Satellites Get a Charge Out of Space
Field lines from inside the auroral ovals thread outward through regions
known as the polar caps and thence away from the sun through regions known
as the lobes of the magnetospheric tail. Conventional wisdom gained from
numerous satellite flights held that the polar caps and lobes are essentially
devoid of plasma. In fact, the plasma density there is very low compared
with either Earth's ionosphere or the solar wind. 
A spacecraft in sunlight becomes charged by the photoelectric effect, the
same effect used in video cameras, the eye, and solar cells. If a photon
of light has enough energy (that is, its wavelength is short enough), it
will free the electron to generate an electric current.
In space, the sun's ultraviolet light charges a spacecraft's exterior in
the same manner. The spacecraft emits electrons and becomes positively charged
by as much as 40 to 50 electron-volts (40 to 50 eV).
The ions scientists want to observe also are positively charged, but at
a lower voltage than the spacecraft. As an added challenge, the plasma density
drops rapidly as it moves from Earth, making measurements even more difficult.
So, just like the laundry with static buildup, the spacecraft repels the
very ions that we want to measure, and our instruments tell us nothing is
there. No wonder that low energy ion plasmas have not been previously observed
far from the polar caps!
To get around this problem, the Polar spacecraft includes a plasma source
instrument (PSI) which produces a gentle spurt of ionized xenon gas. This
holds the the spacecraft's electric charge (its potential) at about +1.8
V, thereby allowing all but the very lowest energy ions to reach the spacecraft.
The result is quite spectacular.
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November 20, 1996
Authors: Dave Dooling, B.L.
Giles
Curator: Linda Porter
NASA Official: Gregory S.
Wilson