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Flares are tremendous explosions on the surface of the Sun which can last from several seconds to a few hours. The largest flares are also the longest in duration, but they are usually quite rare, occurring only a few times a year until solar maximum approaches. Many smaller flares occur down to the limits of detection of modern instruments at about 10²⁷ ergs. These generally last for a short time, down to a few seconds, and they, too, are most frequent near solar maximum, when there may be several dozen flares per day.

Figure 2.33 As the Sun rotates every 27 days, the solar wind becomes a complex spiral of high and low speeds, and high and low densities. When high speed solar wind overtakes slow speed wind, it creates a corotating interaction region. These interaction regions consist of solar wind with very high densities and strong magnetic fields.

(NASA)

Figure 2.34 The corona is threaded with magnetic fields (yellow lines). Areas with closed magnetic fields give rise to a slow, dense solar wind (short, dashed red arrows), while coronal holes with open magnetic fields yield fast, less dense solar wind streams (longer, solid red arrows). In addition to the permanent coronal holes at the poles, coronal holes sometimes occur closer to the equator (right of center). The image was taken by SOHO's Extreme Ultraviolet Imaging Telescope.

(ESA‐NASA)

As flares heat material to a temperature of up to 100 million degrees Celsius, they release energy equivalent to that released by millions of 100‐megaton hydrogen bombs exploding simultaneously. The energy is released in many forms: electromagnetic radiation across virtually the entire spectrum, from radio waves to X‐rays and gamma rays; energetic particles (protons and electrons); and mass flows.

The high‐energy electromagnetic radiation travels at light speed across interplanetary space, sometimes heading towards Earth. Such increases in X‐ray flux enhance the planet's ionosphere, causing it to decrease in altitude, but, fortunately, they are unable to penetrate the lower atmosphere.

Not far behind are the particles that are accelerated to near light speed by the flare. Streaming outward along magnetic field lines, they can reach the Earth–Moon system within 20 to 30 minutes, interfering with short wave radio communications, causing short circuits and computer reboots on satellites, and threatening the health of astronauts who are outside their spacecraft.

Flares are usually difficult to see in visible light against the bright background of the photosphere. Most are detected from the invisible radiation they emit. Radio and optical emissions can be observed with telescopes on the Earth, while orbiting observatories are needed to detect energetic emissions such as X‐rays and gamma rays.

There are typically three stages to a solar flare, each lasting anything from a few seconds to more than an hour. First is the precursor stage, when the release of magnetic energy is triggered. The soft X‐ray emission gradually increases but few, if any, hard X‐rays or gamma rays are detected.

In the second, impulsive stage, protons and electrons are accelerated to energies exceeding 1 MeV, whilst hard X‐rays and gamma rays are emitted, often rising in many short but intense “spikes,” each lasting a few seconds to tens of seconds. The soft X‐ray flux also rises more rapidly during this phase, often synchronizing with the hard X‐ray profile.

In the third, decay stage, hard X‐ray and gamma ray fluxes start to decay rapidly in a matter of minutes, whereas the soft X‐ray flux continues to rise, reaching a peak before declining again, sometimes over a period of several hours.

Flares (and coronal mass ejections) occur near sunspots, usually along the neutral dividing line between areas of oppositely directed magnetic fields. A flare occurs when an enormous amount of magnetic energy that has built up in the corona is suddenly released as the result of magnetic reconnection – when magnetic field lines snap and reconfigure themselves.

When a coronal loop rises to great height, it may become stretched and distorted, so that the two sides of the loop move closer together. When magnetic field lines snap and then reconnect, the original loop splits in two, forming a smaller arch at the surface and a separate loop in the corona. The excess energy is released in an explosive flare, often coinciding with a coronal mass ejection which blasts huge amounts of material into space.

The flare generated in the small arch accelerates electrons down the magnetic field lines, causing them to slam into the denser plasma below, producing X‐rays, microwaves, and a shock wave that heats the surface. The result is a major seismic wave in the Sun's interior, which can be observed as a series of outward moving ripples – much like the ripples that spread from a rock dropped into a pool of water.