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Annales Geophysicae An open-access journal of the European Geosciences Union
Ann. Geophys., 34, 313-322, 2016
https://doi.org/10.5194/angeo-34-313-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Regular paper
29 Feb 2016
Planar magnetic structures in coronal mass ejection-driven sheath regions
Erika Palmerio1, Emilia K. J. Kilpua1, and Neel P. Savani2,3 1University of Helsinki, Department of Physics, P.O. Box 64, 00014 Helsinki, Finland
2Goddard Planetary Heliophysics Institute (GPHI), University of Maryland, Baltimore County, Maryland, USA
3NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
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 h. 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 strong southward magnetic field than the non-planar parts, suggesting that planar sheaths are more likely to drive magnetospheric activity.

Citation: Palmerio, E., Kilpua, E. K. J., and Savani, N. P.: Planar magnetic structures in coronal mass ejection-driven sheath regions, Ann. Geophys., 34, 313-322, https://doi.org/10.5194/angeo-34-313-2016, 2016.
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Short summary
Coronal Mass Ejections (CMEs) are giant clouds of plasma and magnetic field that erupt from the Sun and travel though the solar wind. They can cause interplanetary shocks in the vicinity of Earth. We show in our paper that the region that follows CME-driven shocks, known as sheath region, can obtain a planar configuration of the magnetic field lines (planar magnetic structure, PMS) due to the compression resulting from the shock itself or from the draping of the magnetic field ahead of the CME.
Coronal Mass Ejections (CMEs) are giant clouds of plasma and magnetic field that erupt from the...
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