Plasma of terrestrial origin is produced chiefly by either solar photo-ionization or electron bombardment ionization of the Earth's upper atmosphere, forming the ionosphere. Under many conditions, components of the higher-altitude ionosphere acquire sufficient energy to escape the Earth's gravitational attraction and flow out along geomagnetic field lines into various regions of the magnetosphere, and perhaps beyond, into the solar wind. Over the past two decades, intense observational and modeling efforts by numerous researchers have sought to understand the processes driving these ionospheric outflows and the resulting presence and consequences of these ionospheric plasmas populating the major regions of the magnetosphere.
The matter of ionospheric outflows directly influences
two significant thrusts of current magnetospheric research. First,
the exciting concept of
imaging magnetospheric plasmas
is now believed to be feasible and is planned for future NASA missions.
Some of the envisioned imaging approaches are based on detection of
solar EUV radiation resonantly-scattered from
ionospheric-origin He
and O
ions. Second, there is the major
unresolved issue of whether the ionospheric outflows supply
much of the plasma content of such regions as the ring current and
the plasma sheet, and exactly what the geophysical consequences
of the significant presence of heavy ionospheric He
and O
ions
might be on general wave-particle processes and substorm triggering
and amplification in the tail plasma sheet.
These plasmas of ionospheric origin are important to us in other ways. In the absence of cold dense plasmas as supplied by the ionosphere, the multitudes of communications spacecraft at geosynchronous orbit may be charged to kilovolt negative electrical potentials in the presence of hot magnetospheric electrons there. Subsequent arcing discharges can then cause telemetry upsets or damage to sensitive spacecraft components. Also, it has long been suspected that the terrestrial presence of helium may be controlled by ionospheric plasma outflow loss mechanisms.
We confine our discussion to three topical areas which significantly relate to the above general magnetospheric thrusts: 1. Polar and cleft plasma outflows; 2. Outer plasmasphere/ring current dynamics and interactions; and 3. Predictions and evidence for ionospheric plasma flows into the ring current, plasma sheet and low-latitude boundary layer. Auroral ionospheric outflows are considered by Shelley [[this issue]], while most aspects of waves and wave-particle interactions should be reviewed by Engebretson [[this issue]]. The previous IUGG reports appeared to cover up to about November 1990, and so the coverage here will start at refereed papers for December 1990 through about September 1994. Coverage is restricted to publications involving U. S. authors. To help meet the required reference limit in a somewhat objective way, we chose also to restrict coverage to the prime relevant U. S. journals for magnetosphere-ionosphere research, the Journal of Geophysical Research and Geophysical Research Letters, though this will undoubtedly exclude important publications in monographs and non-U. S. journals.