Solar Flares and How They Work

What you need to know about solar flares

Solar flares are often accompanied by a coronal mass ejection.
Solar flares are often accompanied by a coronal mass ejection. VICTOR HABBICK VISIONS / Getty Images

A sudden flash of brightness on the Sun's surface is called a solar flare. If the effect is seen on a star besides the Sun, the phenomenon is called a stellar flare. A stellar or solar flare releases a vast amount of energy, typically on the order of 1 × 1025 joules, over a broad spectrum of wavelengths and particles. This amount of energy is comparable to the explosion of 1 billion megatons of TNT or ten million volcanic eruptions.

In addition to light, a solar flare may eject atoms, electrons, and ions into space in what is called a coronal mass ejection. When particles are released by the Sun, they are able to reach Earth within a day or two. Fortunately, the mass may be ejected outward in any direction, so the Earth isn't always affected. Unfortunately, scientists aren't able to forecast flares, only give warning when one has occurred.

The most powerful solar flare was the first one that was observed. The event occurred on September 1, 1859 and is called the Solar Storm of 1859 or the "Carrington Event". It was reported independently by astronomer Richard Carrington and Richard Hodgson. This flare was visible to the naked eye, set telegraph systems aflame, and produced auroras all the way down to Hawaii and Cuba. While scientists at the time didn't have the ability to measure the strength of the solar flare, modern scientists were able to reconstruct the event based on nitrate and the isotope beryllium-10 produced from the radiation.

Essentially, evidence of the flare was preserved in ice in Greenland.

How a Solar Flare Works

Like planets, stars consists of multiple layers. In the case of a solar flare, all layers of the Sun's atmosphere are affected. In other words, energy is released from the photosphere, chromosphere, and corona.

Flares tend to occur near sunspots, which are regions of intense magnetic fields. These fields link the atmosphere of the Sun to its interior. Flares are believed to result from a process called magnetic reconnection, when loops of magnetic force break apart, rejoin, and release energy. When magnetic energy is suddenly released by the corona (suddenly meaning over a matter of minutes), light and particles are accelerated into space. The source of the released matter appears to be material from the unconnected helical magnetic field, however, scientists haven't completely worked out how flares work and why there are sometimes more released particles than the amount within a coronal loop. Plasma in the affected area reaches temperatures in the order of tens of million Kelvin, which is nearly as hot as the Sun's core. The electrons, protons, and ions are accelerated by the intense energy to nearly the speed of light. Electromagnetic radiation covers the entire spectrum, from gamma rays to radio waves. The energy released in the visible part of the spectrum makes some solar flares observable to the naked eye, but most of the energy is outside the visible range, so flares are observed using scientific instrumentation.

Whether or not a solar flare is accompanied by a coronal mass ejection is not readily predictable. Solar flares may also release a flare spray, which involves an ejection of material that is faster than a solar prominence. Particles released from a flare spray may attain a velocity of 20 to 200 kilometers per second (kps). To put this into perspective, the speed of light is 299.7 kps!

How Often Do Solar Flares Occur?

Smaller solar flares occur more often than large ones. The frequency of any flare occurring depends on the activity of the Sun. Following the 11-year solar cycle, there may be several flares per day during an active part of the cycle, compared with fewer than one per week during a quiet phase. During peak activity, there may be 20 flares a day and over 100 per week.

How Solar Flares Are Classified

An earlier method of solar flare classification was based on intensity of the Hα line of the solar spectrum.

The modern classification system categorizes flares according to their peak flux of 100 to 800 picometer X-rays, as observed by the GOES spacecraft that orbit the Earth.

ClassificationPeak Flux (Watts per square meter)
A< 10−7
B10−7 – 10−6
C10−6 – 10−5
M10−5 – 10−4
X> 10−4

Each category is further ranked on a linear scale, such that an X2 flare is twice as potent as a X1 flare.

Ordinary Risks from Solar Flares

Solar flares produce what is called solar weather on Earth. The solar wind impacts the magnetosphere of the Earth, producing aurora borealis and australis, and presenting a radiation risk to satellites, spacecraft, and astronauts. Most of the risk is to objects in low Earth orbit, but coronal mass ejections from solar flares can knock out power systems on Earth and completely disable satellites. If satellites did come down, cell phones and GPS systems would be without service. The ultraviolet light and x-rays released by a flare disrupt long-range radio and likely increase risk of sunburn and cancer.

Could a Solar Flare Destroy the Earth?

In a word: yes. While the planet itself would survive an encounter with a "superflare", the atmosphere could be bombarded with radiation and all life could be obliterated. Scientists have observed the release of superflares from other stars up to 10,000 times more powerful than a typical solar flare. While most of these flares occur in stars that have more powerful magnetic fields than our Sun, about 10% of the time the star is comparable to or weaker than the Sun.

From studying tree rings, researchers believe Earth has experienced two small superflares— one in 773 C.E. and another in 993 C.E. It's possible we can expect a superflare about once a millennium. The chance of an extinction level superflare is unknown.

Even normal flares can have devastating consequences. NASA revealed Earth narrowly missed a catastrophic solar flare on July 23, 2012. If the flare had occurred just a week earlier, when it was pointed directly at us, society would have been knocked back to the Dark Ages. The intense radiation would have disabled electrical grids, communication, and GPS on a global scale.

How likely is such an event in the future? Physicist Pete Rile calculates the odds of a disruptive solar flare is 12% per 10 years.

How to Predict Solar Flares

At present, scientists cannot predict a solar flare with any degree of accuracy. However, high sunspot activity is associated with an increased chance of flare production. Observation of sunspots, particularly the type called delta spots, is used to calculate the probability of a flare occurring and how strong it will be. If a strong flare (M or X class) is predicted, the US National Oceanic and Atmospheric Administration (NOAA) issues a forecast/warning. Usually, the warning allows for 1-2 days of preparation. If a solar flare and coronal mass ejection occur, the severity of the flare's impact on Earth depends on the type of particles released and how directly the flare faces the Earth.

Selected References

"Description of a Singular Appearance seen in the Sun on September 1, 1859", Monthly Notices of the Royal Astronomical Society, v20, pp13+, 1859

C. Karoff et al, Observational evidence for enhanced magnetic activity of superflare starsNature Communications 7, Article number: 11058 (2016)

"Big Sunspot 1520 Releases X1.4 Class Flare With Earth-Directed CME". NASA. July 12, 2012 (retrieved 04/23/17)