Determine the Onset Height of the CME–CME Interaction

Determine the heliocentric height at which the interaction between CME1 and CME2 began, including whether the interaction started when CME2’s leading edge reached CME1’s trailing edge or earlier when the CME2-driven shock encountered CME1.

Background

The event involved two successive CMEs launched from the same active region in close succession. White-light observations show that CME2 was already propagating within the central portion of CME1 at approximately 2–3 solar radii, indicating that their interaction had begun by that time.

The available coronagraph observations do not resolve the precise onset of the interaction. The interaction may have begun below the inner boundary of the coronagraph field of view, and data gaps in the STEREO-A/COR2 observations further limit the ability to locate the onset height.

References

The interaction could therefore have begun either when the leading edge of CME2 reached the trailing edge of CME1 or earlier, when the shock driven by CME2 first encountered the trailing edge of CME1. However, the exact height at which the interaction commenced cannot be determined from the available observations, as the onset of the interaction may have occurred below the inner boundary of the coronagraph field of view. In addition, data gaps in the STEREO-A/COR2 observations further limit our ability to determine the precise interaction height.

— Evidence for CME--CME Interaction in a Magnetic-Cloud-Like Ejecta: Insights from Multipoint Observations and Polytropic Analysis  (2609.11554 - Sheoran et al., 10 Sep 2026) in Section 2.2, “Near-Sun EUV and White-Light Observations”