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

Liisa Juusola,Ari Viljanen,Maxim Smirnov,Heikki Vanhamäki

doi:10.5194/angeo-38-983-2020

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).

Highlights

Fast geomagnetic variations at periods from seconds to hours and days are primarily produced by currents in the ionosphere and magnetosphere
The locations of the International Monitor for Auroral Geomagnetic Effects (IMAGE) stations used to construct the maps are shown with black squares, and the Sodankylä (SOD) station is highlighted with a thicker line
Whereas the ionospheric currents are clearly oblivious to the conductivity structure of the Earth, the telluric currents are strongly affected by it

Summary

Introduction

Fast geomagnetic variations at 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. It is possible to fully explain the variation field by two equivalent current systems, namely one at the ionospheric altitude and another just below the Earth’s surface (e.g., Haines and Torta, 1994). This separation is feasible using dense magnetometer networks (Pulkkinen et al, 2003b; Stening et al, 2008; Juusola et al, 2016). The external and internal fields are often approximately in phase, in which case the dynamics of the ionospheric current systems can be estimated reliably without carrying out the separation

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Discussion

Conclusion