Investigating the observational signatures of magnetic cloud substructure

Abstract

[1] Magnetic clouds (MCs) represent a subset of interplanetary coronal mass ejections (ICMEs) that exhibit a magnetic flux rope structure. They are primarily identified by smooth, large-scale rotations of the magnetic field. However, both small-and large-scale fluctuations of the magnetic field are observed within some magnetic clouds. We analyzed the magnetic field in the frames of the flux ropes, approximated using a minimum variance analysis (MVA), and have identified a small number of MCs within which multiple reversals of the gradient of the azimuthal magnetic field are observed. We herein use the term "substructure" to refer to regions that exhibit this signature. We examine, in detail, one such MC observed on 13 April 2006 by the ACE and WIND spacecraft and show that substructure has distinct signatures in both the magnetic field and plasma observations. We identify two thin current sheets within the substructure and find that they bound the region in which the observations deviate most significantly from those typically expected 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 events reveals that they have lower nondimensional expansion rates than nonovertaken magnetic clouds. 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 we conclude that none of these scenarios is able to fully explain all of the aspects of the substructure observations.

Expansion of magnetic clouds in the outer heliosphere

“…When large fluctuations are present in some MCs, then B y,cloud has an odd number of reversals (e.g. Steed et al 2011). Finally, the component B x,cloud has its weakest value when the spacecraft crosses the central part of the MC; its relative magnitude provides an indication of the impact parameter (Gulisano et al 2007;Démoulin & Dasso 2009b).…”

Magnetic Field Configuration Models and Reconstruction Methods for Interplanetary Coronal Mass Ejections

“…Complex ICMEs (Complex ejecta), which may result from the interaction of successive CMEs or from the interaction of CMEs with complex solar wind structures and streams (Burlaga, Plunkett, and St. Cyr, 2002;Wang, Ye, and Wang, 2003;Lugaz, Manchester, and Gombosi, 2005). Double rotation within one MC have also been reported from Ulysses (Rees and Forsyth, 2004) and ACE observations (Steed et al, 2011), showing that even isolated ICMEs may produce complex ejecta.…”

“…The original inclusion of the self-similarly expansion has been found to cause a deviation from the force-free state during expansion. Some refinements proposed by Shimazu and Vandas (2002) have been incorporated into the model to preserve the forcefree state for the duration of the MC travel. As a result, the magnetic field intensity B and the radius r of the fitted MC at a given time as produced by the model are defined as follows:…”

“…MVA is a technique originally developed for solving the normals to a tangential discontinuity (TD) (Sonnerup and Scheible, 1998). In order to obtain the orientation of an ICME, the axis is determined by the intermediate eigenvector, as pointed out by Goldstein (1983).…”

“…Farrugia et al (1993) and Farrugia, Osherovich, and Burlaga (1995) introduced a cylindrical model incorporating self-similarly expansion in two initially force-free constant-α codes, a cylindrical and a spherical models, and noted that the cylindrical configuration did not maintain the force-free state after it starts to expand. Thereafter, Shimazu and Vandas (2002) provided a modification of the mathematical formalism in order to keep the self-similarly expanding cylindrical model as force-free.…”