Plasma cloud expansion in the ionosphere: Three‐dimensional simulation

Abstract

A three‐dimensional time‐dependent model was developed to study the characteristics of a plasma cloud expansion in the ionosphere. The model makes it possible to use realistic ionospheric and cloud parameters. With the model the parallel and perpendicular motion of both the cloud and background ions can be studied. The electrostatic potential is solved in three dimensions taking into account the large parallel‐to‐perpendicular conductivity ratio. The model takes account of ion inertia, pressure gradients, the Lorentz force, the stress tensor (both collisional viscosity and the finite Larmor radius effect), Coulomb collisions and collisions between neutral and ionized species, and the finite parallel conductivity. Three sample simulations are presented: a plasma expansion of a nearly spherical 1 km Ba+ cloud, both with and without a background neutral wind, and a long thin Ba+ cloudlet. With or without the neutral wind the effective potential, which is different from the electrostatic potential if the electron temperature is included, is constant along the magnetic field for typical cloud sizes. The expanding plasma clouds become elongated in the magnetic field direction. The released Ba+ ions push the background O+ ions away along the magnetic field as they expand. Consequently, a hole develops in the background O+ distribution at the cloud location and on the two sides of the cloud O+ bumps form. The entire three‐dimensional structure, composed of the plasma cloud and the background plasma embedded in the cloud, slowly rotates about the magnetic field, with the ions and electrons rotating in opposite directions. The cloud configuration takes the shape of a rotating ellipsoid with a major axis that expands with time. Perpendicular to the magnetic field, in the absence of the neutral wind the motion is insignificant compared to the parallel motion. With a neutral wind the motion along the magnetic field and the rotational motion are qualitatively unchanged, but the cloud and the perturbed background structure move in the direction of the wind, with a speed less than the wind speed. Perpendicular to the magnetic field the deformation of the cloud induced by the wind is characterized by steepening of the backside. For an extended cloudlet along the magnetic field the potential is not constant along B. Also, the parallel expansion of the cloud and the snowplow effect are not important. The sheared rotational motion and diffusion act to smooth initial density perturbations on the cloud surface at early times, but later an instability develops that creates new density structures.