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What is Plasma

Low detail definition:

Plasma is the 4th state of matter, beyond solid, liquid and gas. A plasma is a quasineutral gas containing  positively, negatively and neutrally charged particles. The motions of these particles also depend on the local conditions and also on the plasma conditions away from the region concerned.

When an ionized gas is quasineutral, we mean that the number of electrons is approximately equal to the number of ions (ne ~= ni ~= n, where n is a common density). That is to say that the plasma is almost neutral enough to take this limit, but not neutral enough to remove all the electromagnetic forces.

A plasma is a very hot gas, where some or all of the atoms are separated into ions & electrons. As the negative and positive charges are free to move, plasma is an excellent conductor and is influenced by magnetic and electric fields. Most space plasmas have a very low density, for example the Solar Wind which averages only 10 particles per cubic-cm. Inter-particle collisions are unlikely - hence these plasmas are termed collisionless.

Examples of solar system plasmas include:

  • The Sun's interior plasma (collisional - fusion ) - Fuel and product of solar nuclear fusion.
  • Solar Wind (collisionless) - A stream of particles from the hot solar corona which flows throught the solar system at 400 km/s on average.
  • Cometary Plasma (collisionless at distance from comet) - Outgassing neutral particles are ionized by solar ultraviolet and collisions with the solar wind.
  • Planetary Magnetospheric Plasma (collisionless) - See below
  • Ionospheric Plasma (collisional) - Exterior layer of the atmosphere.

24th May 2006
Andrew Coates

Mullard Space Science Laboratory - Holmbury St. Mary - Dorking - Surrey - RH5 6NT - Telephone: +44 (0)1483 204100 - Copyright © 1999-2005 UCL

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