A geomagnetic storm, also called a magnetic storm, is a short-term change in Earth's magnetosphere caused by interactions between the magnetosphere and large, temporary structures of plasma and magnetic fields from the Sun. These structures come from the Sun and travel through space.

The structures that cause geomagnetic storms include interplanetary coronal mass ejections (CMEs) and corotating interaction regions (CIRs). CMEs usually form in areas of the Sun with high activity, while CIRs form where fast and slow solar wind streams meet. The number of geomagnetic storms changes with the sunspot cycle. During times when the Sun is most active, called solar maxima, geomagnetic storms happen more often, and most are caused by CMEs.

When these structures reach Earth, the increased pressure from the solar wind pushes the magnetosphere inward. The solar wind’s magnetic field interacts with Earth’s magnetic field, adding more energy to the magnetosphere. This interaction increases plasma movement inside the magnetosphere and creates stronger electric currents in the magnetosphere and ionosphere. During the main phase of a geomagnetic storm, stronger currents in the magnetosphere, especially the ring current, weaken Earth’s magnetic field on the side facing the Sun. This causes the magnetopause boundary to move closer to Earth.

Several space weather events are linked to geomagnetic storms. These include solar energetic particle (SEP) events, geomagnetically induced currents (GICs), ionospheric storms, and disturbances that disrupt radio and radar signals. These events can also interfere with magnetic compass navigation and cause auroras to appear at lower latitudes than usual.

The largest recorded geomagnetic storm, the Carrington Event in September 1859, damaged parts of the newly built US telegraph system, causing fires and shocking telegraph operators with electricity. In 1989, a geomagnetic storm caused ground-induced currents that disrupted power supplies across most of Quebec and produced auroras as far south as Texas.

Definition

A geomagnetic storm is described by changes in the Dst (disturbance–storm time) index. The Dst index measures the average change in the horizontal part of Earth's magnetic field at the magnetic equator, using data from several magnetometer stations. Dst is calculated once every hour and shared quickly. During calm times, Dst ranges from +20 to −20 nano-Tesla (nT).

A geomagnetic storm has three stages: initial, main, and recovery. The initial stage happens when Dst (or SYM-H, a one-minute version of Dst) rises by 20 to 50 nT over tens of minutes. This stage is also called a storm sudden commencement (SSC). However, not all geomagnetic storms have an initial stage, and not all sudden increases in Dst or SYM-H lead to a storm. The main stage is when Dst drops below −50 nT. The choice of −50 nT as a storm threshold is not strictly set by rules. During a storm, Dst may reach as low as −600 nT. The main stage usually lasts 2 to 8 hours. The recovery stage occurs when Dst rises from its lowest point back to normal levels. Recovery can take as little as 8 hours or up to 7 days.

The size of a geomagnetic storm is grouped into categories: moderate (Dst minimum between −50 nT and −100 nT), intense (Dst minimum between −100 nT and −250 nT), or super-storm (Dst minimum below −250 nT).

Measuring intensity

Geomagnetic storm intensity is measured using different methods, including:

• K-index
• A-index
• The G-scale used by the U.S. National Oceanic and Atmospheric Administration, which classifies storms into levels from G1 to G5 (G1, G2, G3, G4, G5). G1 is the weakest level (linked to a Kp value of 5), and G5 is the strongest (linked to a Kp value of 9).

History of the theory

In 1930, Sydney Chapman and Vincenzo C. A. Ferraro wrote an article titled A New Theory of Magnetic Storms to explain a natural event. They said that when the Sun releases a solar flare, it also sends out a plasma cloud, now called a coronal mass ejection. They believed this plasma moves fast enough to reach Earth in 113 days, though scientists now know it takes 1 to 5 days. They explained that the cloud pushes against Earth's magnetic field, making it stronger at Earth's surface. Chapman and Ferraro's work was influenced by Kristian Birkeland, who used newly discovered cathode-ray tubes to show that rays bend toward the poles of a magnetic sphere. Birkeland believed a similar process causes auroras, which is why they are more common near Earth's poles.

Occurrences

The first scientific study of a geomagnetic storm happened in the early 1800s. From May 1806 to June 1807, Alexander von Humboldt in Berlin recorded the direction of a magnetic compass. On December 21, 1806, he noticed the compass acted strangely during a bright aurora event.

