Nov 3, 2006

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.

November 20, 1996

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