On the magnetospheric boundary layer and solar wind energy transfer into the magnetosphere

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The magnetopause and the magnetospheric boundary layer constitute the interface between the shocked solar wind plasma and the terrestrial magnetic cavity populated by a predominantly hot plasma in the outer portion and a cold (ionospheric) plasma near the Earth. It is well recognized that this interface region is one through which energy, mass, and momentum are transferred from the solar wind into the magnetosphere. What is less clearly understood are the physical mechanisms governing this transfer. Magneto-fluid dynamics is the basis of the most frequently used method to describe plasma energy and mass transfer. The assumption of plasma being frozen into the magnetic field provides a means of treating the problem by ‘ideal’ MHD. Another method, discussed in more detail in this report, is to consider the energy and mass transfer from a plasma kinetic point of view and utilize a current description. In the kinetic/current model an important consequence of the mass transfer is to make available the kinetic energy of particles. The plasma flow (or pressure) is converted into electrical energy in the interaction region — the boundary layer. The formation of the boundary layer inside the magnetopause is a direct consequence of a mass and/or momentum transfer process. Energy transfer follows from a local exchange of momentum (non-dissipative, to the local plasma) and from a dynamo process driving currents through the resistive (dissipative) terrestrial dayside ionosphere. Large-scale currents and energy dissipation into the dayside ionosphere are, therefore, propelled by boundary-layer dynamo processes where particle kinetic energy is locally converted into electrical energy (polarization). The persistency of the boundary layer and its low altitude footprint, the dayside cleft/cusp, is a strong argument for the boundary-layer model. Measurements in the dayside cleft, assumed to be the magnetic footprint of the low latitude boundary layer, indicates an almost continuous activity there. In this report a review will be made of the boundary-layer morphology, its ‘steady state’ energetics, and its connection to the dayside ionosphere. Small-scale dynamical features such as flux transfer and plasma transfer events (FTEs and PTEs) will also be discussed and related with current structures in the boundary layer.