Aurora borealis.

To enable thermonuclear fusion, matter must be heated to extremely high temperatures. But what happens to matter as it becomes hotter and hotter? In a solid, its constituent particles vibrate about fixed positions. As more thermal energy is supplied, the structure breaks down and the substance melts, becoming a liquid. With further heating, the liquid evaporates and becomes a gas. If enough energy is supplied, molecules may dissociate and atoms become ionised as electrons are removed from them. The gas then turns into plasma. Plasma is an ionised gas consisting of free electrons and ions, and it may also contain neutral atoms or molecules.

Discovery

Plasma was first identified in 1879 by William Crookes, who called it “radiant matter”. The term “plasma”, derived from the Greek word meaning something formed or moulded, was introduced by Irving Langmuir in 1928.

The Universe is full of plasma.

Properties

Because plasma is composed largely of charged particles, it has distinctive properties. It is quasineutral and exhibits collective behaviour. Quasineutrality means that the total positive and negative charge densities are approximately equal, so on scales larger than certain characteristic plasma lengths, the plasma appears electrically neutral. Collective behaviour arises because charged particles interact through electromagnetic fields.

In a magnetic field, charged particles move in helical paths around magnetic field lines. The radius of this motion, called the Larmor radius, depends on the particle’s mass, charge and velocity, as well as on the magnetic field strength. Because ions are much more massive than electrons, their Larmor radii are generally larger under comparable conditions. Moving charged particles also generate electric and magnetic fields of their own.

In a plasma, large numbers of charged particles interact through the electric and magnetic fields they collectively produce. As a result, the plasma can respond as a whole rather than as a collection of independent particles. This collective behaviour gives rise to waves, instabilities and fluid-like motion, and plasma can therefore be described using several different physical models.

Formation

Besides heating, plasma can be created in several other ways: by energetic particles or radiation, by photoionisation when photons have enough energy to remove electrons from atoms, by collisional ionisation in strong electric fields, or by pressure ionisation in extremely dense matter, such as that found in compact stellar objects.

Types of Plasma

Lightning.

Plasma is characterised by its temperature, density, and degree of ionisation. It is very abundant in outer space but very rare on Earth. Typical naturally occurring plasmas on Earth include lightning channels, auroras and the ionosphere. Some flames can also behave as weakly ionised plasmas, depending on their temperature and composition. Lightning channels can reach temperatures of around 30,000 K. The ionosphere is an ionised region of the upper atmosphere extending roughly from about 60 km to several hundred kilometres above the Earth, with its upper boundary depending on the definition used. It is produced mainly by solar ultraviolet and X-ray radiation. In the F region, electron densities can reach around 10¹² m⁻³. Auroras are produced when energetic charged particles guided by the Earth’s magnetic field enter the upper atmosphere and collide with atmospheric atoms and molecules, causing them to emit light.

Plasma is extremely abundant in space: stars, stellar winds, nebulae and much of the interplanetary and interstellar medium consist largely of plasma. More than 99 percent of the observable ordinary matter in the Universe is estimated to be in the plasma state, whereas naturally occurring plasma is relatively rare near the Earth’s surface.