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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 31, issue 9
Ann. Geophys., 31, 1559–1567, 2013
© Author(s) 2013. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 31, 1559–1567, 2013
© Author(s) 2013. This work is distributed under
the Creative Commons Attribution 3.0 License.

Regular paper 10 Sep 2013

Regular paper | 10 Sep 2013

Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions

E. K. J. Kilpua1, H. Hietala2, H. E. J. Koskinen1,3, D. Fontaine4, and L. Turc4 E. K. J. Kilpua et al.
  • 1Department of Physics, P.O. Box 64, University of Helsinki, Finland
  • 2Space and Atmospheric Physics Group, The Blackett Laboratory, Imperial College, London SW7 2AZ, UK
  • 3Finnish Meteorological Institute, P.O. Box 503, Helsinki, Finland
  • 4Laboratoire de Physique des Plasmas, Ecole Polytechnique, Route de Saclay 91128 Palaiseau, France

Abstract. Compressed sheath regions form ahead of interplanetary coronal mass ejections (ICMEs) that are sufficiently faster than the preceding solar wind. The turbulent sheath regions are important drivers of magnetospheric activity, but due to their complex internal structure, relatively little is known on the distribution of the magnetic field and plasma variations in them. In this paper we investigate ultra low frequency (ULF) fluctuations in the interplanetary magnetic field (IMF) and in dynamic pressure (Pdyn) using a superposed epoch analysis of 41 sheath regions observed during solar cycle 23. We find strongest fluctuation power near the shock and in the vicinity of the ICME leading edge. The IMF and Pdyn ULF power have different profiles within the sheath; the former is enhanced in the leading part of the sheath, while the latter is increased in the trailing part of the sheath. We also find that the ICME properties affect the level and distribution of the ULF power in sheath regions. For example, sheath regions associated with strong or fast ICMEs, or those that are crossed at intermediate distances from the center, have strongest ULF power and large variation in the power throughout the sheath region. The weaker or slower ICMEs, or those that are crossed centrally, have in general considerably weaker ULF power with relatively smooth profiles. The strong and abrupt decrease of the IMF ULF power at the ICME leading edge could be used to distinguish the ICME from the preceding sheath plasma.

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