A solar flare is a very strong, focused release of electromagnetic radiation from the Sun's atmosphere. Flares happen in areas of the Sun that are active and are often, but not always, linked to coronal mass ejections, solar particle events, and other types of solar eruptions. The number of solar flares changes based on the 11-year solar cycle.
Solar flares are believed to happen when magnetic energy stored in the Sun's atmosphere speeds up charged particles in the surrounding plasma. This causes electromagnetic radiation to be released across the electromagnetic spectrum. The usual pattern of these emissions has three clear stages: a precursor stage, an impulsive stage where particle acceleration is most active, and a gradual stage where hot plasma injected into the corona by the flare cools through a mix of radiation and energy moving back down to the lower atmosphere. Some flares also have an unexplained extreme ultraviolet (EUV) late stage.
The extreme ultraviolet and X-ray radiation from solar flares is absorbed by Earth's upper atmosphere on the side facing the Sun, especially the ionosphere, and does not reach Earth's surface. This absorption can temporarily increase the ionization of the ionosphere, which might disrupt short-wave radio communication. Predicting solar flares is an ongoing area of scientific study.
Flares also happen on other stars, where the term "stellar flare" is used.
Physical description
Solar flares are bursts of electromagnetic radiation that come from the Sun's atmosphere. They affect all parts of the solar atmosphere, including the photosphere, chromosphere, and corona. During a flare, the plasma in the atmosphere is heated to over 10 million kelvin, and electrons, protons, and heavier ions are accelerated to nearly the speed of light. Flares produce electromagnetic radiation across the entire spectrum, from radio waves to gamma rays.
Flares happen in active regions on the Sun, often near sunspots, where strong magnetic fields connect the corona to the Sun's interior. Flares are caused by the quick release of magnetic energy stored in the corona, which can take minutes to tens of minutes. This same energy can also create coronal mass ejections (CMEs), though scientists do not fully understand how flares and CMEs are related.
Flare sprays are events that send material outward at faster speeds than eruptive prominences, reaching speeds of 20 to 2,000 kilometers per second.
Flares occur when charged particles, mainly electrons, interact with the plasma in the Sun's atmosphere. Evidence suggests that magnetic reconnection—a process where magnetic field lines break and reconnect—causes the rapid acceleration of these particles. On the Sun, magnetic reconnection may happen in solar arcades, which are structures made of coronal loops that follow magnetic field lines. When these lines reconnect, they form a lower set of loops and leave a helix-shaped magnetic field that is no longer connected to the rest of the structure. This sudden release of energy causes the acceleration of particles. The unconnected magnetic field and the material it carries may expand outward, forming a CME. This explains why flares often happen in regions of the Sun where magnetic fields are strongest.
Scientists agree that flares are powered by magnetic energy, but the exact processes that convert this energy into the motion of particles are not fully understood. It is unclear how magnetic energy becomes the kinetic energy of particles or how some particles reach very high speeds, such as the GeV range (10^9 electron volts). There are also questions about the total number of accelerated particles, which sometimes appears to be greater than the number of particles in the coronal loop.
After a flare erupts, hot plasma forms loops across the neutral line where opposite magnetic polarities meet near the flare's location. These loops extend from the photosphere into the corona and grow farther from the source over time. Scientists believe these loops remain hot due to continued heating after the flare and during its decay.
In strong flares, such as those classified as C-class or higher, these loops may join to form a long, arch-like structure called a post-eruption arcade. These structures can last for hours or even days after the flare. In some cases, dark areas of plasma, called supra-arcade downflows, may appear above these arcades.
Frequency
The number of solar flares that happen changes based on the 11-year solar cycle. During the highest activity periods, called solar maxima, several flares can occur each day. During the lowest activity periods, called solar minima, fewer than one flare may happen each week. Stronger flares are rarer than weaker ones. For example, X10-class flares, which are very strong, happen about 8 times each cycle. M1-class flares, which are weaker, happen about 2,000 times each cycle.
In 1984, Erich Rieger and others found a pattern of about 154 days in the timing of gamma-ray flares that occurred at least since the 19th solar cycle. This pattern has been confirmed in many space-related data sets, including the magnetic field in space, and is called the Rieger period. Similar patterns, called resonance harmonics, have also been found in many types of space data.
The way solar flares happen can be described using a type of pattern called a power-law distribution. For example, the highest levels of radio waves, extreme ultraviolet light, and X-ray emissions; the total energy released; and the length of time flares last (see § Duration) have all been found to follow this pattern.
