Sunspots, Rocks and Blackouts: How Magnetic Storms Threaten Power Grids

From the Sun to the power grid

The Sun's activity rises and falls over a solar cycle of about 11 years, seen in the number and size of sunspots — knots of tangled magnetic field. When they abruptly rearrange, the Sun can hurl out superheated, electrically conducting plasma (electrons, protons and ions). If one of these coronal mass ejections heads for Earth, it can set off a magnetic storm: a global disturbance of Earth's magnetic field lasting a day or two.

Into the ground. The storm's electromagnetic disturbance reaches down to Earth's surface and into its interior. How strong it is, and how deep it goes, depends on how fast the field changes and on the electrical conductivity of the rocks:

• a field varying every 10 seconds reaches about a kilometer into wet sedimentary rock (a fairly good conductor), but about 100 kilometers into dry metamorphic or igneous rock (usually resistive);
• slower variations reach hundreds of kilometers or more.

So geoelectric fields at the surface are generally weak over sedimentary basins and strong over metamorphic regions; some storms induce more than 10 volts per kilometer over resistive ground.

Into the grid. A power line crossing such ground offers storm currents an easier path than the resistive rock. Over a 100-km line, 10 V/km adds up to 1,000 volts — enough to drive current in through transformer grounding points at high-voltage substations. These are not the tightly controlled alternating currents grids are built for: they can disrupt operations and even damage or destroy transformers. The strongest ejections cause the most intense storms, and a superstorm could cause prolonged, widespread blackouts, possibly across borders — most likely around solar maximum, when sunspots are numerous.

Sunspots and storms through history

Astronomers have counted sunspots for hundreds of years, and geophysicists have measured magnetic disturbance at ground stations since the middle of the 19th century. Together they show the solar cycle plainly — and that intense storms cluster when sunspots are many and are rare when they are few.

Figure 1. (A) Sunspot number, 1843–2024; (B) geomagnetic disturbance (the aa index, in nanoteslas), 1868–2024. Both show the 11-year cycle and track each other; the 1921 and 1989 storms are labeled. USGS.

• September 1859 — the Carrington storm. Astronomer Richard Carrington, watching sunspots through a telescope, saw a very bright solar flare. Seventeen hours later came one of history's greatest magnetic storms, so strong it overwhelmed most observatories' magnetometers. It disrupted telegraphs and set telegraph stations on fire around the world.
• May 1921. One of the most intense storms ever recorded, in the declining phase of the cycle after the August 1917 maximum. It disrupted telegraph and telephone systems worldwide — then long grounded networks of wire, like today's grids — and set railroad telegraph stations in New York State and City on fire.
• March 1989. Just before the November 1989 sunspot maximum, this storm damaged satellites, disrupted radio and geophysical surveys, caused the complete collapse of the Hydro-Québec grid in Canada, interfered heavily with U.S. grids, and damaged a high-voltage transformer at a nuclear power plant in New Jersey.

The stakes now. With the world ever more dependent on electricity, a repeat of March 1989 could cost the global economy $2.4 trillion to $3.4 trillion — 3.9% to 5.6% of world output. A Carrington-class storm might cost the United States alone $0.6 trillion to $2.6 trillion.

Mapping the hazard

As part of a federal interagency effort to prepare for space weather, the USGS Geomagnetism Program studies storm-induced geoelectric fields using two very different datasets:

1. decades of magnetic-storm records from ground stations run by the USGS and Natural Resources Canada;
2. national surveys of Earth's electrical properties, supported by the National Science Foundation, NASA and the USGS.

Together they allow after-the-fact maps of geoelectric fields across the contiguous United States, and the USGS and the National Oceanic and Atmospheric Administration also present real-time maps.

Figure 2. (A) Geoelectric field amplitudes during the March 1989 storm; (B) where power-grid interference was reported. USGS.

In March 1989, geoelectric fields were strong in the Upper Midwest and the East, over resistive metamorphic and igneous rock, and weak over sedimentary basins such as Michigan and Illinois. Grid interference was worst where the fields were strongest — including the grids serving Boston, New York City, Philadelphia, Baltimore and Washington, D.C., a megalopolis of more than 50 million people.

Maps like these show utilities where to focus on making grids more resilient, and where to be especially watchful during a storm to keep the power flowing.

Sources

Based on "The solar cycle, geology, and geoelectric hazards for power grids," U.S. Geological Survey Fact Sheet, USGS Geomagnetism Program, citing Love and others (2019, 2022), Kelbert and others (2019), Schulte in den Bäumen and others (2014), the National Research Council (2008) and Lloyd's of London (2013), among others; a work of the United States government in the public domain.