Title: Planar magnetic structures in coronal mass ejection-driven sheath regions,
Abstract: Planar magnetic structures (PMSs) are periods in the solar wind during which
interplanetary magnetic field vectors are nearly parallel to a single plane.
One of the specific regions where PMSs have been reported are coronal mass
ejection (CME)-driven sheaths. We use here an automated method to identify PMSs
in 95 CME sheath regions observed in-situ by the Wind and ACE spacecraft
between 1997 and 2015. The occurrence and location of the PMSs are related to
various shock, sheath and CME properties. We find that PMSs are ubiquitous in
CME sheaths; 85% of the studied sheath regions had PMSs with the mean duration
of 6.0 hours. In about one-third of the cases the magnetic field vectors
followed a single PMS plane that covered a significant part (at least 67%) of
the sheath region. Our analysis gives strong support for two suggested PMS
formation mechanisms: the amplification and alignment of solar wind
discontinuities near the CME-driven shock and the draping of the magnetic field
lines around the CME ejecta. For example, we found that the shock and PMS plane
normals generally coincided for the events where the PMSs occurred near the
shock (68% of the PMS plane normals near the shock were separated by less than
20° from the shock normal), while deviations were clearly larger when PMSs
occurred close to the ejecta leading edge. In addition, PMSs near the shock
were generally associated with lower upstream plasma beta than the cases where
PMSs occurred near the leading edge of the CME. We also demonstrate that the
planar parts of the sheath contain a higher amount of strongly southward
magnetic field than the non-planar parts, suggesting that planar sheaths are
more likely to drive magnetospheric activity.