On September 1–2, 1859, the largest known geomagnetic storm occurred. Between August 28 and September 2, 1859, many sunspots and solar flares were seen on the Sun, with the biggest flare on September 1. This event is called the solar storm of 1859 or the Carrington Event. Scientists believe a large coronal mass ejection (CME) from the Sun reached Earth in 18 hours, a trip that usually takes three to four days. The Colaba Observatory recorded a 1600 nT drop in the horizontal magnetic field. Dst levels were estimated at about −1760 nT. Telegraph wires in the United States and Europe had increased voltage, causing shocks to operators and fires. Auroras were seen as far south as Hawaii, Mexico, Cuba, and Italy, areas where auroras are rarely visible. Ice cores show similar events happen roughly once every 500 years.

After 1859, smaller storms occurred, such as the aurora on November 17, 1882, and the May 1921 geomagnetic storm, both causing telegraph issues and fires. In 1960, radio disruptions were reported globally.

In early August 1972, a series of solar flares and storms peaked with an X20 flare, the fastest CME recorded. This caused severe geomagnetic and proton storms, disrupting Earth’s electrical systems, communications, and satellites. It also triggered U.S. Navy mines in North Vietnam.

The March 1989 geomagnetic storm caused the Hydro-Québec power grid to collapse in seconds due to equipment failures. Six million people lost power for nine hours. Auroras were visible as far south as Texas and Florida. The storm was caused by a CME from the Sun on March 9, 1989, with a Dst minimum of −589 nT.

On July 14, 2000, an X5 flare (the Bastille Day event) erupted, sending a coronal mass directly toward Earth. A geomagnetic super storm occurred from July 15–17, with a Dst minimum of −301 nT. No power failures were reported, but the event was observed by Voyager 1 and Voyager 2, the farthest solar storm observations in the Solar System.

From October 19 to November 5, 2003, 17 major solar flares occurred, including an X28 flare, the largest ever recorded. This caused extreme radio blackouts and three geomagnetic storms. Dst values reached −151, −353, and −383 nT. A fourth storm had a Dst minimum of −69 nT. The final storm was weaker because the Sun’s active region rotated away from Earth. This event became known as the Halloween Solar Storm. The WAAS system, used by the FAA, was offline for 30 hours. The Japanese ADEOS-2 satellite was damaged, and many others had communication issues.

Interactions with planetary processes

The solar wind carries the Sun's magnetic field. This field can point either north or south. When the solar wind is very strong, causing the magnetosphere to shrink and grow, or when the solar wind points south, geomagnetic storms may happen. A south-pointing magnetic field causes magnetic lines to connect on the dayside of Earth's magnetosphere, quickly sending magnetic and particle energy into the magnetosphere.

During a geomagnetic storm, the F2 layer of the ionosphere becomes unstable, breaks apart, and might not be visible anymore. In Earth's northern and southern polar regions, auroras can be seen.

Instruments

Magnetometers monitor the auroral zone and the equatorial region. Two types of radar, coherent scatter and incoherent scatter, are used to study the auroral ionosphere. By sending signals that bounce off irregularities in the ionosphere, scientists can track how these irregularities move and learn about magnetospheric convection.

Spacecraft instruments include:

• Magnetometers, often of the flux gate type. These are usually placed at the ends of booms to avoid interference from the spacecraft and its electrical systems.
• Electric sensors at the ends of opposing booms measure differences in electric potential between points to determine electric fields linked to convection. This method works best in high-density plasma near Earth; longer booms are needed in space to prevent electric forces from being blocked.
• Ground-based radio sounders send radio waves of different frequencies toward the ionosphere. By timing how long it takes for the waves to return, scientists can determine the electron density profile up to the peak, where radio waves no longer return. Radio sounders on the Canadian satellites Alouette 1 (1962) and Alouette 2 (1965) sent radio waves toward Earth to study the electron density profile of the "topside ionosphere." Other methods, such as those used on the IMAGE satellite, were also tested.
• Particle detectors include Geiger counters, which were used in early studies of the Van Allen radiation belt. Later, scintillator detectors and channeltron electron multipliers were developed. To determine the charge, mass, and energy of particles, various mass spectrograph designs were used. For energies up to about 50 keV, which make up most of the magnetospheric plasma, time-of-flight spectrometers (such as the "top-hat" design) are commonly used.