Classification
The modern system for classifying solar flares uses the letters A, B, C, M, or X. This classification is based on the highest strength, measured in watts per square meter (W/m²), of soft X-rays with wavelengths between 0.1 and 0.8 nanometers (1 to 8 ångströms). These measurements are taken by GOES satellites that orbit Earth at a fixed distance.
Each class has a number added to it to show the strength of the flare. The number ranges from 1 to 9. For example, an X2 flare is twice as strong as an X1 flare, and an X3 flare is three times as strong as an X1 flare. M-class flares are one-tenth the size of X-class flares with the same number. An X2 flare is four times stronger than an M5 flare. If an X-class flare has a peak strength greater than 10 W/m², its number may be 10 or higher.
This system was first created in 1970 and included only C, M, and X classes. These letters were chosen to avoid confusion with other classification systems. The A and B classes were added in the 1990s as tools improved and could detect weaker flares. Around the same time, the terms "moderate" and "extreme" were used to describe M-class and X-class flares, respectively.
An older system, sometimes called "flare importance," used H-alpha spectral observations. This method considered both the brightness of the flare and the area it covered. Brightness was described as faint (f), normal (n), or brilliant (b). The area was measured in millionths of the hemisphere, with the total hemisphere area (A H) equal to 15.5 × 10 km².
Flares were classified using a letter for brightness and a number or letter for size. For example, Sn represents a normal-sized flare.
The duration of a flare is often measured using the full width at half maximum (FWHM) time of its X-ray flux. This is the time between when the flare’s strength reaches halfway between its peak and the background level and when it returns to that level. Flares can last from tens of seconds to several hours. On average, flares last about 6 minutes in the 0.05 to 0.4 nm X-ray band and about 11 minutes in the 0.1 to 0.8 nm band.
Flares are also grouped by duration into impulsive or long duration events (LDEs). There is no clear time limit for separating these groups. The Space Weather Prediction Center (SWPC) considers flares that take 30 minutes or more to decay to half their maximum strength as LDEs. Belgium’s Solar-Terrestrial Centre of Excellence defines LDEs as flares lasting more than 60 minutes.
Effects
The light and energy from a solar flare travel away from the Sun at the speed of light. The strength of this energy decreases as the distance from the Sun increases, following a pattern where it becomes weaker with the square of the distance. The extra energy from solar flares, such as X-rays and extreme ultraviolet (XUV) light, can affect the atmospheres of planets. This is important for understanding space exploration and the search for life beyond Earth.
Solar flares also influence other objects in the Solar System. Scientists have studied their effects mainly on Mars and, to a lesser degree, on Venus. Research on how flares affect other planets, like Mercury, Jupiter, and Saturn, is limited. For Mercury, studies have focused on how ions in its magnetosphere respond to flares. For Jupiter and Saturn, research has only looked at how X-rays bounce off their upper atmospheres.
During solar flares, increased XUV light can cause more ionization, breaking apart molecules, and heating the ionospheres of Earth and similar planets. On Earth, these changes, called sudden ionospheric disturbances, can disrupt short-wave radio communication and systems like GPS. Over time, the expansion of the upper atmosphere can also increase drag on satellites in low Earth orbit, causing them to slowly lose altitude.
XUV light from solar flares interacts with the neutral gases in planetary atmospheres through a process called photoionization. This process frees electrons, which are called photoelectrons to distinguish them from other electrons in the atmosphere. These photoelectrons gain energy equal to the energy of the incoming XUV light minus the energy needed to break free from atoms. In the lower ionosphere, where solar flare effects are strongest, photoelectrons lose energy by transferring it to surrounding gases and by causing further ionization through collisions. This energy transfer heats the atmosphere and causes it to expand. The greatest ionization occurs in the lower ionosphere, where X-ray wavelengths are absorbed, such as Earth’s E and D layers and Mars’s M1 layer.
The temporary increase in ionization on Earth’s daylight side, especially in the D layer of the ionosphere, can disrupt short-wave radio communications. These communications rely on the ionosphere to reflect radio waves back to Earth. When ionization is higher than normal, radio waves lose energy more quickly due to frequent collisions with free electrons, causing signals to weaken or disappear.
The level of ionization in a planet’s atmosphere is linked to the strength of a solar flare, measured by its soft X-ray intensity. The Space Weather Prediction Center, part of the United States National Oceanic and Atmospheric Administration, uses this measurement to classify radio blackouts caused by solar flares.
