Formation of thin current sheets in magnetospheres.

The formation of thin current sheets is believed to play an important role in many space and astrophysical plasma phenomena, including transport and conversion of magnetic to kinetic energy in stellar and magnetospheric activity. The Earth's magnetosphere is an appropriate environment for the in-situ study of these phenomena. Thin current sheets form typically when a smooth current sheet is perturbed by external forces. The result is a double structure with a thin current sheet embedded in a broader one. A simple example leading to a double structure is analyzed. Resistive instabilities typically involve thin current sheets also. When the Earth's magnetapause becomes resistivelly unstable due to localized resistive "patches", a fast unstable dynamical process occurs which leads to magnetic opening and complicated 3D magnetic structures involving thin current sheets. As magnetic flux is transferred to the magnetotail the plasma sheet compresses and the tail current density increases. However, a smoothly distributed current cannot explain the observed signatures of a resistive (or a corresponding kinetic) instability. Recently, observations have indicated that a double structure forms. That processes can be recovered in a simple model based on slow adiabatic evolutions, and the mechanism can be identified. Thin current sheet forming in the magnetotail before onset of the tail instability has played the role of a "missing link" in the theory of magnetospheric activity. [ABSTRACT FROM AUTHOR]