Forecasting

The complex interactions between successive CMEs are identified as major players in the onset of severe geomagnetic storms, stressing their importance for space weather forecasting. Such interactions are particularly notable during solar maximum events, inducing prolonged periods of geomagnetic storming.

Lugaz et al. underscore the necessity for detailed and continuous measurements, emphasizing the potential contributions of upcoming missions like Solar Probe+ and Solar Orbiter. By providing new data closer to the Sun, these missions will enhance our understanding of CME propagation and interaction dynamics.

Conclusion

This review by Lugaz et al. comprehensively contextualizes our understanding of successive CME interactions in the solar magnetic environment and their broader implications for solar-terrestrial relations. It advances the idea that CME-CME interactions hold critical importance for both theoretical advancements in heliophysics and practical considerations in space weather forecast modeling. This paper effectively sets a foundation for future research in elucidating the mechanisms behind CME interactions and their extensive impacts on the heliosphere and geomagnetic conditions.