The longitudinal spatial coherence near 1 AU of the magnetic field in sheath regions driven by interplanetary coronal mass ejection (ICME) is studied by investigating ACE and Wind spacecraft measurements of 29 sheaths. During 2000-2002 Wind performed prograde orbits, and the non-radial spacecraft separation varied from 0.001 to 0.012 AU between the studied events. We compare the measurements by computing the Pearson correlation coefficients for the magnetic field magnitude and components and estimate the magnetic field coherence by evaluating the scale lengths that give the extrapolated distance of zero correlation between the measurements. The correlation is also separately examined for low-and high-pass filtered data. We discover magnetic fields in ICME sheaths have scale lengths that are larger than those reported in the solar wind but that, in general, are smaller than the ones of the ICME ejecta. Our results imply that magnetic fields in the sheath are more coherently structured and well correlated compared to the solar wind. The largest sheath coherence is reported in the GSE y-direction that has the scale length of 0.149 AU while the lengths for B x , B z , and |B| vary between 0.024 and 0.035 AU. The same sheath magnitude ordering of scale lengths also apply for the low-pass filtered magnetic field data. We discuss field line draping and the alignment of preexisting discontinuities by the shock passage giving reasoning for the observed results.

• Spatial coherence length of magnetic field in ICME sheaths is larger than in the solar wind and typically smaller than in ICME ejecta. • High frequency fluctuations are localized in ICME sheaths. • Large correlation length for B y is consistent with field line draping and shock deflection.

Sheath regions of interplanetary coronal mass ejections (ICMEs) are formed when the upstream solar wind is deflected and compressed due to the propagation and expansion of the ICME. Small-scale flux ropes found in the solar wind can thus be swept into ICME-driven sheath regions. They may also be generated locally within the sheaths through a range of processes. This work applies wavelet analysis to obtain the normalized reduced magnetic helicity, normalized cross helicity, and normalized residual energy, and uses them to identify small-scale flux ropes and Alfvén waves in 55 ICME-driven sheath regions observed by the Wind spacecraft in the near-Earth solar wind. Their occurrence is investigated separately for three different frequency ranges between 10–2 − 10–4 Hz. We find that small scale flux ropes are more common in ICME sheaths than in the upstream wind, implying that they are at least to some extent actively generated in the sheath and not just compressed from the upstream wind. Alfvén waves occur more evenly in the upstream wind and in the sheath. This study also reveals that while the highest frequency (smallest scale) flux ropes occur relatively evenly across the sheath, the lower frequency (largest scale) flux ropes peak near the ICME leading edge. This suggests that they could have different physical origins, and that processes near the ICME leading edge are important for generating the larger scale population.