Space plasma phenomena have attracted particular interest since the beginning of the exploration of space about half a century ago. Already a first set of pioneering observations (e.g., Ness, 1969) discovered that matter and electromagnetic fields in space have a complex structure, which was largely unpredicted. Terrestrial and, particularly, spacecraft observations of solar plasmas and fields point in the same direction. In fact, our present picture of the plasma and the electromagnetic fields in space throughout the solar system (and beyond) is that of an extremely complex medium with spatial and temporal variations on large ranges of scales. The wealth of dynamical phenomena observed in space plasmas has steadily increased as more and more refined observational techniques have become available, and it can be expected that important processes still await their detection.

An outstanding class of space plasma phenomena is addressed here under the notion of space plasma activity. Quite generally, in the area of space and astrophysical plasmas the term activity is used for a set of particular magnetospheric, stellar or galactic phenomena, which, although vastly different regarding their space and time scales and their dominant physical processes, have an important characteristic property in common. In all cases they show sudden transitions from relatively quiet states with less pronounced time-dependence to dynamic states in a strongly time-dependent evolution. (Note that this property by no means is restricted to plasma phenomena, volcanic activity being a prominent example from another discipline.)