Stability and Structure of ­Superthin" Sheets: 2D and 3D Full Particle Simulations

Current sheets are a central feature of magnetospheric structures and play a significant role in the transfer of plasma, momentum and energy from the solar wind. Our recent hybrid (fluid electrons, kinetic ions) simulations for northward IMF demonstrate that magnetospheric structures go through well defined transition points as the magnetic dipole strength is increased in value. Further, for weak dipole fields, which may be relevant for asteroids and smaller planets, the magnetopause takes the form of a very thin current layer with Ÿ/L ~ 7 where Ÿnis the ion gyroradius and L is the current sheet half-thickness. The properties of such thin current sheets are largely unknown. We have used linear kinetic theory as well as 2D and 3D full particle simulations to study the structure and stability of such thin sheets. Some of the important issues that we address include, (i) what is the lower limit of current sheet thickness in the presence of instabilities such as the lower hybrid drift instability that will broaden the sheet? (ii) Can magnetic reconnection occur in this limit and what form will it take? Thin sheets also enable us to settle important issues regarding the stability of thicker current sheets observed at Earth. The study of such current sheets at has been difficult due to small growth rates, and many important issues remain unresolved. Even using unrealistically small mass ratios, relevant simulations remain out of reach of present computational capabilities. Through our study of thin sheets we will address issues that are of direct interest to the Earth's magnetosphere, using realistic mass ratios. Examples include (i) nonlinear interaction of tearing and lower hybrid drift instability and (ii) saturation mechanism of guide field tearing in three-dimensions.