An Introduction to Geomagnetically Induced Currents

Earth‐directed space weather is a serious concern that is recognized as one of the top priority problems in today’s society. Space weather‐driven geomagnetically induced currents (GICs) can disrupt operation of extended electrically conducting technological systems. This threat to strategic technological assets, like power grids, oil and gas pipelines, and communication networks, has rekindled interest in extreme space weather. To improve national preparedness, it is critical that we understand the physical processes related to extreme events in order to address key national and international objectives. This paper serves to provide a basic introduction to space weather and GICs, and highlights some of the major science challenges the GIC community continues to face. 1 Department of Physics, The Catholic University of America, Washington, DC, USA 2 Goddard Space Flight Center, Space Weather Laboratory, National Aeronautics and Space Administration, Greenbelt, MD, USA 3 Now at Atmospheric and Space Technology Research Associates, Louisville, CO, USA Key Points • Geomagnetically induced currents (GICs) is a space weather‐driven phenomena. • It is a threat to strategic technological assets, such as power grids, oil and gas pipelines, and communication networks. • This paper serves to provide basic introduction on space weather and GICs, and the major science challenges the GIC community continues to face. 0004382678.INDD 3 8/23/2019 6:47:15 PM CO PY RI GH TE D M AT ER IA L 4 GEOMAGNETICALLY INDUCED CURRENTS FROM THE SUN TO THE POWER GRID chapter provides a high‐level summary of space weather and GICs. While some of the topics touched on cover a broad range of space weather domains, the discussions are oriented/biased towards the geophysical facet of GICs. For more insight on specific GIC aspects, the reader is urged to consult other sections of this volume. 1.2. THE SPACE WEATHER CHAIN The Sun is the primary source of all space weather in the heliosphere. Sudden, violent eruptions of solar material from the Sun’s atmosphere (the corona) called coronal mass ejections (CMEs), mark the beginning of major space weather events that eventually produce geomagnetic storms (disturbances) in the Earth’s upper atmosphere. The Sun’s activity is closely governed by the solar activity cycle, which has an average length of about 11 years. The cycle is defined by the number of visible active sunspots on the solar surface. During solar maximum period when solar activity is high, the Sun can launch multiple CMEs towards Earth per day. A CME can be perceived as a cloud of plasma with the solar magnetic field known as the interplanetary magnetic field (IMF) embedded within it. Upon arriving at Earth, CMEs interact with the magnetosphere, a low‐ density partially ionized region around the upper atmosphere dominated by Earth’s magnetic field. This interaction then triggers geomagnetic disturbances (GMDs) that lead to violent global magnetic field variations. Orientation of the IMF varies with time and is important for interaction between the solar wind and the magnetosphere. Historically, the most intense disturbances have been recorded when the IMF Bz component, which is parallel to the solar rotation axis is oppositely directed to the Earth’s magnetic field, a condition often referred to as a southward or negative IMF. Under southward condition, the coupling between the solar wind and the magnetosphere is enhanced and the transfer of CME plasma, momentum, and energy into the near‐Earth space environment is increased. This enhanced energy flow stimulates a chain of complex processes within the magnetosphere–ionosphere (M–I) coupled system that regulate phenomena such as storm enhanced density, ionospheric irregularities, substorms, GICs, and auroral displays at high‐latitude locations. In addition to these effects, space weather can also compromise the integrity and performance of our technology (Lanzerotti, 2001). Figure 1.1 highlights some of the key technological assets affected by space weather. Per the purpose of this book, we now focus our discussions exclusively on GICs that occur at the end of the space weather chain. 1.3. GEOMAGNETICALLY INDUCED CURRENTS Overall the GIC problem can be categorized by a two‐step approach (Pirjola, 2000, 2002a). In step 1, the geophysical facet involving the estimation of the geoelectric field based on M‐I currents and the ground conductivity is considered. Step 1 is fundamentally a science piece and the connection to space weather phenomenon. In step 2 (“engineering piece”) the current flowing on the system is calculated based on the estimated geoelectric field and detailed information about the particular ground system (e.g., Lehtinen and Pirjola, 1985; Molinski et al., 2000; Pirjola, 2000). In other words, the magnitude of GICs flowing through a network is generally determined by a combination of the horizontal surface geoelectric field, the geology, and elements of a given network (e.g., Molinski et al., 2000; Pirjola, 2000). We now briefly examine each of these three components. 1.3.1. The Geoelectric Field The ground geoelectric field is the actual link to space weather through M‐I processes. The primary feature of geomagnetic storms that pertains to GICs is the variation of electric currents in the M‐I mode. Intense time‐varying magnetosphere and ionosphere currents lead to rapid fluctuation of the geomagnetic field on the ground. Faraday’s law of induction is the basic principle related to the flow of GICs on ground networks: a changing magnetic field induces an electric field through geomagnetic induction in the earth. In turn this electric field is responsible for currents that flow on ground conductors, such as power grids, according to Ohm’s law J = σ E, where J is the current density, σ is the conductivity, and E is the electric field. The key processes for the creation and flow of GICs are illustrated in Figure 1.2. Mathematically, Faraday’s law of induction can be expressed as: