Characteristic scales of complexity and coherence within interplanetary coronal mass ejections: Insights from spacecraft swarms in global heliospheric simulations

Many aspects of the 3D structure and evolution of interplanetary coronal mass ejections (ICMEs) remain unexplained. Here, we investigate two main topics: (1) the coherence scale of magnetic fields inside ICMEs, and (2) the dynamic nature of ICME magnetic complexity. We simulate ICMEs interacting with different solar winds using the linear force-free spheromak model incorporated into the EUHFORIA model. We place a swarm of similar to 20,000 spacecraft in the 3D simulation domain and characterize ICME magnetic complexity and coherence at each spacecraft based on the simulated time series. Our simulations suggest that ICMEs retain a lower complexity and higher coherence along their magnetic axis, but that a characterization of their global complexity requires crossings along both the axial and perpendicular directions. For an ICME of initial half angular width of 45 degrees that does not interact with other large-scale solar wind structures, global complexity can be characterized by as little as 7-12 spacecraft separated by 25 degrees, but the minimum number of spacecraft rises to 50-65 (separated by 10 degrees) if interactions occur. Without interactions, ICME coherence extends for 45 degrees, 20 degrees-30 degrees, 15 degrees-30 degrees, and 0 degrees-10 degrees for B, B ( phi ), B ( theta ), and B ( r ), respectively. Coherence is also lower in the ICME west flank compared to the east flank due to Parker spiral effects. Moreover, coherence is reduced by a factor of 3-6 by interactions with solar wind structures. Our findings help constrain some of the critical scales that control the evolution of ICMEs and aid in the planning of future dedicated multispacecraft missions.