A two-dimensional model of plasma expansion in the ionosphere

Abstract

The motion of artificial plasma clouds injected perpendicular to the geomagnetic field ( B) in the ionosphere was studied using a time-dependent, two-dimensional, electrostatic model. Various geophysical and gas release conditions were considered, including different solar cycle, seasonal, and geomagnetic activity levels and different cloud sizes, injection velocities, release altitudes, and species. The effects of both magnetospheric electric fields and thermospheric winds on the cross-field motion were also considered. This systematic study led to the following conclusions: (1) initially, the newly created plasma clouds (Ba + and Li +) move across B with the injected neutral clouds (for t ≲ 1 s), but the ions are decelerated much faster than the neutrals and eventually become tied to the magnetic field; (2) at high altitudes (~400 km), the shape of a plasma cloud becomes long and narrow because it is formed via photoionization from the more rapidly moving neutral cloud. However, the precise cloud configuration and deceleration rate are determined by the ionospheric and release conditions; (3) the deceleration of plasma clouds is faster in summer than in winter, but the cloud Pedersen conductivities have similar magnitudes; (4) solar cycle and geomagnetic activity variations have a similar effect on the cross-field motion of plasma clouds. At solar maximum and high geomagnetic activity, the plasma clouds are decelerated faster and the cloud Pedersen conductivities are greater than at solar minimum and low geomagnetic activity; (5) at high altitudes, structure forms on the rear side of Ba + clouds in winter at solar minimum and low geomagnetic activity, but not for other geophysical conditions or for lithium; (6) the electrodynamic drift associated with the magnetospheric electric field acts to distort the shape of plasma clouds, change their direction of motion, and change their orientation relative to the neutral clouds; (7) at low altitudes (~200 km), the cross-field motion of both the plasma and neutral clouds is much slower, the cloud Pedersen conductivity is much greater (by two orders of magnitude), and the plasma cloud configuration is more isotropic than at high altitudes; (8) a thermospheric wind has an important effect on the cross-field motion of plasma clouds at 200 km, but its effect is small at 400 km. At low altitudes, the neutral wind acts to polarize the plasma cloud and eventually it moves in the direction of the thermospheric wind (a well-known result); (9) for high injection velocities and high cloud densities, the plasma cloud moves a greater distance across B and the cloud tends to become longer and narrower, particularly at high altitudes; (10) increasing the initial cloud size has an effect that is similar to lowering the cloud injection velocity; (11) if a cloud is partially ionized at the time of release, the motion of the initial ionization is affected by the electric field associated with the subsequent photoionization of the more rapidly moving neutral cloud; and (12) comparing comparable barium and lithium releases, the lithium ion cloud has a lower Pedersen conductivity and is decelerated faster because of the lower photoionization rate and smaller mass for lithium.