Boundary layers of the Earth's outer magnetosphere

The magnetospheric boundary layer and the plasma-sheet boundary layer are the primary boundary layers of the earth's outer magnetosphere. The magnetospheric boundary layer occurs everywhere near the outer magnetospheric boundary or magnetopause. Any plasma, momentum or energy transport from the solar wind to the magnetosphere is carried out by way of this boundary layer. More than 98% of the high-β plasma of the shocked solar wind or magnetosheath is deflected around the magnetosphere. Although the plasma flow is predominantly tangential to the magnetopause surface, some 1–2% of the oncoming solar wind plasma gains entry into the magnetosphere and initially provides a source for the magnetospheric boundary layer. This boundary layer is generally intermediate in number density, mean energy and flow speed with respect to its high-β source region and the low-β hot plasma of the frontside outer magnetosphere. On the tailward side, the plasma-sheet boundary layer is also intermediate in number density and mean energy with respect to the low-β lobe region and the medium to high-β central plasma sheet. This boundary layer is identified by high-speed ion beams which are dominantly field-aligned and flowing sunward and, occasionally, antisunward. Counter-streaming ion beams are also frequently observed within the plasma-sheet boundary layer. Low-energy ion beams, likely of ionospheric origin, are also commonly observed within this boundary layer. Recent satellite observations of these two boundary layer regions indicate that they provide for more than 50% of the plasma and energy transport in the outer magnetosphere although they constitute less than 5% by volume. Relative to the energy density in the source regions plasma in the magnetospheric boundary layer is predominantly de-energized whereas plasma in the plasma-sheet boundary layer has been accelerated. The reconnection hypothesis continues to provide a useful framework for comparing data sampled in the highly dynamic magnetospheric environment. Reconnection provides for fairly tractable models in which dissipative effects are localized to a “diffusion” region so that the idealized MHD relations can be assumed valid throughout most of the system. However, observations of the boundary layers of the outer magnetosphere suggest that dissipative effects here are ubiquitous. For this and other reasons, steady-state reconnection is not generally applicable to the earth's magnetosphere. However, observations of “flux transfer events” and other detailed features near the boundaries have been recently interpreted in terms of non-steady-state reconnection. Alternative hypotheses are also being investigated. More work needs to be done, both in theory and observation, to determine whether reconnection actually occurs in the magnetosphere and, if so, whether it is important for overall magnetospheric dynamics.