A semikinetic model was used to describe the steady state collisionless flow of the polar wind along diverging geomagnetic field lines at high latitudes. Emphasis was given to studying the behavior of O+ ions for a wide range of boundary conditions at the baropause (4500 km). The main result obtained was that for high electron temperatures (Te ∼ 10,000 K) O+ is not gravitationally bound and significant escape fluxes (∼107 cm−2 s−1) of suprathermal (∼2 eV) O+ ions occur. The O+ flow is supersonic over most of the altitude range considered, and O+ Mach numbers as large as 20 are predicted at 10 RE. Also, depending on the boundary conditions, the O+ density can be comparable to or greater than the H+ density at high altitudes above the baropause when the electron temperature is high.

In this review we attempt to present a unified approach to the study of transport phenomena in multicomponent anisotropic space plasmas. In particular, a system of generalized transport equations is presented that can be applied to widely different plasma flow conditions. The generalized transport equations can describe subsonic and supersonic flows, collision-dominated and collisionless flows, plasma flows in rapidly changing magnetic field configurations, multicomponent plasma flows with large temperature differences between the interacting species and plasma flows that contain anisotropic temperature distributions. In addition, if Maxwell's equations of electricity and magnetism are added to the system of transport equations, they can be used to model

A semikinetic model was used to study the effect that hot electron populations have on the polar wind. The model was used to describe the steady state plasma flow along diverging geomagnetic field lines in the collisionless regime at high altitudes. The plasma contained O+ and H+ ions and both hot and cold electron populations. Several hot electron populations were considered, including the polar rain, polar showers, and polar squall. Estimates of hot electron parameters based on characteristic energy and flux measurements indicate that the hot/cold electron temperature ratio varies from 10 to 104 and that the percentage of hot electrons varies from 0.1% to 10% at 4500 km. For ratios at the lower ends of these ranges the polar wind solutions with hot electrons are similar to those obtained previously for supersonic H+ outflow without hot electrons. For higher hot electron temperatures and a greater percentage of hot electrons, there is a discontinuity in the kinetic solution, which indicates the presence of a sharp transition. This transition corresponds to a contact surface between the hot and cold electrons. Along this surface, a double‐layer potential barrier exists which reflects the cold ionospheric electrons and prevents their escape. The presence of the hot electrons acts to increase the supersonic H+ outflow velocity and H+ energy but does not affect the already saturated H+ escape flux. The H+ energy gain may be as large as 1 to 2 keV. With regard to O+, the hot electrons act to reduce the potential barrier, thereby allowing more O+ ions to escape. A significant enhancement in the O+ escape flux can occur depending on the hot electron density and temperature.

We have computed ion velocity distributions for the auroral ionosphere with the use of a Monte Carlo computation scheme that includes a detailed description of the interaction between ions and neutrals. This means that both the speed and angular dependence of the interaction are accounted for in a serf-consistent manner. We have used the model to deal with several types of interactions all at once and to describe quantitatively how the velocity distribution evolves in the presence of a mixture of neutral constituents. The model also contains an ad hoc procedure to deal with the effect of chemistry, which is important for periods during which N• ions are abundant. We have found that the shape of NO + ion velocity distributions reaches an asymptotic limit for electric fields reaching about 125 mV/m, that the shape of O + and N• + ion velocity distributions may be very difficult to model analytically for very large electric field strengths, but that conversely, analytical theories should do an adequate job for fields up to 100 or 150 mV/m for these two ion species. We have, among other things, presented our results in terms of an ion temperature anisotropy, showing how this anisotropy, or the partition parameters that are sometimes used to describe it, change with electric field strength. We have found in the process that the anisotropy of O + ions is noticeably greater than what has been assumed to be the case in the past. Finally, we have found that the shape of the distribution is sensitive to the presence of a secondary neutral constituent. For example, the presence of just 25% atomic oxygen is sufficient to give the O + distribution a detectably greater toroidal character.

The polar wind is an ambipolar plasma outflow from the terrestrial ionosphere at high latitudes. As the ions drift upward along geomagnetic flux tubes, they move from collision‐dominated (ion barosphere) to collisionless (ion exosphere) regions. A transition layer is embedded between these two regions where the ion characteristics change rapidly. A Monte Carlo simulation was used to study the steady‐state flow of H+ ions through a background of O+ ions. The simulation domain covered the collision‐dominated, transition, and collisionless regions. The model properly accounted for the divergence of magnetic field lines, the gravitational force, the electrostatic field, and H+‐O+ collisions. The H+ velocity distribution, f(H+), was found to be very close to Maxwellian at low altitudes (deep in the barosphere). As the ions drifted to higher altitudes, f(H+) formed an upward tail. In the transition layer, the upward tail evolved into a second peak with a kidney bean shape, and hence, f(H+) developed a double‐humped shape. The second peak grew with altitude and eventually became dominant as the ions reached the exosphere. This behavior is due to the interplay between the electrostatic force and the velocity‐dependent Coulomb collisions. Moreover, the H+ heat flux, q(H+), was found to change rapidly with altitude in the transition layer from a positive maximum to a negative minimum. This remarkable feature of q(H+) is closely related to the coincident formation of the double‐humped structure of f(H+). The double‐hump distribution might destabilize the plasma or, at least, cause enhanced thermal fluctuations. The double‐hump f(H+), and the associated wave turbulence, have several consequences with regard to our understanding of the polar wind and similar space physics problems. The plasma turbulence can significantly alter the behavior of the plasma in and above the transition region and, therefore, should be considered in future polar wind models. The wave turbulence can serve as a signature for the formation of the double‐hump f(H+). Also, more sophisticated (than the existing bi‐Maxwellian 16‐moment) generalized transport equations might be needed to properly handle problems such as the one considered here.