Field‐aligned current loop model on formation of sporadic metal layers

Abstract

This paper proposes a new mechanism for the formation of the sporadic layers by metallic ions as well as metallic atoms in the lower thermosphere and upper mesosphere. The mechanism is based on sporadic occurrence of an electric current loop consisting of the magnetic field‐aligned current (FAC) and horizontal Pedersen current in the ionosphere connected with the FAC. The essential point of the present mechanism is the existence of the horizontal electric field convergence at the foot of the upward FAC region in the current loop, where the positive ions including the metallic ions may converge by the horizontal Pedersen flow. The accumulated metallic ions are reacted to the clustered metallic ions, which are transferred to the metallic atoms through the dissociative recombination with electrons. The density profile of the sporadic layer for metallic ions has been obtained in the analytical form by solving the continuity equation. The density profile of the sporadic layer for metallic atoms has been obtained also in the analytical form by assuming that 100% of the loss of the metallic ions is converted to metallic atoms. The simulation based on the present mechanism has been examined with respect to three species of metallic components: sodium, iron, and calcium. The sporadic sodium layer (SSL) derived from the present model is compared with the SSL observed by the sodium lidar installed at the European Incoherent Scatter radar site in Tromsø (69.6°N, 19.2°E), Norway, and reasonable agreement has been attained with respect to the density profile of the SSL.