Tail pressure release through auroral acceleration

Earthward convecting plasma has to circumvent the Earth on its way from the tail reconnection region to the dayside magnetosphere. This leads to radially sheared flows between the corotating plasmasphere and the low-latitude boundary layer. Consequently, embedded magnetic field lines also become sheared and set-up field-aligned currents which must close in the highly conducting auroral ionosphere. Associated electric fields and plasma motions are relatively small, the auroral ionosphere thus exerts a drag on the magnetic flux tubes embedded in the magnetospheric plasma which in turn affects the pressure distribution in the equatorial magnetosphere. We investigate the role of parallel potential drops in the acceleration region in this context with the help of a steady convection model. Parallel potential drops provide a means to decouple the magnetosphere from the ionosphere, thus they can reduce ionospheric drag to avoid high pressure build-up tailward of the Earth. Relationships between field-aligned currents, pressure gradients, and parallel potential drops are derived and compared with observations. In particular, the current-voltage relationship allows for a determination of the field-aligned resistance which is in good agreement with others estimates.