  Many aspects of the three-dimensional (3-D) 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
~20000 spacecraft in the 3-D simulation domain and characterize ICME magnetic
complexity and coherence at each spacecraft based on 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 $45^\circ$ 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 $25^\circ$,
but the minimum number of spacecraft rises to 50-65 (separated by $10^\circ$)
if interactions occur. Without interactions, ICME coherence extends for
$45^\circ$, $20^\circ$-$30^\circ$, $15^\circ$-$30^\circ$, and
$0^\circ$-$10^\circ$ 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 multi-spacecraft missions.
