Induced currents due to 3D ground conductivity play a major role in the interpretation of geomagnetic variations

Abstract

Abstract. Geomagnetically induced currents (GICs) are directly described by ground electric fields, but estimating them is time-consuming and requires knowledge of the ionospheric currents and the three-dimensional (3D) distribution of the electrical conductivity of the Earth. The time derivative of the horizontal component of the ground magnetic field (dH∕dt) is closely related to the electric field via Faraday's law and provides a convenient proxy for the GIC risk. However, forecasting dH∕dt still remains a challenge. We use 25 years of 10 s data from the northern European International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network to show that part of this problem stems from the fact that, instead of the primary ionospheric currents, the measured dH∕dt is dominated by the signature from the secondary induced telluric currents at nearly all IMAGE stations. The largest effects due to telluric currents occur at coastal sites close to high-conducting ocean water and close to near-surface conductivity anomalies. The secondary magnetic field contribution to the total field is a few tens of percent, in accordance with earlier studies. Our results have been derived using IMAGE data and are thus only valid for the stations involved. However, it is likely that the main principle also applies to other areas. Consequently, it is recommended that the field separation into internal (telluric) and external (ionospheric and magnetospheric) parts is performed whenever feasible (i.e., a dense observation network is available).

Modeling the Geomagnetic Response to the September 2017 Space Weather Event Over Fennoscandia Using the Space Weather Modeling Framework: Studying the Impacts of Spatial Resolution

“…In this study the goal is to compare the results from multiple SWMF runs, so the conclusions should be unaffected. Nevertheless, it may be important for readers and future studies to mention that Juusola et al (2020) have recently shown, using IMAGE magnetometer data, that the horizontal dB/dt is dominated by telluric currents nearly at all IMAGE stations. This means that much of the spatial and temporal structure seen in the total measured dB/dt comes from underground.…”

Spatio-temporal development of large scale auroral electrojet currents relative to substorm onsets

“…Using the the Swarm data in the substorm context provided by SuperMAG will enable this study to focus on the substorm time DF currents. Swarm also provides a different view of the currents compared to ground based magnetometers as the latitudinal coverage of the auroral oval crossing is not dependent on the network density and the effect of ground induced currents to the magnetometer measurements, which can sum up to tens of percents of the total field strength at ground level (Juusola et al (2020)), is subdued at Swarm altitudes.…”

Comparing three approaches to the inducing source setting for the ground electromagnetic field modeling due to space weather events

“…With the help of the SECS technique, it is possible to separate the measured magnetic field into external and internal parts, which are represented by two equivalent sheet currents placed in the ionosphere and underground (Juusola et al, 2020).…”

“…Two models approximate the source by a laterally varying sheet current flowing above the Earth's surface. One of the models is built using the results of physics-based 3-D MHD simulation of the near-Earth space, another model uses the IMAGE magnetometer data and the Spherical Elementary Current Systems (SECS) method (Juusola et al, 2020;. The third modeling is based on a "plane wave" approximation which assumes that the source is locally laterally uniform.…”