Investigating the observational signatures of magnetic cloud substructure

STEED K.; OWEN C.J.; DÉMOULIN P.; DASSO S.

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2011 vol. 116 p. 1106 - 1118

0148-0227

Magnetic clouds (MCs) represent a subset of interplanetary coronal mass ejections(ICMEs) that exhibit a magnetic flux rope structure. They are primarily identified bysmooth, large‐scale rotations of the magnetic field. However, both small‐ and large‐scalefluctuations of the magnetic field are observed within some magnetic clouds. Weanalyzed the magnetic field in the frames of the flux ropes, approximated using a minimumvariance analysis (MVA), and have identified a small number of MCs within whichmultiple reversals of the gradient of the azimuthal magnetic field are observed. We hereinuse the term ?substructure? to refer to regions that exhibit this signature. We examine, indetail, one such MC observed on 13 April 2006 by the ACE and WIND spacecraft andshow that substructure has distinct signatures in both the magnetic field and plasmaobservations. We identify two thin current sheets within the substructure and find that theybound the region in which the observations deviate most significantly from those typicallyexpected in MCs. The majority of these clouds are followed by fast solar wind streams,and a comparison of the properties of this magnetic cloud with five similar eventsreveals that they have lower nondimensional expansion rates than nonovertaken magneticclouds. We discuss and evaluate several possible explanations for this type of substructure,including the presence of multiple flux ropes and warping of the MC structure, but weconclude that none of these scenarios is able to fully explain all of the aspects of thesubstructure observations.