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A plasma instability involves plasma waves that grow exponentially or faster. For proper discription of a particular instability, one should be able to define
Because of all these points of view, the nomenclature for plasma instabilities is even more cumbersome than for the wave modes themselves. A solid understanding of plasma theories is needed in order to study the formation of different instabilities. One important way to classify different instabilities is to devide them into macroinstabilities and microinstabilities. A macroinstability is driven by the structure of the medium in configuration space. A familiar example of a macroinstability is for a convectively unstable system: when the temperature gradient is superadiabatic, internal gravity waves grow to large amplitude and cause a large-scale convection of the fluid, which tends to reduce the temperature gradient. Other familiar examples are the Rayleigh-Jeans instability, in which a denser fluid is supported by a less dense fluid, and the Kelvin- Helmholtz instability, in which one fluid flows over another fluid, e.g., wind over water, causing surface waves to grow. In plasmas the macroinstabilities occur in the low-frequency regime and usually involve the magnetic field. Examples include flute (or interchange) and ballooning instabilities. The latter ones are used, for example, in some substorm models.
Microinstabilities, on the other hand, are usually driven by a velocity space anisotropy in the plasma. A consequence of a microinstability is a greatly enhanced level of fluctuations in the plasma associated with the unstable mode. These fluctuations are called microturbulence. Microturbulence can lead to enhanced radiation from the plasma and to enhanced scattering of particles resulting in 'anomalous' transport coefficients, e.g., anomalous electric and thermal conductivities. The simplest example of a microinstability is a beam-driven instability in a unmagnetized plasma.