Identification of the magnetic cloud boundary layers

Abstract

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, Δφ ∼ 180°, and latitudinal changes, Δθ ∼ 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 β) 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. Copyright 2003 by the American Geophysical Union.