Key Points
- A geomagnetic storm is a major disturbance of Earth’s magnetosphere caused by energy from the solar wind, most often after a coronal mass ejection (CME) from the sun.
- NOAA rates these storms on a five-level G-scale, from G1 (Minor, Kp 5) to G5 (Extreme, Kp 9), based on the planetary Kp index.
- Strong storms can disturb power grids, GPS, high-frequency radio and satellites; a March 13, 1989 storm caused a nine-hour blackout in Quebec, Canada.
- The same storms push the northern lights far south; NOAA says G5 aurora has been seen as low as Florida and southern Texas.
- NOAA’s Space Weather Prediction Center issues the official U.S. watches, warnings, alerts and 3-day forecasts for geomagnetic storms.
A geomagnetic storm is a major disturbance of Earth’s magnetic environment that happens when a burst of energy from the sun slams into the planet’s protective magnetic bubble. These storms are part of what scientists call space weather. Most pass with nothing more than a beautiful aurora, but the strongest ones can disrupt power grids, GPS and radio. Here is what causes a geomagnetic storm, how NOAA’s G1 to G5 scale works, and what a storm can and cannot do to you.
What Is a Geomagnetic Storm?
NOAA’s Space Weather Prediction Center (SWPC) defines a geomagnetic storm as a major disturbance of Earth’s magnetosphere that occurs when there is a very efficient exchange of energy from the solar wind into the space environment surrounding Earth. The magnetosphere is the region around the planet controlled by Earth’s magnetic field. It normally deflects the steady stream of charged particles, called the solar wind, that flows out from the sun.
During a storm, that shield is shaken hard. SWPC explains that the storm produces major changes in the currents, plasmas and fields in the magnetosphere. A ring of westward electric current around Earth strengthens and produces magnetic disturbances that instruments on the ground can measure. Other currents flow along magnetic field lines into the polar upper atmosphere, where they feed intense currents called auroral electrojets.
A geomagnetic storm is not a weather storm in the usual sense. It brings no wind, rain or snow at the surface, and it is not felt by people. Its effects show up in technology and in the night sky.
What Causes Geomagnetic Storms?
The trigger is always the sun, but the type of solar activity matters.
Coronal mass ejections
The largest storms come from coronal mass ejections. SWPC describes a CME as a large expulsion of plasma and magnetic field from the sun’s corona, its outer atmosphere. A CME can carry billions of tons of material, and it travels at speeds from slower than 250 kilometers per second to near 3,000 kilometers per second. Most take several days to reach Earth, but SWPC notes the fastest Earth-directed CMEs can arrive in as little as 15 to 18 hours.
High-speed solar wind streams
A second source is a high-speed solar wind stream. When fast wind runs into slower wind ahead of it, it creates a compressed region called a co-rotating interaction region. SWPC says these storms are usually less intense than CME storms, but they can deposit more energy into the magnetosphere over a longer period.
Why the magnetic direction matters
Not every CME produces a big storm. According to SWPC, the most effective condition is a sustained, southward-directed magnetic field in the solar wind, pointing opposite to Earth’s own field at the dayside of the magnetosphere. That orientation lets energy pour in. A CME whose field points mostly northward can pass with relatively mild effects, which is one reason storm forecasts carry real uncertainty until the cloud is close.
The NOAA G-Scale: G1 to G5 Explained
NOAA introduced its Space Weather Scales to tell the public how severe a space weather event is, much like the Saffir-Simpson Hurricane Scale ranks hurricanes. There are three scales: R for radio blackouts, S for solar radiation storms and G for geomagnetic storms. The G-scale is based on Kp, a planetary index from 0 to 9 built from magnetic measurements around the world. The table below summarizes the official NOAA scale, where one solar cycle is about 11 years.
| Level | Kp | Average frequency per cycle | Lowest typical aurora sightings |
|---|---|---|---|
| G1 Minor | 5 | 1,700 (900 days) | High latitudes such as northern Michigan and Maine |
| G2 Moderate | 6 | 600 (360 days) | As low as New York and Idaho |
| G3 Strong | 7 | 200 (130 days) | As low as Illinois and Oregon |
| G4 Severe | 8, including 9- | 100 (60 days) | As low as Alabama and northern California |
| G5 Extreme | 9 | 4 (4 days) | As low as Florida and southern Texas |
The frequency column is useful context. Minor storms are common, while G5 storms happen on only about four days in an average 11-year cycle. The aurora column describes where the lights have been seen, not a guarantee for any given storm.
What Can a Geomagnetic Storm Do?
NOAA lists effects that grow with each level. At G1, weak power grid fluctuations can occur and migratory animals can be affected. At G5, the scale warns of widespread voltage control problems, possible grid collapse or blackouts and transformer damage.
