Solar flares are intense bursts of radiation that erupt from the sun’s surface, releasing energy equivalent to millions of 100-megaton hydrogen bombs detonating simultaneously. They occur when tangled magnetic fields in the sun’s atmosphere suddenly snap and reconnect, converting stored magnetic energy into light, heat, and accelerated particles across the entire electromagnetic spectrum. A large flare can release up to 10²⁵ joules of energy, and the radiation it produces reaches Earth in just over eight minutes.
How Solar Flares Form
The sun’s surface is threaded with powerful magnetic fields that constantly shift, twist, and tangle as hot plasma churns beneath. In regions of intense magnetic activity, called active regions, these field lines can become heavily stressed and twisted over days or weeks, storing enormous amounts of energy like a rubber band being wound tighter and tighter.
The release happens through a process called magnetic reconnection. When opposing magnetic field lines are forced close together, they can suddenly break apart and snap into a new, simpler configuration. That reorganization converts the pent-up magnetic energy into three things at once: extreme heating of the surrounding plasma, acceleration of charged particles to near light speed, and an intense flash of radiation spanning radio waves through X-rays and gamma rays. Once reconnection begins, it can cascade rapidly, with the initial break destabilizing the broader magnetic structure and triggering an explosive outward eruption.
What Happens During a Flare
A solar flare unfolds in distinct stages. First, small brightenings appear in the active region roughly five minutes before the main event, a precursor phase that signals the magnetic field is becoming unstable. Then comes the impulsive phase, the most dramatic part, where energy release peaks in seconds. During this stage, the flare’s brightness can spike in under 20 seconds, with the most energetic bursts lasting only 40 to 50 seconds. High-energy electrons slam into the denser lower atmosphere, producing intense X-ray and white-light emission.
After that initial spike, the flare transitions into a gradual phase. The plasma continues to glow as it slowly cools, but the rise and fall of brightness is far more gentle, with some regions remaining visible for over 10 minutes. The gradual emission comes primarily from thermal heating rather than the high-energy particle bombardment of the impulsive phase. From start to finish, a typical flare lasts anywhere from a few minutes to over an hour, depending on its size.
How Scientists Classify Flares
Solar flares are classified by the peak intensity of X-rays they produce, measured by satellites orbiting Earth. The system uses five letter grades, each one representing a tenfold increase in power:
- A and B class: Background-level activity. These happen constantly and have no noticeable effect on Earth.
- C class: Small flares with minor or no measurable impact on the planet.
- M class: Moderate flares. These can cause brief radio blackouts over polar regions and minor radiation storms.
- X class: The most powerful category. X-class flares can trigger planet-wide radio disruptions and radiation storms. The scale is open-ended, so an X10 flare is ten times stronger than an X1.
Within each class, a number from 1 to 9 provides finer detail. An M5 flare, for example, sits halfway up the M-class range. The X class has no upper cap, and the strongest flares on record have exceeded X20.
Effects on Earth and Technology
Because flare radiation travels at the speed of light, effects begin about eight minutes after the eruption. The most immediate impact is on radio communications. X-rays from a flare ionize the upper atmosphere on the sunlit side of Earth, absorbing high-frequency radio signals. Pilots flying polar routes, maritime operators, and amateur radio users can all lose communication during these events. NOAA rates these radio blackouts on an R1 to R5 scale, with R1 causing minor signal degradation and R5 producing a complete high-frequency radio blackout lasting hours.
GPS accuracy also suffers. The same atmospheric disturbance that blocks radio waves introduces errors in the timing signals that GPS satellites rely on, which can degrade positioning accuracy from meters to tens of meters. For everyday navigation this is a nuisance, but for precision applications like aviation approaches and surveying, it matters significantly.
Flares are often accompanied by solar particle events, bursts of high-energy protons that arrive at Earth within minutes to hours. These particles pose a radiation hazard to astronauts and can damage satellite electronics. They also increase radiation exposure on high-altitude polar flights, though the doses for airline passengers remain small.
Solar Flares vs. Coronal Mass Ejections
Flares and coronal mass ejections (CMEs) frequently occur together, but they are different phenomena. A flare is a flash of radiation, essentially light and energy that arrives at Earth in minutes. A CME is a massive cloud of magnetized plasma, billions of tons of charged particles physically ejected from the sun’s outer atmosphere. CMEs travel much more slowly, typically reaching Earth in one to three days at speeds exceeding 600 miles per second.
When a CME strikes Earth’s magnetic field, it can trigger geomagnetic storms that produce auroras, disrupt power grids, and interfere with satellite operations. A flare, by contrast, primarily affects radio communications and the upper atmosphere. The largest solar events often involve both: the flare delivers the first punch as a burst of radiation, and the CME follows a day or two later with a sustained geomagnetic disturbance. Not every flare launches a CME, and not every CME is accompanied by a flare, but the most consequential space weather events tend to involve both.
Radiation Risks for Astronauts
Outside the protection of Earth’s magnetic field, solar particle events from large flares pose a serious health risk. A single intense event can deliver a concentrated dose of radiation in just a few hours, potentially causing acute effects like nausea and an increased long-term cancer risk. NASA currently limits career radiation exposure for astronauts to 600 millisieverts. The European, Russian, and Canadian space agencies set their career limit at 1,000 millisieverts.
For a potential Mars mission lasting around 650 days, background cosmic radiation alone approaches these limits. A major solar particle event during transit, when astronauts would lack the shielding provided by Earth’s magnetic field, could push exposure well beyond safe thresholds. This is one of the central engineering challenges for deep-space human exploration, and current spacecraft designs include dedicated shielding areas where crews can shelter during solar storms.
The Solar Cycle and Current Activity
Solar flare frequency follows an approximately 11-year cycle tied to the sun’s magnetic activity. During solar minimum, flares are rare and generally weak. During solar maximum, the sun can produce multiple M- and X-class flares per week. The current cycle, Solar Cycle 25, was predicted by NOAA’s Solar Cycle Prediction Panel to reach its maximum around July 2025, with a peak sunspot number of about 115. The expected range places the peak between November 2024 and March 2026.
In practice, Cycle 25 has been more active than many early forecasts anticipated. This means the period from 2024 through 2026 carries the highest likelihood of major flare events and associated space weather impacts. Solar Cycle 26 is expected to begin sometime between 2029 and 2032, but no predictions for its intensity have been issued yet.
How Flares Are Monitored
NOAA’s Space Weather Prediction Center tracks solar flares in real time using X-ray sensors on geostationary satellites. When a flare is detected, alerts go out to airlines, power grid operators, satellite companies, and government agencies within minutes.
NASA’s Solar Dynamics Observatory (SDO) provides detailed imaging of flares across ten different wavelengths of extreme ultraviolet light. Each wavelength reveals plasma at a different temperature, letting scientists map the structure and evolution of a flare from its cooler outer edges to its superheated core. The hottest wavelengths, capturing material at over 11 million degrees Fahrenheit, are specifically tuned to highlight the most intense flare activity. Together, these instruments give scientists and forecasters the data needed to understand each event and predict its consequences for Earth.