A large number of indices exist to categorize space weather in statistical, global, time- and space-averaged ways. The reasons for attempting to develop approximate indices include the following: (1) the details of most phenomena differ from event to event, so that only approximate representations are sought at first, (2) many of the phenomena depend strongly on the observer's location and local time, so that both a large number of observers and approximate global descriptions are necessary, (3) rapid time variations occur and attempts must be made to disentangle the effects of movement of the phenomenon from intrinsic time variations.
Here only a small number of indices are mentioned. One is the index, which measures the strength and time variations of the ring current in terms of the associated changes measured in Earth's magnetic field at latitudes near to (but not at) the magnetic equator. Another is the Kp (or ) index which nominally measures the range of variation of particular magnetic field components associated with phenomena other than diurnal changes, yearly variations, and the slow recoveries associated with longterm storm variations. Both and are averaged over many observation sites to obtain ``planetary'' measures of activity.
Another is the AE (Auroral Electrojet) index which measures changes in the magnetic field observed at Earth's surface at auroral latitudes. Related indices are the AU and AL indices, which measure the upper and lower envelopes of the disturbance at a given site, which are then averaged together to form AE. These indices all measure geomagnetic activity; i.e., variations in the magnetic field observed at Earth.