Meridian chains of magnetometers as a powerful “remote sensing” tool in determining electromagnetic quantities in the ionosphere on a global scale

The meridian chains of magnetometers have proven to be a powerful tool in determining the distribution of the ionospheric currents, the field-aligned currents, the electric potential, the Joule heat production rate, and the auroral particle injection rate over the entire polar region, with a time resolution of about 5 minutes or even less. A brief history behind the development of this remote sensing method is given, which includes the pioneering efforts by K. Birkeland and S. Chapman, the Second Polar Year projects, IGY efforts, IMS meridian chains, and finally, the computer code developments during and after the IMF Data Analysis Phase. There is still much to improve in this remote sensing method, but it has become complementary to in-situ satellite observations.