A solar storm struck Earth in May 2024, delivering dazzling northern lights to places we don’t typically see them—as far south as Oklahoma and Florida. It was the most intense storm in two decades and the largest display of aurorae in 500 years—and we still haven’t reached the peak of the current activity cycle (that’s in 2025).
The sun’s constant magnetic activity builds over an average 11-year cycle; at the end of the process, eruptions of energy unleash charged particles that rush along solar winds and collide with Earth's magnetic field. While the northern and southern lights are amazing to behold and pose little direct danger to humans, a strong solar storm can have dire consequences for our technological systems, including power, navigation, communications, and more.
Here are 12 facts related to solar storms—how they happen, what they wreak, and when they’ll be back.
- Solar storms start with a sunspot.
- There are three kinds of solar storms.
- Solar winds carry the sun’s storms to Earth.
- A clash of magnetic fields produces aurorae.
- Aurora borealis was coined in the early 17th century.
- Aurorae appear in a palette of colors and sounds.
- Solar storms can fry technology.
- NOAA categorizes geomagnetic storms on a scale from G1 to G5.
- The Carrington Event is the largest solar storm ever recorded.
- It’s not just Earth—the rest of the solar system is also affected by solar storms.
- Scientists have evidence of solar storms going back thousands of years.
- Experts are always scanning the skies for solar storms.
The sun is a giant ball of always-active gases. Occasionally, the magnetic turbulence within the sun creates dark, planet-sized regions on the surface—a.k.a. sunspots—that are capable of causing solar flares (massive eruptions of radiation) or coronal mass ejections (or CMEs, which are explosions of plasma from the sun’s interior).
Sunspots appear darker in contrast to the rest of the sun because they’re cooler than the surrounding mass. A typical sunspot on its own would be about as bright as Earth’s moon. Each sunspot has a dark, central area (umbra), which is roughly 6300°F compared with the rest of the sun’s photosphere at 10,000°F. The boiling hotbed of magnetic activity within the sunspot causes its pressure to increase, which prevents interior heated gas from rising to the surface and further lowers the sunspot’s temperature. When the pressure becomes too great, a solar flare might erupt and a CME may explode, shooting charged particles outward that typically take three to five days to reach Earth.
There are three kinds of solar storms.
The sun sends Earth space weather in three forms: a radio blackout, a solar radiation storm, or the strongest of all, a geomagnetic storm. A radio blackout, caused by eruptions of radiation from solar flares, is the most common and occurs approximately 2000 times per solar cycle. These storms also reach Earth quickest, making them hard to plan for; their effects are less severe and more limited to issues with aviation and marine equipment. A solar radiation storm involves a significant number of energized protons, which may reach Earth in as quickly as 10 minutes. Radiation near Earth then increases to dangerous levels, posing a health risk to astronauts and potentially disrupting satellite electronics from hours to days. Geomagnetic storms are the most powerful, and they’re also the most noticeable—producing colorful aurorae and causing electronic systems to go haywire (which is why they’re the focus of this list).
In 1957, University of Chicago astrophysicist Eugene Parker theorized that the sun must emit charged particles from its corona, wherein the corona becomes so hot that the particles eventually escape the sun’s gravity. Though widely criticized at the time, Parker’s description of solar wind was accurate: It’s plasma containing protons and electrons that soars away from the sun continuously, yet remains within a protected bubble that encapsulates the entire solar system called the “heliosphere.” The winds that form this windsock-shaped bubble protect us from dangerous cosmic rays, but they’re also what carry a geomagnetic storm toward Earth. As NASA associate administrator Nicky Fox once poetically put it, “if the sun sneezes, Earth catches a cold.”
Norwegian scientist Kristian Birkeland was the first to offer a scientific explanation for aurorae as a product of geomagnetism in 1906. It wasn’t until the 1970s and the advent of satellite technology that his theory could be confirmed correct.
A clash of magnetic fields produces aurorae.
Different areas of the sun produce solar wind at speeds that range from 250 to 500 miles per second, faster at the Sun’s poles versus its equator, which determine how quickly a solar storm reaches Earth (it can take from 15 hours to several days). Earth’s magnetosphere deflects most of the charged particles brought by the constant solar wind. However, when a substantial CME occurs, the material that reaches Earth contains its own magnetic field. This field makes contact with Earth’s geomagnetic field, causing changes in the upper atmosphere (roughly 53–375 miles above our planet) and creating a complex undulation of electrical currents.
Protons and radiation rain along our planet’s magnetic field lines, which send the stream of particles toward Earth’s north and south poles. There, the material collides with oxygen and nitrogen atoms, which fuel the particles with energy and make them glow as northern or southern lights.
Galileo described the lights and named them aurora borealis in his Discourse on the Comets, published in 1619. Combining the Latin word for “dawn” and the Greek word for “north,” Galileo wrote, “the sky at nighttime [is] illuminated in its northern parts in such a way that its brightness yields nothing to the brightest dawn and closely rivals the sun … forming for us this northern dawn [questa boreale aurora]” [PDF]. The French astronomer and mathematician Pierre Gassendi witnessed a display of the aurora borealis in France in 1621, but did not use the phrase in writing until 1649. The first appearance of aurora borealis in English occurred in a letter written by John Flamsteed, Britain’s astronomer royal, in 1703.
Seventy years later, Captain James Cook became the first to describe the aurora australis, or southern lights, on his trip around the tip of South America.
Aurorae appear in a palette of colors and sounds.
