Title: 50 Years of Space Plasma Observations: What Next?

Dr. Tom Moore
Goddard Space Flight Center

Before the space age of orbiting spacecraft, we thought the ionosphere was a thin layer confined to altitudes below 1000 - 2000 km altitude, based on sounding rocket probes. The magnetosphere appeared to be a cavity into which leaked a small amount of solar wind plasma. Beginning with the Russian Lunik 2, K Gringauz found that the ionosphere extends out to about 4 Earth radii before dropping steeply in density, and this was confirmed by remote sensing using radio waves from lightning strokes by D Carpenter. Since this pioneering work at the beginning of the space age, we have known that ionospheric plasmas expand into the magnetosphere at high altitudes inside and beyond the plasmapause. These plasmas were thought to be limited to the lighter ions not bound by gravity to the Earth, and a theory was developed of the plasmasphere and the light ion polar wind that has been largely confirmed by high altitude observations. But the first plasma composition measurements showed that oxygen ions not only reach high altitudes but also acquire energies comparable to those of alpha particles that most likely originate from the solar wind. Then observations of the auroral zones showed that ions of all masses, including at times molecular species, are heated and accelerated within one Earth radius altitude, causing their escape. Diverse processes have been shown to act from the F-region peak up to several Earth radii, including i) electron heating by soft electron precipitation and ii) electromagnetic energy inputs that drive plasma through the neutral gas and heats the ions resonantly. These processes are well enough understood to specify ionospheric plasma circulation throughout the magnetosphere. From this we are learning that ionospheric plasmas are not just a thin layer for sinking solar wind energy into the neutral gas of the upper atmosphere. They also expand into the magnetosphere and fill it during active periods owing to large amounts of solar wind energy dissipation. These plasmas are heated further as they circulate in the magnetosphere. Their pressure is at times observed, and can now be simulated, to exceed the pressure of the solar wind. Thus the magnetosphere acts as a pressure vessel that is filled by the heated expanding ionosphere, inflating it substantially as it seeks escape from both the gravity of Earth and its geomagnetic plasma trap.

UT Arlington Physics