The size of Ihe terrestrial magneto sphere is determined by the balance between the solar wind dynamic pressure and the pressure exerted by the magnetosphere. principally that of its magnetic field. The shape of the magnetosphere is additionally influenced by the drag of the solar wind, or tangential stress, on the magnetosphere. This drag is predominantly caused by the mechanism known as reconnection in which the magnetic Held of the solar wind links with the magnetic field of the magnetosphere. The factors that control the rate of reconnection of the two fields are not understood completely but a southward direction of the interplanetary field is critical to enabling reconnection with the dayside low latitude magnetosphere, resulting in magnetic flux transfer to the niagnetoiail. Numerical simulations suggest that the conductivity of the ionosphere controls the rate of reconnection but this has not been verified observationally. While solar wind properties ultimately control the interaction, the properties of the plasma that makes direct contact with the magnetosphere are different than those of the solar wind, having been altered by a standing bow shock wave. This standing shock is necessitated by the fact that the flow velocity of the solar wind far exceeds the velocity of the compressional wave that diverts the solar wind around the Earth. The upper atmosphere is the final recipient of all the energy and momentum that enters the magnetosphere. Coupling takes place along the magnetic field lines principally in the polar and auroral region via current systems thai close across the magnetic field both at low and high altitudes and flow parallel to the magnetic field between high and low altitudes.