Magnetotail Aurora Connection: The Role of Thin Current Sheets

Connections between magnetotail structure and dynamics and auroral forms are investigated on the basis of fluid and particle simulations, as well as Vlasov theory. Our focus is on possible mechanisms to generate or enhance perpendicular electric fields associated with quasistatic electric potentials of "U" shape or "S" shape, relevant for auroral arcs. At small scales (less than 10 km), such electric fields are associated with Hall currents and electron E x B drift rather than plasma flow shear, indicating the relevance of thin current sheets as possible source regions. Two-dimensional and 3-D MHD theory and simulations demonstrate that moderate deformations of the magnetotail may lead to critical states characterized by the loss of neighboring equilibrium and the formation of thin current sheets embedded in the wider plasma sheet. This provides a suitable scenario for the onset of reconnection in the tail, as well as a potential mechanism to generate or intensify auroral arcs, even prior to the onset of reconnection. Particle-in-cell simulations confirm this mechanism and further demonstrate that thin embedded electron current sheets may form from moderate compression, as well as expansion, of a thicker current sheet. This suggests that such structures might be more typical for the tail current sheet than a smooth solution and may represent a possible source region for quiescent arcs within the plasma sheet footpoint region and at the plasma sheet/polar cap boundary. One-dimensional Vlasov models of such structures indicate association with either S- or U-shaped potentials (and intermediate structures), depending on the magnitude of the magnetic field jump and the plasma beta.