Ionospheric and magnetospheric current systems create disturbances in the geomagnetic field. Thus, by measuring these effects with ground based magnetometers, one can monitor the magnetospheric activity (e.g., the development of geomagnetic storms and substorms) continuosly. For example, several geomagnetic indices are calculated from these measurements. It is customary to arrange the instruments in meridional chains, like is the case with the IMAGE chain in Finland (managed by the Finnish Meteorological Institute), and SAMNET chain in United Kingdom. CANOPUS is a big Canadian array of magnetometers, while Intermagnet is a project to promote the exchange of magnetic observatory data around the world.
The magnetic field perturbations are usually resolved along a geomagnetically north-south (positive north), east-west (positive east), and and parallel to B directions (in the northern hemisphere) and are donoted by H, D, and Z components, respectively. Sometimes a geographic coordinate system is used, in which case the symbols X, Y, and Z denote the magnetic perturbations in the north, east, and vertical (positive down) directions.
It is impossible to derive the true horizontal ionospheric current distribution uniquely from ground magnetic perturbations, since they are a superposition of contributions from the horizontal ionospheric currents, field-aligned currents, distant currents in the magnetosphere, and currents induced in the Earth's surface. For these reasons the ground magnetic perturbations are usually expressed in terms of "equivalent" ionospheric currents.
The time resolution of magnetometers are typically about 10 seconds. Pulsation magnetometers, which are used to study geomagnetic pulsations at ULF range, have much better resolution, about 0.1 seconds.