Ionospheric and telluric currents are equally important causes of geomagnetic variations

Fast changes of the magnetic field seen at the ground surface are primarily caused by ionospheric currents related to aurora, but secondary induced currents flowing in the electrically conductive ground and oceans play a role too. That role has now been found to be larger than previously believed.

By measuring variations in the magnetic field at the surface of the Earth it is possible to estimate the spatial and temporal behavior of the ionospheric currents flowing at approximately 100 km altitude. This is an important topic in Space research, but the estimates are obscured the fact that changes in the external ionospheric field always create secondary induced currents in the surface layers of the Earth, which modify the measured magnetic field. Mathematically, it's possible to separate these internal and external sources from the measured total magnetic field, if there are enough measurements from an extended area. The IMAGE magnetometer network operating in the Nordic countries provides such data. By analyzing 25 years of IMAGE measurements, it was found that in slow variations the ionospheric currents are the dominant source, as expected. However, a new finding was that in fast variations the contribution from the internal induced currents was systematically larger than that of the ionospheric currents. Details depend on the geological and geophysical conditions in the vicinity of each IMAGE magnetometer stations. It is very likely that similar behavior of the magnetic field variations can be seen also elsewhere in the world, and this should be taken into account when estimating ionospheric electrical currents from magnetic measurements.

The study was carried out in collaboration with the Finnish Meteorological Institute and the Luleå University of Technology. It was funded by the Academy of Finland.

Original article:
Juusola L., H. Vanhamäki, A. Viljanen, and M. Smirnov.: Induced currents due to 3D ground conductivity play a major role in the interpretation of geomagnetic variations, Ann. Geophys., 38, 983-998,, 2020.

Last updated: 1.2.2021