1. Potsdamer erdmagnetische Kennziffern;Bartels;Zeitschrift für Geophysik,1938
2. The three-hour-range index measuring geomagnetic activity;Bartels;Terrestrial Magnetism and Atmospheric Electricity,1939
3. BCMT Bureau central de Magnetisme Terrestre2014Available at: http://www.bcmt.fr/ (Last accessed 20 June 2016)
4. Visualization of earth main magnetic field maps on a spherical display;Berezko;Russian Journal of Earth Sciences,2011
5. The evolution of the Earth’s magnetic field;Bloxham;Scientific American,1989