Solar Flares

Schematic diagram of a solar flare. Red and blue lines represent magnetic fields, carrying solar material off the surface. Flares occur when these field lines meet and "reconnect", producing huge explosions, and heating and acceleration of solar material. Credit: NASA Marshall Space Flight Center.

NASA SOHO Image Solar Flare

Solar flares are intense, temporary releases of energy. They are seen at ground-based observatories as bright areas on the Sun in optical wavelengths and as bursts of noise at radio wavelengths; they can last from minutes to hours. The primary energy source for flares appears to be the tearing and reconnection of strong magnetic fields. They radiate throughout the electromagnetic spectrum through visible light out to kilometer-long radio waves.

Massive Solar Storms Video From The National Geographic

A flare is defined as a sudden, rapid, and intense variation in brightness. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to x-rays and gamma rays at the short wavelength end. The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time.

This "lightbulb" Coronal Mass Ejection (CME) shows the three classical parts of a CME: leading edge, void, and core. Taken on February 27, 2000 by the LASCO C3 coronagraph.

As the magnetic energy is being released, particles, including electrons, protons, and heavy nuclei, are heated and accelerated in the solar atmosphere. The energy released during a flare is typically on the order of 1027 ergs per second. Large flares can emit up to 1032 ergs of energy. This energy is ten million times greater than the energy released from a volcanic explosion. On the other hand, it is less than one-tenth of the total energy emitted by the Sun every second. There are typically three stages to a solar flare. First is the precursor stage, where the release of magnetic energy is triggered. Soft x-ray emission is detected in this stage. In the second or impulsive stage, protons and electrons are accelerated to energies exceeding 1 MeV. During the impulsive stage, radio waves, hard x-rays, and gamma rays are emitted. The gradual build up and decay of soft x-rays can be detected in the third, decay stage. The duration of these stages can be as short as a few seconds or as long as an hour. Solar flares extend out to the layer of the Sun called the corona. The corona is the outermost atmosphere of the Sun, consisting of highly rarefied gas. This gas normally has a temperature of a few million degrees Kelvin. Inside a flare, the temperature typically reaches 10 or 20 million degrees Kelvin, and can be as high as 100 million degrees Kelvin. The corona is visible in soft x-rays, as in the above image. Notice that the corona is not uniformly bright, but is concentrated around the solar equator in loop-shaped features. These bright loops are located within and connect areas of strong magnetic field called active regions. Sunspots are located within these active regions. Solar flares occur in active regions. The frequency of flares coincides with the Sun's eleven year cycle. When the solar cycle is at a minimum, active regions are small and rare and few solar flares are detected. These increase in number as the Sun approaches the maximum part of its cycle.

The Classification of  Solar Flares

A solar flare is an explosion on the Sun that happens when energy stored in twisted magnetic fields (usually above sunspots) is suddenly released. Flares produce a burst of radiation across the electromagnetic spectrum, from radio waves to x-rays and gamma-rays. 

Scientists classify solar flares according to their x-ray brightness in the wavelength range 1 to 8 Angstroms. There are 3 categories: X-class flares are big; they are major events that can trigger planet-wide radio blackouts and long-lasting radiation storms. M-class flares are medium-sized; they can cause brief radio blackouts that affect Earth's polar regions. Minor radiation storms sometimes follow an M-class flare. Compared to X- and M-class events, C-class flares are small with few noticeable consequences here on Earth.

Each category for x-ray flares has nine subdivisions ranging from, e.g., C1 to C9, M1 to M9, and X1 to X9. In this figure, the three indicated flares registered (from left to right) X2, M5, and X6.

Peak (W/m2)between 1 and 8 Angstroms

NASA: The Mystery of the Aurora

People in the Northern Hemisphere have particular reason to keep an eye on the sun, both because of geography and geology. The region is relatively close to the magnetic North Pole, where solar disturbances often enter the Earth's atmosphere, and the granite bedrock below readily conducts surges of electricity. 

Though NASA scientists say the solar maximum this cycle may  be more potent as the last one, society is more vulnerable to damage today because so much more technology orbits the Earth. Hundreds of satellites, most of them used for telecommunications, have been launched since 1989.

Geomagnetic Substorm over Asia NASA Polar Satellite image

In a worst-case scenario, they say solar storms could knock out all high-frequency radio on the sunlit side of the Earth or expose airplane passengers in northern latitudes (near the North Pole) to the equivalent of 100 chest X-rays.