A macroinstability is an instability that has a length scale comparable with an equilibrium length. Since there is a considerable variety of macroinstabilities, even a brief description of each instability would break the present scope. So we concentrate on instabilities that seem to play an important role in space plasma activity.

We begin with a brief discussion of stability concepts and then turn to particular dynamical models and resulting instabilities.

Since changes of the magnetic topology are believed to play an important role for activity phenomena, considerable room is given to instabilities that involve such changes, covering both fluid and kinetic models.

An instability that changes magnetic topology is particularly relevant if the system considered is stable with respect to topology-conserving instabilities. This motivates the inclusion of ideal MHD modes.

Stability concepts

In general terms, a stable steady state is characterized by its robustness against external perturbations, while for an unstable steady state there exists at least one perturbation that leads to substantial changes, which in some cases have dramatic consequences. Turning such qualitative statements into quantitative notions requires operational definitions of stability and instability. There are several possibilities of such definitions.

One line of approach is based on exponential modes as used in our discussion of microinstabilities (Section 9.3.1). This approach considers timedependence of the form exp(–iωt); stability corresponds to Im(ω) ≤ 0, instability to Im(ω) > 0.