Computers have enabled scientists to combine years of separate magnetic observations and find average patterns of electrical currents and typical responses to changes in the solar wind. They also run simulations of the global magnetosphere and its responses by solving magnetohydrodynamics (MHD) equations on a numerical grid. Additional adjustments are needed to model the inner magnetosphere, where magnetic drifts and ionospheric conduction must be considered. In polar regions, which are directly connected to the solar wind, large-scale ionospheric anomalies can be modeled successfully, even during intense geomagnetic storms. At smaller scales, such as those comparable to a degree of latitude or longitude, results are harder to interpret, and assumptions about high-latitude forces are often necessary.

Impacts

A geomagnetic storm as strong as the 1859 solar storm could cause billions or trillions of dollars in damage today. This would harm satellites, power grids, and radio communications. It might also cause large-scale electrical blackouts that could take weeks, months, or even years to fix. These blackouts could harm food production.

When magnetic fields move near a conductor, like a wire, they create a current in the conductor. This happens during geomagnetic storms, which also affected telephone and telegraph lines before fiber optics were used. Long power lines, which can be many kilometers long, are especially vulnerable to this effect. Operators in China, North America, and Australia are most at risk, especially in modern high-voltage, low-resistance lines. The European grid uses shorter lines, which are less likely to be damaged.

Currents caused by geomagnetic storms can harm electrical equipment, especially transformers. These currents can cause transformers to overheat, reducing their performance or even breaking them. In extreme cases, this could lead to a chain reaction that overloads transformers. Most generators are connected to the grid through transformers, which protect them from these currents. However, if a transformer is damaged, it can cause problems for the generator, such as overheating parts of the generator.

A 2008 study by Metatech Corporation said a storm as strong as the 1921 event could destroy over 300 transformers in the United States, leaving more than 130 million people without power and costing several trillion dollars. Some reports suggest outages could last indefinitely until transformers are fixed. However, a report from the North American Electric Reliability Corporation said a geomagnetic storm would cause temporary grid problems but not widespread damage to transformers. It noted that the 1998 Quebec blackout was not caused by overheating transformers but by seven relays failing at once. In 2016, the U.S. Federal Energy Regulatory Commission required electric utilities to follow new rules to test equipment and upgrade systems to protect against geomagnetic storms within four years.

Electricity companies can also be indirectly affected by geomagnetic storms. For example, Internet service providers may stop working during storms, which could disrupt electricity distribution if companies rely on the Internet to operate.

Power companies can reduce damage by using alerts from organizations like the Space Weather Prediction Center or satellites like SOHO and ACE. These warnings allow companies to disconnect transformers or create temporary blackouts to protect equipment. Other measures include preventing geomagnetically induced currents from entering the grid through connections between neutral wires and ground.

High-frequency radio systems (3–30 MHz) use the ionosphere to send signals over long distances. Ionospheric storms can disrupt these signals, causing them to fluctuate or take unexpected paths. TV and commercial radio are less affected, but ground-to-air, ship-to-shore, shortwave, and amateur radio (mainly below 30 MHz) often face disruptions. Radio operators use solar and geomagnetic alerts to keep their communication systems working.

Military systems that use high-frequency radar, like over-the-horizon radar, can be disrupted by geomagnetic storms. These systems rely on the ionosphere to detect aircraft and missiles, but storms can cause interference. Some submarine detection systems also use magnetic signals, which can be masked by geomagnetic storms.

The Federal Aviation Administration gets alerts about solar radio bursts to prepare for communication issues. When aircraft and ground stations align with the Sun, noise can interfere with air-control radio frequencies. This can also happen with satellite communications. AirSatOne provides live updates about space weather from NOAA to help flight crews and maintenance teams avoid unnecessary repairs.

In the past, telegraph lines were affected by geomagnetic storms. These systems used long wires and the ground as return paths, making them sensitive to changes caused by geomagnetic activity. Induced currents could weaken or distort signals. Some operators disconnected batteries and used the induced current instead. In extreme cases, the current caused fires or electric shocks. Long-distance telephone lines, including undersea cables, are also affected unless they use fiber optics.

Damaged communication satellites can disrupt global telephone, television, radio, and Internet services. A 2008 study by the National Academy of Sciences warned that a solar superstorm could cause months-long global Internet outages. Researchers suggest solutions like mesh networks and new protocols to improve Internet resilience.

Global navigation systems, like GPS, and older systems like LORAN and OMEGA, can be disrupted by solar activity. These systems rely on signals that can be distorted during geomagnetic storms, leading to inaccurate navigation. If navigators received warnings about solar events, they could switch to backup methods.