A. W. Yau

Sources of Ion Outflow in the High Latitude Ionosphere

Ion composition observations from polar-orbiting satellites in the past three decades have revealed and confirmed the occurrence of a variety of ion outflow processes in the high-latitude ionosphere. These processes constitute a dominant source of ionospheric plasma to the Earths magnetosphere. We review the current state of our observational knowledge on their occurrence, energy, composition, variability, interrelationships, and quantitative contributions to the overall mass input to the magnetosphere. In addition, we identify the prevalent sources and the gaps of our current understanding of these sources.

Theories and Observations of Ion Energization and Outflow in the High Latitude Magnetosphere

A review is given of several mechanisms causing outflow at high latitudes of ionospheric ions to the terrestrial magnetosphere. The upward ion motion along the geomaagnetic field can be divided into several categories, including polar wind, bulk ion outflow in the auroral region, upwelling ions and ion conics and beams. More than one ion energization mechanism can be operating within each category, and a combination of categories is important for the total ion outflow.

EXOS D (Akebono) suprathermal mass spectrometer observations of the polar wind

We report observations of the H+, He+, and O+ polar wind ions in the polar cap (>80° invariant latitude, ILAT) above the collision-dominated altitudes (>2000 km), from the suprathermal mass spectrometer (SMS) on EXOS D (Akebono). SMS regularly observes low-energy (a few eV) upward ion flows in the high-altitude polar cap, poleward of the auroral oval. The flows are typically characteristic of the polar wind, in that they are field-aligned and cold (Ti 80° ILAT), the average H+ velocity reached 1 km/s near 2000 km, as did the He+ velocity near 3000 km and the O+ velocity near 6000 km. At Akebono apogee (10,000 km), the averaged H+, He+, and O+ velocities were near 12,7, and 4 km/s, respectively. Both the ion velocity and temperature distributions exhibited a day-to-night asymmetry, with higher average values on the dayside than on the nightside.

Altitude profile of the polar wind velocity and its relationship to ionospheric conditions

The authors report recent results from the Akebono satellite. They present data on polar wind velocities, examined in conjunction with electron properties, as a function of altitude in the ionosphere. This data came from the Suprathermal ion Mass Spectrometer and the Thermal Electron energy Distribution instruments. The measurements show a vertical component to the polar wind, consistent with model results, when measured in terms of H[sup +] ions. There was a definite altitude dependence of the velocity of the hydrogen ions, and there was also a positive correlation of this velocity with the measured electron temperature.

Solar-minimum quiet time ion energization and outflow in dynamic boundary related coordinates

We report hemispheric average fluxes and energies of outflowing energetic (0.015 < E/q < 33 keV) H + , O + , and He + ions in dynamic boundary-related coordinates, from observations obtained by the Polar/TIMAS instrument near 6,000 km altitude in the southern hemisphere during quiet geomagnetic intervals at solar minimum. We discuss our observations in terms of known energization and transport processes. We find that only a small fraction of energetic ions escape from the ionosphere directly into the polar cap and at quiet times the characteristic energies of escaping H + are between 30 and 300 eV in the cusp region and between 30 eV and 1.2 keV in the midnight sector. For O + we conclude the characteristic energy in the cusp is ~100 eV and between 150 and 600 eV in the midnight sector. Our data suggest that the relative energization and acceleration of O + is significantly different in the noon quadrant. The observations and analysis presented here also suggest that O + has activity dependent transport paths from the ionosphere to the ring current that have not previously been identified.

Rotation-vibration level energies of the hydrogen and deuterium molecule-ions

Bound and quasi-bound rotation-vibration level energies, calculated in the adiabatic approximation, are tabulated for H 2 + and D 2 + together with the resonance widths of the less stable quasibound states. Bound levels for HD + , calculated from a single adiabatic internuclear potential, are also listed, together with an estimate of their likely range of validity. Expectation values for the internuclear separations are also presented, graphically.

