  The sheaths of compressed solar wind that precede interplanetary coronal mass
ejections (ICMEs) commonly display large-amplitude magnetic field fluctuations.
As ICMEs propagate radially from the Sun, the properties of these fluctuations
may evolve significantly. We have analyzed magnetic field fluctuations in an
ICME sheath observed by MESSENGER at 0.47 au and subsequently by STEREO-B at
1.08 au while the spacecraft were close to radial alignment. Radial changes in
fluctuation amplitude, compressibility, inertial-range spectral slope,
permutation entropy, Jensen-Shannon complexity, and planar structuring are
characterized. These changes are discussed in relation to the evolving
turbulent properties of the upstream solar wind, the shock bounding the front
of the sheath changing from a quasi-parallel to quasi-perpendicular geometry,
and the development of complex structures in the sheath plasma.
