The Lyon-Fedder-Mobarry (LFM) global MHD magnetospheric simulation code

Initial results from the coupled magnetosphere ionosphere thermosphere model: magnetospheric and ionospheric responses

The polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes to the magnetosphere along geomagnetic field lines. The polar wind plasma consists mainly of H+, He+, and O+ ions and electrons. Although it was initially believed that O+ ions play a major role only at low altitudes, it is now clear from observations that relatively large amounts of suprathermal and energetic O+ ions are present in the polar magnetosphere. Recently, thermal O+ outflow has been observed at altitudes of 5000–10,000 km together with H+ and He+ ions. The polar wind undergoes four major transitions as it flows from the ionosphere to the magnetosphere: (1) from chemical to diffusion dominance, (2) from subsonic to supersonic flow, (3) from collision-dominated to collisionless regimes, and (4) from heavy to light ion composition. The collisions are important up to about 2500 km, after which the ions and electrons exhibit temperature anisotropies. The direction of the anisotropy varies with geophysical conditions. The polar wind outflow varies with season, solar cycle, and geomagnetic activity. The O+ flux exhibits a summer maximum, while the H+ flux reaches a maximum in the spring. The He+ flux increases by a factor of 10 from summer to winter. At both magnetically quiet and active times the integrated H+ ion flux is largest in the noon sector and smallest in the midnight sector. The integrated upward H+ ion flux exhibits a positive correlation with the interplanetary magnetic field. In the sunlit polar cap the photoelectrons can increase the ambipolar electric field, which in turn increases the polar wind ion outflow velocities. The outflowing polar wind plasma flux tubes also convect across the polar cap. When the flux tubes cross the cusp and nocturnal auroral regions, the plasma can be heated and become unstable. Similar mixing of hot magnetospheric plasma with cold polar wind may result in instabilities. A number of free energy sources in the polar wind, including temperature anisotropy, relative drift between species, and spatial inhomogeneities, feed various fluid and kinetic instabilites. The instabilities can produce plasma energization and cross-field transport, which modify the large-scale polar wind outflow.

The polar wind

In February 1996, the POLAR spacecraft was placed in an elliptical orbit with a 9 R E geocentric distance apogee in the northern hemisphere and 1.8 R E perigee in the southern hemisphere. The Thermal Ion Dynamics Experiment (TIDE) on POLAR has allowed sampling of the three-dimensional ion distribution functions with excellent energy, angular, and mass resolution. The Plasma Source Instrument (PSI), when operated, allows sufficient diminution of the electric potential to observe the polar wind at very high altitudes. In this paper, we describe the results of a survey of the polar wind characteristics for H + , He + , and O + as observed by TIDE at ∼5000 km and ∼8 R E altitudes over the polar cap during April-May 1996. At 5000 km altitude, the H + polar wind exhibits a supersonic outflow, while O + shows subsonic downflow, which suggests a cleft ion fountain origin for the O + ions in the polar cap region. Dramatic decreases of the 5000 km altitude H + and O + ion densities and fluxes are seen as the solar zenith angle increases from 90° to 100° for the ionospheric base, which is consistent with solar illumination ionization control. However, the polar cap downward O + flow and density decline from dayside to nightside in magnetic coordinates suggest a cleft ion fountain origin for the polar cap O + . Cleft ion fountain origin O + density plumes could also be partially responsible for a similar day-night asymmetry in H + , owing to the charge-exchange reaction. At 8 R E altitude, both H + and O + outflows are supersonic and H + is the highly dominant ion species. The average bulk ion field-aligned velocities are in the typical ratio V O+ : V He+ : V H+ ∼ 2: 3: 5, which may suggest a tendency toward comparable energy gains, such as via an electric potential layer.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JA02662

Polar wind survey with the Thermal Ion Dynamics Experiment/Plasma Source Instrument suite aboard POLAR

/pdf/the-polar-wind-recent-observations-4ygpwya2f7.pdf

The polar wind: Recent observations

: We have developed a numerical model to study the steady state behavior of a fully ionized plasma (H+, O+ and the electrons) encompassing the geomagnetic field lines. The theoretical formulation is based on the 16-moment system of transport equations. The electron gas is collision is dominated below 2500 km. Above this altitude electron temperature anisotropy develops with temperature perpendicular to the field line being higher than that parallel to the field line. The H+ ion temperature anisotropy shows H+ temperature parallel to the field line being higher than that perpendicular to the field line. H+ ion temperature also exhibits adiabatic cooling as to the supersonic ion gas cools down as it expands in a diverging magnetic field. Our results are in good agreement with the pervious theoretical studies of the polar wind and recent experimental observations. This is the first successful steady state solution to the 16-moment set of transport equations. Keywords include: Polar wind, Temperature anisotropy, and Adiabatic cooling.

S G Ganguli

Papers

Behavior of Ionized Plasma in the High Latitude Topside Ionosphere.