Polar/Toroidal Imaging Mass‐Angle Spectrograph observations of suprathermal ion outflow during solar minimum conditions

We present observations of the magnitude and variability of escaping suprathermal ions in the energy per charge range of 15 eV/e to 33 keV/e. The data were obtained from the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) on the Polar spacecraft from April 1996 to September 1998 over the Earths southern Polar cap during solar minimum conditions. The net outflow rates of ionospheric ions derived from this data set are significantly different from those inferred from analysis of similar data obtained at higher altitudes from the Dynamics Explorer (DE) 1 satellite. The data present a clear picture of the seasonal variation of ion outflow as a function of solar illumination (i.e., season). We conclude that the differences between the present results and previous DE 1 estimates of the magnitude of escaping suprathermal ions can be explained by energization of the H+ component of the Polar wind above the 6000–8000 km altitude region, where the Polar data were acquired. We also note that seasonal variations in He+ outflow presented here are not as large as those reported previously.

Quiet time solar illumination effects on the fluxes and characteristic energies of ionospheric outflow

[1] We report on the characteristic energy, intensity, and flow rate of escaping ionospheric ions as a function of solar illumination. The data presented here were acquired with the Toroidal Ion Mass-Angle Spectrograph (TIMAS) instrument on the Polar satellite at altitudes of 6000 to 9000 km, during solar minimum. To obtain uniform coverage under various solar illumination conditions, data were restricted to geomagnetically quiet intervals when the Dst index was above -50 nT. We explicitly report data for four magnetic local time ranges. Our investigation confirms many of the characteristics of ion outflows deduced from earlier episodic studies and identifies an anticorrelation in the dependence of beam and conic fluxes on solar illumination, which we attribute to variations in the altitude at which auroral acceleration processes occur. We find that the cusp is an important but not dominant source of ionospheric plasma for the magnetosphere. We conclude that significantly different plasma energization and/or transport mechanisms are dominant in the cusp and the midnight sectors. In addition, we conclude that variations in the solar EUV and geomagnetic energy inputs into the ionosphere, rather than the longer timescale seasonal and annual variations in solar illumination, determine the global rates of H + and O + outflow. The data presented here provide comprehensive and realistic boundary conditions for large-scale magnetospheric models during nonstorm times.

EXOS D (Akebono) observations of molecular NO+ and N2 + upflowing ions in the high‐altitude auroral ionosphere

The authors report on observation of molecular ion drift upward in the ionosphere in regions near to the cleft or auroral oval. In particular they observed NO{sup +}, N{sub 2}{sup +}, and even O{sub 2}{sup +} molecular ion flow upward. These fluxes were typically 5 to 15% of the total ion flux. Molecular ion drift is not observed in all passes through the polar region, and seems to correlate with periods of more intense activity. Their observations are compared with, and correlated with other observations at lower altitudes.

Thermal ion observations of depletion and refilling in the plasmaspheric trough

Thermal (0–25 eV) ion observations in the altitude range 5000 km to 10,000 km and at invariant latitudes greater than ∼60°, made by the Suprathermal Ion Mass Spectrometer (SMS) on the EXOS-D satellite, are used to estimate temperatures, densities, composition, and drift velocities of the local major thermal ion population during a 22-day period in February 1990. This preliminary study indicates that equatorward of a high-latitude boundary, higher-density cold ions corotate with Earth and field-aligned drift velocities are low (<1 km/s). These observations are consistent with a plasmaspheric origin for these ions; however, densities measured near the boundary (10-100 cm−3) suggest that this boundary lies in or near the trough region. Poleward of this boundary, convection patterns deviate from corotation, and large upward directed field-aligned flows of ionospheric plasma, consistent with a “polar wind like” source mechanism, are detected. Typically, parallel drift velocities of 12, 7, and 3 km/s for H+, He+, and O+ are observed. Correlations of these plasma parameters, which were obtained approximately once per day, with magnetic activity (Kp) suggest response times of 2–3 days between the onset of Kp changes and the establishment of new, large-scale, steady state conditions. The implications of these results with regard to ionospheric plasma dynamics associated with depletion and replenishment of high-latitude flux tubes are discussed.