Power grids
A changing magnetic field induces electric current in long conductors. During a storm, these geomagnetically induced currents flow into power lines and transformers. SWPC explains that the currents can push transformers out of their designed operating range, saturating their cores, heating internal parts and confusing protective equipment. Its list of documented grid impacts includes a 1958 blackout, equipment tripping in August 1972, a nine-hour blackout in Canada with a transformer loss on March 13, 1989, and a blackout in Sweden during the October 2003 storm.
The National Weather Service says the 1989 storm left 6 million people without electricity for 9 hours and, citing the North American Electric Reliability Corporation, that it disrupted transmission from the Hydro Quebec generating station and damaged some power transformers in New Jersey.
GPS and radio
Storms heat the ionosphere unevenly. SWPC says this creates strong variations in ionospheric density that can bend radio signals and cause errors in GPS positioning. On the G-scale, satellite navigation can be degraded for hours at G4 and for days at G5, and high-frequency radio may be impossible in many areas for one to two days at G5.
Satellites
Extra energy heats and expands the upper atmosphere, which increases drag on satellites in low Earth orbit and can change their orbits. NOAA’s scale also lists surface charging and orientation problems for spacecraft at the higher levels.
Pipelines
The same induced currents can flow along long metal pipelines. NOAA notes that pipeline currents can reach hundreds of amps in a G5 storm, which matters for the systems that protect pipes from corrosion.
Geomagnetic Storms and the Northern Lights
The aurora is the friendly side of a geomagnetic storm. SWPC explains that the northern and southern lights appear when energized electrons follow Earth’s magnetic field down toward the poles and collide with oxygen and nitrogen in the upper atmosphere, typically 80 to 500 kilometers above the surface. Normally the lights sit in two ovals around the magnetic poles, best seen between about 60 and 75 degrees latitude.
During major storms those ovals expand toward the equator, and the aurora can be seen over much of the United States. On May 10, 2024, SWPC announced that G5 conditions were first observed at Earth at 6:54 p.m. EDT, the top level of the scale. To see the lights you still need clear, dark skies away from city glow, and SWPC notes that auroral arcs often brighten and twist near midnight.
The Biggest Geomagnetic Storms on Record
The National Weather Service calls the September 1859 Carrington Event the strongest geomagnetic storm on record. It is named for British astronomer Richard Carrington. During that storm, excess currents on telegraph lines shocked technicians and in some cases set telegraph equipment on fire, and aurora was seen as far south as Cuba and Hawaii.
Modern examples include the March 1989 storm that blacked out Quebec, the October 2003 storms that led to a blackout in Sweden, and the May 2024 G5 storm. Each one has helped grid operators, satellite companies and forecasters plan for a repeat of something on the scale of 1859.
Are Geomagnetic Storms Dangerous to People?
For people on the ground, the storm itself is not a direct health threat. The National Weather Service says Earth’s magnetosphere, ionosphere and atmosphere do a great job of protecting us from the most hazardous effects of space weather. The realistic risks are indirect and come through technology. The weather service notes that power outages from space weather are rare, but a significant outage could cascade into water systems, refrigerated food and medicine, heating and cooling, communications and fuel distribution.
That makes basic preparedness the same as for any long outage: water, nonperishable food, flashlights and batteries, a plan for medications that need power or refrigeration, and a battery or hand-crank radio. Radiation risk to astronauts and high-altitude flights belongs to a different NOAA scale, the S-scale for solar radiation storms, rather than the G-scale.
How Geomagnetic Storms Are Forecast
SWPC, part of the National Weather Service, is the official U.S. source for space weather forecasts. Forecasters use coronagraph images from the NASA SOHO and STEREO spacecraft to judge a CME’s size, speed and direction, then predict storm levels in SWPC’s 3-day forecast and forecast discussion. The DSCOVR satellite, located in the L1 orbital area on the sun side of Earth, detects the shock ahead of an arriving CME and, according to SWPC, often provides 15 to 60 minutes of advance warning before it reaches Earth.
SWPC issues watches, warnings and alerts for geomagnetic storms and offers a subscription service, an aurora 30-minute forecast and dashboards for power, aviation, GPS and emergency managers. The National Weather Service also keeps a plain-language space weather safety page. Earthly hazards follow different rules, as our explainer on how a bomb cyclone forms shows.
The Bottom Line
A geomagnetic storm is the sun’s energy shaking Earth’s magnetic field, usually after a coronal mass ejection with a southward magnetic field arrives. NOAA’s G-scale runs from common G1 storms to rare G5 events that can strain power grids, degrade GPS and push the northern lights as far south as Florida. The storm will not hurt you directly, but strong ones can affect the technology you rely on. For storms, alerts and severe weather closer to the ground, follow our U.S. Weather Live center.
Sources
- NOAA Space Weather Prediction Center, Geomagnetic Storms and NOAA Space Weather Scales
- NOAA SWPC, Coronal Mass Ejections and Aurora
- NOAA SWPC, Electric Power Transmission
- NOAA SWPC, G5 Conditions Observed! (May 10, 2024)
- National Weather Service, Space Weather and Safety
