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Plasmas Can't Hide

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Plasmas Can't Hide From Neutralized TIDE

As its name suggests, the Polar spacecraft orbits over the Earth's poles. The orbit is tailored so the apogee (highest point) is over the North Pole. Because this area is of great interest, scientists have ringed it with ground-based instruments and launch sites for research rockets so we can study the magnetosphere from several angles at once. (Other satellites, like Wind, stationed out in the solar wind, and Geotail, probing deep into the tail of the magnetosphere, are at work, too.)
As Polar loops to its apogee, it crosses the cusp region where solar wind plasma enters directly and penetrates along field lines to the dayside auroral oval. To &quotsee"these plasmas, we take measurements with the Thermal Ion Dynamics Experiment (TIDE) instrument on Polar and then convert the data into color graphs. These could pass for abstract art, but plasma physics scientists can read them almost as easily as most people can read satellite weather pictures.

In the plot shown here, the cusp appears at the left side as a bright white region (high fluxes over a wide range of angles, for both hydrogen ions and doubly ionized helium ions). This figure includes more than 10 hours of data as Polar slowly arcs to apogee at nearly 9 Earth radii (about 57,600 km [36,000 miles]) from the dayside to the nightside of the polar cap .

Remember, too, that Polar carries its own miniature plasma source instrument (PSI) to eliminate static buildup. When the plasma source - a trickle of xenon stored on the satellite and ionized before it is released - is turned off, the polar cap appears to be devoid of any observable plasma flux because the spacecraft's own charge repels the low-energy plasmas.

When the spacecraft plasma source is on, we see a supersonic ionospheric plasma flowing out of the polar cap (although the atmospheres is too thin to carry sound, supersonic is still an important measure because it relates to how even thin gases compress and form shock waves). These outflows are depicted in the figure as a series of bright green strips that descend in energy (see the right edge of the plots). Because Polar rotates, TIDE faces into the line of flight - and you see the bright green stripe - then away, then back into it (the data are adjusted to cancel Polar's own motion so the plot shows what you would get if you were hovering in space).

We see significant plasma flux only when TIDE looks down along the local magnetic field (not always the same as down at the Earth because the field lines curve), not when it looks sideways or up. This means the low is up, away from the Earth's atmosphere and into deep space. Higher energy, narrowly collimated flows of oxygen and helium are also seen. The hydrogen flows have a flow velocity of about 108,000 km/h (67,500 mph). And within that flow, the temperature corresponds to atoms moving at 36,000 km/h (57,600 mph).

Thus, the polar caps, though low in plasma density, are anything but empty. They carry substantial flows of ionospheric plasma upward and then down the geotail to where plasma storms are energized.

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November 20, 1996

Authors: Dave Dooling, B.L. Giles
Curator: Bryan Walls
NASA Official: John M. Horack