During the Gjøa and Discovery expeditions (1901–1906), weak geomagnetic signatures were observed simultaneously with visual polar cap auroras.  Main types are transpolar arcs, poleward-moving substorm arcs, and Sun-aligned arcs.  The first two are infrequent, while the last one occurs often. They appear like the regular magnetic solar Sq variation.  It is suggested that they are embedded in polar rain and dependent on the density and energy of photoelectrons.

The DC-powered service of the Berlin suburban railways disturbed the observations in Potsdam and Seddin. A suitable location for the establishment of a new observatory was found near the small town of Niemegk. The "Adolf-Schmidt-Observatorium für Erdmagnetismus Niemegk" was officially opened on 23 July 1930, Adolf Schmidt’s 70^th.  The observations started on 1 January 1932. The Niemegk measurement series spans 92 years, only interrupted at the end of World War II.

The explanation of thunder and lightning in encyclopedias from ancient times to Wikipedia is described and discussed.

In December 2021, the agonic line crossed the Royal Observatory of Madrid on its westward drift. The previous crossing by the agonic line, drifting eastward, is not accurately dated because declination measurements of that time are scarce. After collecting the data obtained in the Iberian Peninsula over the last 500 years and reducing them to ROM coordinates, a geomagnetic declination curve was generated. As a result, the previous cross ing been the agonic line was dated around the year 1652.