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Image: Figure from Nature Vol. 414 13 Dec 2001 p. 724. Background: an aurora seen from the Space Shuttle


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Jan 12, 2005
Seeing Circuits (1)

Caption: The blue lines represent the auroral current system. Together with the ionospheric closure current and the magnetospheric generator these form the complete auroral current circuit. The figure shows a north–south section through the structures that usually extend several hundreds of kilometers in the east–west direction.

Before the first satellites were put into orbit, most scientists believed electrical phenomena couldn't exist in space. When those first satellites encountered streams of charged particles, the inertia of that prior belief prevented the scientists from seeing the streams as electrical currents: The streams were named "radiation belts".

Magnetic fields in space are easy to measure; electric currents are not. So the movements of particles in the radiation belts were described in terms of magnetic fields, and the fact that moving charges constitute a current was ignored. But if charges move from one place to another, they will soon build up an excess of charge in that other place and repel any additional charges, therewith stopping the movement. For particles to continue to move, their path must close in a circuit. And if a circuit exists, the description of the particles' state at any point depends not only on the local conditions but also on what's happening in the rest of the circuit. For example, if a double layer forms and explodes, all the energy of the circuit will be released in it, an amount that can be vastly greater than the energy of the double layer itself.

Hannes Alfven, the father of plasma cosmology, has identified several interacting circuits in the Earth's magnetosphere. The aurora is the visible part of one of these. Because the circuits radiate--that is, lose--energy, they must be powered by some source of energy (see diagram above). That source is a current from the Sun, a part of a larger circuit. Again, the inertia of prior belief prevented astrophysicists from seeing the moving charges of this current as a current, and it was named the "solar wind."

Double layers--capacitor-like formations in plasma--are particularly interesting because they have no noticeable magnetic effects. They can only be identified by sending probes through them. Astrophysicists who map magnetic fields and assume there's no electricity in space (or if there is, "it doesn't do anything") will have no idea the double layers exist.

But the double layers' electrical effects are large and many. They store and discharge energy; they radiate "noisily" (over broad bands of frequencies); they accelerate charged particles; and they divide plasma into cells of like properties, often separating small variations in such parameters as temperature or density or chemical composition. When astrophysicists observe these effects, they have to posit a multitude of mechanical devices to explain them: black holes, magnetic reconnection, neutron stars, frozen-in magnetic field lines, shock waves, etc.

All these effects consume energy. They act as loads in a circuit and hence must be supplied with power from some other part of the circuit or through a coupling with a more powerful circuit. Where is the power source?

(To be continued in Part 2)


David Talbott, Wallace Thornhill
Amy Acheson
  CONTRIBUTING EDITORS: Mel Acheson, Michael Armstrong, Dwardu Cardona,
Ev Cochrane, C.J. Ransom, Don Scott, Rens van der Sluijs, Ian Tresman
  WEBMASTER: Michael Armstrong

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