As the atmosphere becomes thinner with altitude, it becomes easier for ultraviolet rays to ionize atoms of hydrogen, helium, nitrogen and oxygen. When this happens, the atoms become charged and they eject some of their electrons. This mixture of ionized atoms and electrons is called a
States of Matter: The Mysterious Plasma State
It sounds like a mysterious word, and unless you watch a lot of science fiction programs on TV you have probably not heard this word very often. The truth of the matter is that, even though it is a rare word, in fact, all of the visible universe is in this so-called fourth state of matter beyond the familiar Solid - Liquid - Gas phases.
If you heat a gas hot enough, the molecules in it will collide so violently that they will be shaken apart. The individual atoms like oxygen and nitrogen will lose one or more of their many electrons that orbit their nuclei. Every atom consists of a heavy nucleus that contains one or more protons and neutrons. This dense core is what gives an atom nearly all of its mass. Surrounding the nucleus is a cloud of electrons; one electron for every proton in the nucleus. You now have a 'neutral' atom because there are just as many negative electrons as there are positively-charged protons.
If you hit this atom hard enough with another atom, energetic particle or even a particle of light (photon) you can knock one or more of the electrons out. If the atom had 8 protons and 8 electrons (oxygen for example), knocking one of the electrons out, makes 8 protons and 7 electrons. The atom now has a net charge of +1 because the 7 electrons cancil the positive charges of 7 of the protons in the nucleus, but there is one remaining proton with a positive charge. If you removed 6 electrons from the oxygen atom, it would have a positive net charge of +6. Atoms that have a net charge are called ions. So, a plasma is a gas in which some, but not necessarily all, of the atoms have been converted into ions.
It is an invisible, donut-shaped zone barely distinct from the upper atmosphere, where low-energy particles trapped on particular magnetic field lines bounce and drift around Earth. This continues until some space storm strips them off their lines of magnetism, but there are always more ionosphere ions ready to fill these field lines back up again when the storm passes. It takes about a day or so for the plasmasphere to heal itself after a solar storm.
The plasmasphere can be detected in satellite images such as those provided by IMAGE, out to a distance of about 3 times the Earth's radius ( 3Re or 21,000 km) from Earth. Although the innermost edge seems permanent, its outer edges come and go with solar storms as its contents are shed and refilled from day to day. At a distance of about .... Re, we encounter a boundary to the plasmasphere called the plasmapause. Here, and interior to it, plasma trapped on magnetic field lines is forced to rotate exactly in step with Earth as its magnetic field sweeps 360 degrees every 24 hours. No spacecraft controlled by gravity alone could ever keep up with this motion...(check this)
Here's a great page at Perdue University that shows the differences between solid-liquid-gas better.
Visit NASA's Space Place for a great discuussion about ions. Did you know that the Deep Space 1 spacecraft used an Ion Engine to propell itself through space, just like in science fiction stories?