During times when the Sun is quiet, small electric currents in the ionosphere’s E layer create slight daily changes in Earth’s magnetic field. These currents can grow stronger during large solar flares because the increased ionization makes the E and D layers more conductive. This leads to temporary changes in Earth’s magnetic field, called solar flare effects or magnetic crochets. The term "magnetic crochet" comes from the French word for "hook," describing the hook-like shape of magnetic field changes observed from Earth. These changes are small, lasting only a few minutes, and are much weaker than those caused by major geomagnetic storms.
For astronauts in low Earth orbit, the radiation from solar flares delivers a dose of about 0.05 gray, which is not immediately deadly. However, the particle radiation from solar particle events poses a greater risk to astronauts.
The effects of solar flare radiation on Mars are important for exploring the planet and searching for life. Models of Mars’s atmosphere suggest that the most powerful solar flares recorded could have delivered harmful or near-lethal radiation doses to mammals or other complex life forms on the planet’s surface. While flares strong enough to cause lethal radiation doses have not yet been observed on the Sun, they are believed to occur and have been seen on other stars similar to the Sun.
Observational history
Flares release radiation across the electromagnetic spectrum, but with varying strength. They are not strong in visible light, but they can be bright at certain wavelengths. They usually produce bremsstrahlung in X-rays and synchrotron radiation in radio waves.
Solar flares were first seen by Richard Carrington and Richard Hodgson on September 1, 1859, using an optical telescope with a broad-band filter. This flare was extremely bright in visible light, emitting a large amount of light in the visual spectrum.
Because flares emit a lot of radiation at H-alpha, adding a narrow (≈1 Å) passband filter centered at this wavelength to an optical telescope allows small telescopes to observe flares. For many years, Hα was the main, if not the only, source of information about solar flares. Other passband filters are also used.
During World War II, on February 25 and 26, 1942, British radar operators noticed radiation that Stanley Hey believed came from the Sun. This discovery was not made public until after the war ended. Around the same time, Southworth also observed the Sun in radio waves, but his findings were not widely known until after 1945. In 1943, Grote Reber became the first to report radioastronomical observations of the Sun at 160 MHz. The rapid growth of radioastronomy revealed new aspects of solar activity, such as storms and bursts linked to flares. Today, ground-based radiotelescopes observe the Sun from about 15 MHz up to 400 GHz.
The Earth's atmosphere blocks much of the electromagnetic radiation from the Sun with wavelengths shorter than 300 nm. Space-based telescopes have allowed scientists to observe solar flares in high-energy spectral lines that were previously unobserved. Since the 1970s, the GOES series of satellites have continuously monitored the Sun in soft X-rays, and their data have become the standard way to measure flares, reducing the importance of the H-alpha classification. Space-based telescopes also enable observations of extremely long wavelengths—up to several kilometers—that cannot pass through the ionosphere.
The most powerful flare ever recorded is believed to be the one linked to the 1859 Carrington Event. Although no soft X-ray measurements were taken at the time, ground-based magnetometers recorded a magnetic crochet associated with the flare. These readings allowed scientists to estimate the flare’s strength after the event. Based on magnetometer data, its soft X-ray class is estimated to be greater than X10 and around X45 (±5).
In modern times, the largest solar flare measured by instruments occurred on November 4, 2003. This event overwhelmed the GOES detectors, so its classification is only approximate. Initially, the GOES data suggested it was X28. Later analysis of ionospheric effects increased this estimate to X45. This flare provided the first clear evidence of a new spectral component above 100 GHz.
Prediction
Current methods for predicting solar flares have challenges, and there is no guarantee that an active region on the Sun will produce a flare. However, certain characteristics of active regions and their sunspots are linked to flare activity. For example, regions with complex magnetic fields, known as delta spots, often create the largest flares. A basic classification system for sunspots, based on the McIntosh method or related to a region's fractal complexity, is often used as a starting point for flare predictions. Predictions are usually expressed as chances of flares above M- or X-class occurring within 24 or 48 hours. The U.S. National Oceanic and Atmospheric Administration (NOAA) provides these forecasts. MAG4 was created at the University of Alabama in Huntsville with support from the Space Radiation Analysis Group at Johnson Space Flight Center (NASA/SRAG) to predict M- and X-class flares, coronal mass ejections (CMEs), fast CMEs, and solar energetic particle events. A physics-based method for predicting large solar flares was introduced by the Institute for Space-Earth Environmental Research (ISEE) at Nagoya University in Japan.