More than 95% of the geomagnetic field at Earth’s surface is created by dynamo action in the Earth’s fluid outer core; the remaining few percent are due to magnetized rocks in the Earth’s crust and due to electric currents in the Earth’s ionosphere and magnetosphere. Tidal winds in the ionosphere move a conductor (the ionospheric plasma) through the field lines of the Earth’s magnetic field, which causes electric currents in the ionosphere (“ionospheric dynamo”). These currents cause a magnetic field variation that is measured by geomagnetic stations. The tidal winds (and the corresponding currents and magnetic field variations) consist of a solar component (mainly due to thermal forcing, with heating on the dayside and cooling on the nightside, and a small contribution due to gravitational forcing by the sun) and a lunar component (which is entirely due to gravitational forcing by the moon). The solar contribution is about 20 times larger than the lunar contributions and can be directly seen in the magnetogram of individual days; the lunar contribution is only visible after stacking of data from several months or years of data.
Professor -afdelingsleder for Geomagnetisme