Electric currents flowing along polar geomagnetic field lines are an important mechanism to transfer energy between the magnetosphere at several Earth radii and the ionosphere, the ionised part of the upper atmosphere at 100-300 km altitude. These currents cannot be detected on ground, but they produce significant signatures in magnetic field observations on board Low-Earth-Orbiting satellites. High precision magnetic field missions, like CHAMP and Swarm, have been used in the past to characterise these currents. An international consortium takes now the initiative to calibrate data from satellites caring magnetometers that have not been designed for scientific applications but for navigation purpose (Cryosat, GOCE, GRACE-FO, and many others). Still, the current signature can be extracted after careful calibration. The combination of data from several satellites provides un-precedented global coverage and is expected to strongly enhance the monitoring of the polar ionosphere and its interaction with the magnetosphere. For this purpose, we have with a continuous global data set from the year 2000 until today.
Combining data of different sampling rate, signal amplitude, noise level, and latency in one mapping procedure requires special care for data handling and inter-calibration to achieve an unbiased result uniformly valid over the globe. The amplitude of the signal detected may change over the last decades due to descending satellite orbits, architectural settings of missions and by naturally varying solar flux levels.