Plasma--The Fourth State of Matter

In science classes, only three states of matter are commonly described: 1) solids, which have definite size and shape; 2) liquids, which have a definite volume and no definite shape; and 3) gases which have no definite size or shape. Ice is a solid. It turns to water, the liquid state, when heated and if heated further, the water turns to steam, the gaseous state.

What happens if the temperature of a gas is raised above temperatures that exist on the sun? As a gas' temperature is raised to over 10,000°, its molecules collide so violently that they are broken apart into individual atoms. The negatively charged electrons are knocked completely off the atoms. It is at this point that the plasma state is reached.

So besides being very hot, a plasma is distinguished from a gas by the fact that a plasma's movements are affected by electric and magnetic fields. When the positively charged ions and electrons of a plasma are knocked off the atoms, they can create long-range electric fields. Moreover, the interaction between the ions and the electrons form an electric current; this, in turn, is a source of a magnetic field. The fact that a plasma can generate electric and magnetic fields and, in turn, be affected by them gives rise to a wide variety of new phenomena for which there is no parallel in the other three states of matter.

The earth is composed of all four states of matter. We are familiar with solid rocks and liquid oceans. The air that constitutes our atmosphere is in the gaseous state. Above the atmosphere, however, matter exists in the plasma state. The plasma that is dominated by the earth's magnetic field constitutes the magnetosphere. Through a study of the dynamics of the plasma in the magnetosphere we are beginning to understand the phenomenon responsible for the aurora.