A time‐dependent gyro‐kinetic model of thermal ion upflows in the high‐latitude F region

Abstract

Ample evidence supports the significance of the high‐latitude ionospheric contribution to magnetospheric plasma. Assuming flux conservation along a flux tube, the upward field‐aligned ion flows observed in the magnetosphere require high‐latitude ionospheric field‐aligned ion upflows of the order of 108 to 109 cm−2 s−1. Since radar and satellite observations of high‐latitude F region flows at times exceed this flux requirement by an order of magnitude, the thermal ionospheric upflows are not simply the ionospheric response to a magnetospheric flux requirement. Several ionospheric ion upflow mechanisms have been proposed, but simulations based on fluid theory do not reproduce all the observed features of ionospheric ion upflows. Certain asymmetries in the statistical morphology of high‐latitude F region ion upflows suggest that the ion upflows may be generated by ion‐neutral frictional heating. We developed a single‐component (O+), time‐dependent gyro‐kinetic model of the high‐latitude F region response to frictional heating in which the neutral exobase is a discontinuous boundary between fully collisional and collisionless plasmas. The concept of a discontinuous neutral exobase and the assumption of a constant and uniform polarization electric field reduce the ion guiding center motion in the frame of a convecting flux tube to simple one‐dimensional ballistic trajectories. We thus are able to analytically calculate a time and height‐dependent ion velocity distribution function, from which we can compute the ion density, parallel velocity, parallel and perpendicular temperature, and parallel flux. Using our model, we simulated the response of a convecting flux tube between 500 km and 2500 km to various frictional heating inputs; the results were both qualitatively and quantitatively different from fluid model results, which may indicate an inadequacy of the fluid theory approach. The gyro‐kinetic frictional heating model responses to the various simulations were qualitatively similar: (1) initial perturbations of all the modeled parameters propagated rapidly up the flux tube, (2) transient values of the ion parallel velocity, temperature, and flux exceeded 3 km s−1, 2 × 104 K, and 109 cm−2 s−1. respectively, (3) a second transient regime developed wherein the parallel temperature drops to very low values (a few hundred Kelvins), and (4) well after heating ceased, large parallel temperatures and large downward parallel velocities and fluxes developed as the flux tube slowly returned to diffusive equilibrium. The ion velocity distributions during the simulation are often non‐Maxwellian and are sometimes composed of two distinct ion populations.

On the generation/decay of the storm‐enhanced density plumes: Role of the convection flow and field‐aligned ion flow

“…They suggested that the cusp moved equatorward of the radar site, and the radar was measuring the ballistic return of upward flowing ions. The ballistic return of outflowing ions has been observed using the DE 2 satellite [Loranc et al, 1991] and studied using simulations [Loranc and St.-Maurice, 1994]. In our cases, five out of six plumes are observed near the equatorward boundary of the convection pattern and electron temperature decrease is observed within the SED; therefore, it is unlikely that plumes extended to or beyond the cusp in these cases and so the ballistic return explanation is not applicable for these cases.…”

Ion temperature anisotropy effects on threshold conditions of a shear-modified current driven electrostatic ion-acoustic instability in the topside auroral ionosphere

“…Interestingly, the effects of shears in the convection flow on F-region ion velocity distributions, which were studied by St-Maurice et al (1994), showed three-dimensional anisotropies. At higher altitudes in the topside, temperature anisotropies were also observed above the neutral exobase, which corresponds to the transition region from a weakly ionized plasma (ions collide with neutrals) to a fully ionized one (ions do not collide with neutrals), as a result of simulations from a single-component (O + ) time-dependent gyro-kinetic model of the high-latitude F-region response to frictional heating, between 500 km and 2500 km (Loranc and St-Maurice, 1994). Namely, this ion upflow model simulates the response of the passage of a flux tube, under various conditions, through a spatially localised heating region for which the neutral exobase is a discontinuous boundary between fully collisional and collisionless plasmas.…”

Mechanisms of ionospheric mass escape

“…The gravitational escape velocity of O + being approximately 11 km/s in the F region, bulk escape of ionospheric O + is thought to require the average O + ion to be given over 10 electron volts, implying a temperature of ∼100,000 K. Loranc and St.-Maurice [1994] studied transient frictional heating as a method of producing ion up-flows, and they showed that transient effects produce nonthermal velocity distributions that are not well described in fluid approaches. However, they did not concern themselves with processes that turn up-flows into outflows by accelerating ions to escape velocity.…”