Poleward arcs of the auroral oval during substorms and the inner edge of the plasma sheet

An analytical model for the connection between the near‐Earth edge of the plasma sheet at substorm onset and the auroral arcs at the poleward edge of the auroral oval is presented. The connection is established through the existence of a Boström type I current system. Its generator is assumed to be constituted by a narrow high‐beta plasma layer located at the interface between the near‐Earth plasma sheet and the outer edge of the near‐dipolar field of the magnetosphere. The energy balance between the downward Poynting flux and the energy conversion in the auroral acceleration region and ionosphere provides a relation for the electric fields as a function of the upward field‐aligned current. Only the upward current region is being considered in this work. An interesting effect, incorporated in the energy balance, is the feedback of the auroral electrojet on the magnetospheric plasma by dragging the latter eastward from below under the action of a Hall generator. Thereby a relation arises between the westward electric field, tangential to the arc, and the equatorward polarization field. Quantitative solution of the energy equation is achieved by using the empirical relations between auroral energy flux and electron energy and the integrated Hall and Pedersen conductivities. Accommodation of the downward energy flux requires the existence of a minimum arc length. The resulting quantities are consistent with typical auroral data sets. Relating the downward energy flux to the parameters of the generator reveals a strong dependence of polarization electric field, overall energy dissipation, and total current strength on the plasma beta of the generator. The dumping of excess energy from the high‐beta plasma layer into the auroral arc(s) allows the stretched tail field lines to transform into dipolar field lines. It opens, so‐to‐speak, the gate into the outer magnetosphere.