Lessons from laboratory experiments on reconnection

Abstract

Magnetic reconnection has been studied in a laboratory experiment designed to model the basic two-dimensional neutral sheet configuration. However, the focus has been put on the inner region of the neutral sheet where the ions are effectively unmagnetized and MHD concepts are violated. In this parameter regime driven reconnection is governed by the fast dynamics of electrons. In true neutral sheets (B z ⋍ 0) the current is carried by electrons. Thin current sheets (Δz ≳ c/ω pe) rapidly form multiple X and 0 points due to the onset of the collisionless electron tearing mode. Magnetic energy is transported along the separator at the speed of whistler waves rather than Alfvén waves. Due to space charge separation the reconnection electric field E y is, in general, not constant along the separator but localized near boundaries, nonuniformities in density and magnetic fields which limit the current I y. This leads to localized particle acceleration, formation of anisotropic velocity distributions and instabilities. Reconnection and energization can be spatially separated which shows the importance of investigating both the global current system as well as critical local plasma properties. Experiments of current sheet disruptions are performed which demonstrate the processes of magnetic energy storage, transport, conversion and dissipation. Double layers and shock waves can be produced by current disruptions. The laboratory experiments show new dynamic features of reconnection processes not considered in MHD models yet relevant to narrow current sheets or the center of thick sheets.