Identification of the magnetic cloud boundary layers

Abstract

[1] Based on a statistical analysis of the boundary physical states of 80 magnetic clouds reported in the literature from the years 1969 to 2001, we suggest a new identification of the magnetic cloud boundary by describing it as front and tail boundary layers (BLs) formed through the interaction between the magnetic cloud and the ambient medium. In our identification the outer boundary of the layer often displays the properties of magnetic reconnection, which could be characterized by a ''three-high state'' (relatively high proton temperature, high proton density, and high plasma beta) and the corresponding magnetic signatures (the intensity drop and the abrupt azimuthal changes, Áf $ 180°, and latitudinal changes, Áq $ 90°, in the magnetic field). The inner boundary of the layer exhibits a ''three-low state'' (relatively low proton temperature, low proton density, and low plasma b) and separates the magnetic cloud body, which has not basically been affected by the interactions, from the boundary layers. The front boundary layer could be associated with the outer loops of CMEs and its average time scale is 1.7 hours; the tail boundary layer seems not be a filament and its average time scale is 3.1 hours. The distribution function of magnetic fluctuations in the boundary layer is significantly different from those in the ambient solar wind and the cloud body itself. The preliminary numerical simulation in principle confirms this new identification and could qualitatively explain most of the observations of the cloud boundary. This work could help partly overcome some inconsistencies in identifying the boundaries of magnetic clouds.

Coronal mass ejections and their sheath regions in interplanetary space

“…In most cases the boundary is far from that clear. A statistical study of 80 MCs by Wei et al (2003a) showed that the majority of them were bounded by boundary layers with average passage durations of 1.7 and 3.1 h at the front and rear edges, respectively. The right-hand side of Fig.…”

“…In the left-hand panels the dashed vertical line indicates the sharp boundary and in the right-hand panels the pair of dashed lines marks a wider boundary layer. Both cases are discussed in Wei et al (2003a) magnetic field direction. As noted by Wei et al (2003a) these are common features of boundary layers and can be interpreted as signatures of magnetic reconnection.…”

Analysis of the substructure within a complex magnetic cloud on 3–4 September 2008

“…Regions of depressed magnetic fields are also occasionally observed at the leading edges of magnetic clouds (Wei et al, 2003). These depressions are localized regions where the IMF magnitude is depressed well below average accompanied by a simultaneous increase in plasma pressure.…”

“…As a significant fraction of MCs occur close to the interplanetary magnetic field (IMF) sector boundaries and are followed by high-speed streams (Crooker et al, 1998;Fenrich and Luhmann, 1998), the distortion of the MC structure by the following high-speed stream is a common scenario. Substructures have also been used to define MC front boundaries (Wei et al, 2003) and to separate MCs from ICME-related plasma ahead (Farrugia et al, 2001;Kilpua et al, 2013a). According to above-described studies the analysis of MC substructures may yield important information on the history of the interaction between the MC and the ambient solar wind as well as on the CME release process.…”

A scheme for finding the front boundary of an interplanetary magnetic cloud

“…However, a boundary layer of a MC is an extended region (see Wei et al, 2003) and the actual MC boundary is usually rather abrupt as the spacecraft flies through it, so an identification of a boundary layer should have limited usefulness for our method. It is mainly for this reason that we did not add the possible existence of a boundary layer to our scheme.…”