A heuristic model of the geomagnetic cavity is introduced. The model includes a unipolar induction current JU, that is driven by the solar wind. The current is generated by the solar wind plasma flowing in the magnetopause boundary layer. This current closes in the ionosphere. The electromagnetic forces associated with JU drive the magnetospheric convection. Even though the solar wind plasma at the magnetopause is basically collisionless, the plasma penetrates more deeply into the geomagnetic cavity than it would if the magnetopause were an ideal Chapman-Ferraro sheath because the boundary layer flow is non-adiabatic in the regions where JU is nonzero. Downstream from the earth the resulting expanded boundary layer forms the plasma sheet and the geomagnetic tail. Since the generation of JU does not depend on the existence of an interplanetary magnetic field, the geomagnetic cavity is closed when the interplanetary field is zero. The tangential drag is developed in the part of the boundary layer that lies forward of the midnight-meridian intersection of the leading edge of the neutral sheet. The tail is produced by the inflation of the downstream part of the modified cavity that is formed by the superposition of the geomagnetic field and the magnetic field of JU. Transient phenomena produced by changes in JU include substorm expansions and injections of ring current particles which are initiated by decreases in JU. Changes in JU are caused by changes in the solar wind velocity (i.e., by changes in the unipolar emf) and by changes in the effective resistance to the flow of JU. If there is a critical value of |JU| above which the current is unstable, then there is a corresponding critical magnitude of the unipolar emf and therefore of the solar wind velocity. Such an instability in JU would account for the positive correlation between solar wind velocity and Kp. When the interplanetary magnetic field is greater than zero, shear stresses are set up between the solar wind plasma in the magnetopause boundary layer and that in the magnetosheath. The unipolar induction, i.e., the unipolar emf, and the resulting tangential drag then depend on the direction and magnitude of the interplanetary field as well as on the velocity of the solar wind and the electrical (Pedersen) resistivity of the magnetospheric plasma. The many attractive features of the model, especially the simplicity of the tangential drag mechanism, suggest that unipolar induction is a competitive alternative to magnetic field line reconnection and the other mechanisms that have been proposed to account for the tangential drag on the cavity and the various transient cavity phenomena.
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