EHD/EMHD Transport Processes and Electric Reconnection in Dusty and Dirty Plasmas

Abstract

ABSTRACT: For dusty and dirty plasmas, fully or partially ionized, collisionless or collisional, containing dust grains or aerosols, charged and/or uncharged, an electrohydrodynamics (EHD)/electromagnetohydrodynamics (EMHD) is presented on the basis of a new equation of electric field transport in a charged fluid, supplemented by the equations of fluid vorticity, magnetic field, energy transport, and heat transfer extended for this new regime. The ‘electric Reynolds number’, RE=ɛμvL/T (ɛ: dielectric constant; μ: permeability; v: fluid velocity; L: characteristic length; T: characteristic time) plays an important role in the EHD/EMHD regime as a criterion of relative importance of convection, diffusion, dissipation, propagation, and radiation. The equation of electric field transport recovers the Kelvin‐Helmholtz theorem for high electric Reynolds numbers with a space‐charge related frozen‐in field concept. Tenuous cosmic plasmas where dust particles can be highly charged, need to be considered a multi‐component fluid, composed of electrons, ions, charged and/ or uncharged dust, and specifications are given briefly to a multi‐ component fluid model. By analogy with fluid vortex merging in the HD regime and magnetic reconnection in the MHD regime, electric reconnection is newly introduced in the EHD/EMHD regime. Following dust‐related electric reconnection, subsequent processes are divided into two cases, depending on whether the local electric fields on the grain surface are below or beyond the breakdown threshold of the background gas. For the former case, some of the electrostatic energy tends to be converted into the kinetic energy of the particle, leading to its rapid acceleration. For the latter case, a local surface discharge will take place due to sufficiently high local electric fields, forming a corona around the dust or object and leading to critical ionization flow in the form of streamer and leader followed by an eventual main discharge, or rarely by a plasma layer formation around the dust grain. Applications of EHD/EMHD and electric reconnection to a number of examples are indicated, including the formation of diffuse dust layers or dust distributions between the clouds and the ionosphere with reference to pre‐earthquake effects, natural and triggered lightning, tornadic thunderstorms, cloud‐to‐ionosphere discharges, ball lightning, nebular lightning, Chondrule production in meteorites, and a new universal electric‐cusp type plasma reactor.