Earth environment, encompassing the ionosphere, magnetosphere, and solar wind, is filled with dynamically varying plasma through which electric currents are continually flowing. Changes in the state of this system result in changes in its electrical current that can be monitored remotely through the magnetic perturbations they produce. For this reason it is common practice to use geomagnetic data obtained at the earth's surface to diagnose processes occurring in our plasma environment.
In the first half of this century the pioneering work of K. Birkeland, soon followed by S. Chapman and H. Alfvén, attempted to estimate the electric current system responsible for ground magnetic disturbances. Studies of ground-based magnetic records have been vital in understanding the processes occurring in the magnetosphere-ionosphere system, especially the growth and decay of the three-dimensional current distribution, which consists of ionospheric currents and field-aligned currents. In the past, studies on this subject were primarily based on the statistically obtained, so-called ‘equivalent’ current systems, which can be determined by assuming all currents flow in a spherical shell (i.e., the ionosphere) concentric with the earth. Owing to the significant improvement of the ground magnetic networks, as well as recent direct measurements of the field-aligned currents and electric field by satellites and radars, it is now possible to infer the three-dimensional current system over the polar region during individual magnetospheric substorms with a sufficient time resolution. Meanwhile, various numerical techniques have been developed to analyze global magnetometer data for the purpose of facilitating this inference. It has taken more than half a century to quantify these pioneering concepts of Birkeland, Chapman, and Alfvén, although much improvement is still needed for better accuracy.