A plasma is often called the 'fourth state of matter' because of its unique properties that distinguish it from the other three.
The dominant form of matter in our universe is actually plasma.
A plasma is created by taking an ordinary gas of atoms and heating it so that the atoms collide with enough energy to shed one or more of their electrons. the new 'gas' is a mixture of electrons and ionized atoms. Plasmas can be easily found on the surface of the sun where temperatures are above 5,770 Kelvins and hydrogen atoms collide with enough energy to 'collisionally ionize' some fraction of the neutral hydrogen atoms. In interplanetary space, the ejected plasma from the solar corona can persist as a plasma because the electrons and ions are few and far between and rarely collide to re-form neutral hydrogen atoms.
Plasmas are also found within the magnetospheres of many of the planets where the solar wind and other planetary sources of ions, create a trapped population of particles held in 'magnetic bottles' formed by the planetary field. The ionosphere of the earth is also a weak plasma consisting of a mixture of neutral atoms, ions and a thin layer of free electrons generated by the ultraviolet light from the sun.
When plasmas are threaded by a magnetic field, as they often are in many locations in the solar system and in stars, the 'magnetoplasma' system can behave in a complex way as the flow of plasma produces its own magnetic fields which interact with existing ones. Plasmas of a given density and magnetic field have a set of natural frequencies that they prefer to oscillate at, and when electromagnetic waves impact these plasmas, they can behave as mirrors or amplifiers of the incident energy.
For more about plasmas, visit the American Institute of Physics The Pervasive Plasma State illustrated primer.
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