Aurorae colors are different in the two hemispheres. Northern lights tend to show greener and whiter shades, and southern lights feature mostly green and pink. Blues, reds, and purples make a cameo at both poles. There are also reports that aurorae emit sound—often a hissing, crackling noise. NASA wants aurora fans to track their sightings on Aurorasaurus, a citizen science database, because each verified sighting contributes to better modeling of the marvels.
The intense bombardment of electromagnetic particles in a geomagnetic storm can shove a satellite out of orbit, undermine undersea cables transmitting the internet, take down power grids, disrupt circuits and signals, and more. A 1989 storm led to a 12-hour electrical event in Quebec, casting more than 6 million people into the dark.
Ocean monitoring systems are also at risk. Comprised of thousands of sensors that span the Arctic to Antarctic, these observatories measure ocean currents and record climate data. In May 2024, ocean-borne compasses captured significant swings related to the messy magnetic field. Many shared GPS outages on social media during the solar storm, and one South Dakota farmer described his GPS-powered tractor as uncontrollable and moving in circles.
NOAA categorizes geomagnetic storms on a scale from G1 to G5.
NASA’s Solar Dynamics Observatory (SDO) was created in 2010 to understand the sun’s behavior better. A solar flare can be billions of times more powerful than a single nuclear bomb, with the strongest flares (X-class) occurring about 10 times each year. The biggest CME may house billions of tons of powerful solar stuff that gets blown our way. As it monitors solar activity, the National Oceanic and Atmospheric Administration (NOAA) ranks incoming geomagnetic storms as G1 (minor), G2 (moderate), G3 (strong), G4 (severe), or G5 (extreme). To determine their severity, NOAA measures multiple currents—some circling Earth, some produced in the magnetosphere, and other within aurorae—along with magnetic disturbances on the ground. A solar storm can run for hours or days, with impacts lasting for weeks.
The Carrington Event was an extreme geomagnetic storm that struck Earth in early September 1859. Though many skywatchers were gazing upward at increasing sunspot activity, amateur astronomer Richard Carrington, stationed in Surrey, England, spotted a sudden flash of brilliant light that he considered to be the cause of the resulting catastrophe.
In fewer than 20 hours, a CME traveled 90 million miles and arrived at Earth in the form of an incredibly powerful solar storm. The event was unprecedented: Nearly all of the world’s 125,000 miles of telegraph lines were affected, with outages lasting eight hours in some areas. The telegraph machines shocked their operators and ignited paper, while other equipment ran on ambient energy, even with their batteries disconnected. Aurorae lit up the sky almost as far south as the equator. For the first time, scientists began to see the relationship between the sun and geomagnetic storms, as well as the effects of those storms on modern technology.
If a solar storm on the same scale as the Carrington Event were to happen again, many predict an internet-wide crash, halting online commerce and operations. Researchers have estimated that economic damages could tally up to $2.6 trillion just in the U.S.
It’s not just Earth—the rest of the solar system is also affected by solar storms.
Everything in space—systems, crafts, humans—is susceptible to the damage brought by a solar storm. Astronauts above Earth’s atmosphere need a protective shield to avoid health risks from radiation, for example. Other planets, especially the gas giants Jupiter and Saturn, have strong magnetic fields that interact with solar particles and allow them to experience aurorae.
In contrast, scientists think solar storms stripped Mars of its atmosphere. Its former oceans, seas, and rivers were protected by a thick atmospheric blanket wrapped around the planet, but modern Mars has an atmosphere that’s just 1 percent as dense as Earth’s. One theory to explain this suggests that approximately 3.7 billion years ago, Mars’s core cooled, which weakened its magnetic power and made it more susceptible to solar storms.
NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter was present for a CME that clocked the planet in 2015. It observed that Mars’s sparse remaining atmosphere vaporized about 10 times faster than normal during the CME, suggesting that this was the likely mechanism behind its loss of atmosphere over time.
A 2022 study published in Nature Communications reported that ice cores from Greenland and Antarctica contained an abnormal abundance of solar energetic particles, evidence that points to a massive solar storm affecting Earth about 9200 years ago. And, from analysis of carbon-14 in tree rings, NASA proposed that a solar storm with a similar severity to Carrington hit Earth in 774 CE.
The earliest known written description of aurorae comes from China in 2600 BCE. Aristotle also wrote about aurorae roughly 2000 years ago.
Researchers have theorized that prehistoric rock art containing seemingly abstract shapes and lines may actually depict atmospheric phenomena, particularly aurorae, and may have spiritual meaning. Vikings thought an aurora meant Odin, the Norse god of death and war, had sent the Valkyries to take fallen warriors to Valhalla. Cree people saw aurorae as spirits communicating with their living loved ones. Finnish beliefs held that an aurora resulted from a “fire fox” sprinting across the snow and spraying sparks across the dark sky (the Finnish word for aurora, revontulet, means “fire fox”).
Experts are always scanning the skies for solar storms.
NOAA’s Space Weather Prediction Center (SWPC) is constantly supervising the skies from its base in Colorado. Telescopes on the ground and above Earth keep an active eye out for variations in space between the sun and Earth that could indicate solar storms brewing. This early warning system was critical for protecting sensitive equipment prior to the May 2024 event, and it’s a key to preventing future damage as we progress through the 11-year sunspot cycles.
Monitoring of sunspot cycles began in 1755 with Solar Cycle 1. Today, we’re in the midst of Solar Cycle 25, which the National Weather Service predicts will peak in July 2025 with 115 sunspots. Though that number is below the average of 179 spots, the ramp-up to the solar maximum next year may give skywatchers more opportunities to see aurorae in unexpected places.
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