Magnetospheric Plasma Regions and Boundaries

Abstract

Several magnetospheric regions and their boundaries are discussed in critical terms. Serious questions are raised by recent observations that suggest that the magneto sheath plasma on the day-side penetrates deep into the region of closed magnetic field lines. This deduction, and the evidence that the polar cap field lines interconnect with the interplanetary magnetic field lines, suggest that the magnetopause be defined as a plasma boundary, rather than in terms of open or closed magnetic field lines. The magnetopause is here defined as that surface where there is an abrupt reduction in the phase space density from the values typical of the magneto sheath plasma. On the night side this definition, coupled with the evidence of closed field lines poleward of the auroral oval, requires a modification to Dungey’s model of the magnetosphere in which there is a gap or space between the end of the plasma sheet and the X-line. These considerations imply that the usual model of magnetic merging is not applicable to the magnetosphere. This implication is supported by the failure to detect the energy dissipation at the dayside magnetopause that would be a consequence of the tangential component of the electric field characteristic of the merging process. Careless use of words or phrases such as interconnection and generation is discussed. It is suggested that the resolution of these serious questions can be achieved only by turning to a more primitive description that avoids the shortcomings of the fluid theory.