Fast geomagnetic variations of periods from seconds to hours and days are primarily produced by currents in the ionosphere and magnetosphere. There is always an associated secondary (internal, telluric) current system induced in the conducting ground and contributing to the total variation field measured by ground magnetometers. Mathematically, it is possible to fully explain the variation field by two equivalent current systems, one at the ionospheric altitude and another just below the ground. In practice, this separation is feasible using dense magnetometer networks.
A common way in space physics has been to implicitly neglect the internal part and interpret the ground field only in terms of ionospheric currents. As known from previous studies, this is often a reasonable assumption, since a typical internal contribution is about 30%. However, the situation is much different when the time derivative of the magnetic field (dB/dt) is considered. For the north European IMAGE magnetometer network, the internal part exceeds the external one nearly at all stations. The largest effects due to telluric currents occur at coastal sites close to highly-conducting ocean water and at inland locations close to highly-conducting near-surface anomalies.
This finding gives a new viewpoint for studies of geomagnetically induced currents (GIC), which are closely related to dB/dt. One key question is to understand which are the ionospheric drivers of big GIC events. We will demonstrate how the telluric currents can strongly modify field variations and especially dB/dt, and how this is correspondingly seen in equivalent current patterns. Consequently, it is recommended that the field separation is performed whenever it is feasible, i.e. a dense observation network is available.
