Abstract
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 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 larger scale
lengths in ICME sheaths than those reported for the solar wind but, in
general, smaller than for 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 -direction that has the scale length of 0.149 AU
while the lengths for Bx, Bz, 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 pre-existing discontinuities by
the shock passage giving reasoning for observed results.