On the importance of investigating CME complexity evolution during interplanetary propagation

This white paper brings to light the need for comprehensive studies on the evolution of interplanetary coronal mass ejection (ICME) complexity during propagation. To date, few studies of ICME complexity exist. In this white paper, we define ICME complexity and accompanying changes in complexity, describe recent works and their limitations, and outline key science questions that need to be tackled. In order to be able to address these science questions from a fundamental physics perspective, our main recommendations for the Decadal Survey Committee for Solar & Space Physics (Heliophysics) 2024-2033 are: * Prioritize spacecraft missions that can obtain comprehensive in situ ICME measurements at multiple heliocentric distances in the inner heliosphere. * Explore options, through simulations, for optimal spacecraft configuration in terms of radial, longitudinal, and latitudinal spacing to achieve the necessary spatial resolution to fully investigate ICME complexity evolution. * Once the data are available, focus efforts on comprehensive/holistic studies of ICME complexity evolution, integrating the results from many vantage points. Fundamental research on ICME complexity changes from the solar corona to 1 AU and beyond is critical to our physical understanding of the evolution and interaction of transients in the inner heliosphere. Furthermore, a complete understanding of such changes is required to understand the space weather impact of ICMEs at different planets.