Solar flare

A solar flare is an intense, localized burst of electromagnetic radiation. This energy release happens within the Sun's atmosphere. These eruptions occur in active regions where magnetic fields are very strong. Often, flares are accompanied by other events like coronal mass ejections or solar particle events. They affect all layers of the solar atmosphere, including the photosphere, chromosphere, and corona.

Solar flares are not unique to our Sun. They also occur on other stars, where they are called stellar flares.

The mechanism behind a flare involves the release of stored magnetic energy. Scientists believe a process called magnetic reconnection causes this. This likely happens on solar arcades, which are series of coronal loops. These loops follow magnetic lines of force. During reconnection, these lines snap and join into new, lower loops. This sudden change releases massive amounts of energy. This energy accelerates charged particles, such as electrons and protons, in the surrounding plasma. Some particles reach near the speed of light.

These emissions typically follow three identifiable stages. First, there is a precursor phase. Next comes the impulsive phase, where particle acceleration is the main driver. Finally, a gradual phase occurs. In this stage, hot plasma injected into the corona cools down. It cools through radiation and by conducting energy back to the lower atmosphere. Some flares also show an extreme ultraviolet (EUV) late phase. This specific phase is currently unexplained by scientists.

Scientists classify solar flares using a specific system. This system is based on the peak flux of soft X-rays. The letters used are A, B, C, M, and X. These letters represent different ranges of watts per square metre (W/m2). An X-class flare is the most powerful, while an A-class is the weakest.

You can add a number to the letter to show intensity. An X2 flare is twice as strong as an X1 flare. This system was created in 1970 using only C, M, and X. The A and B classes were added later in the 1990s.

The frequency of these events changes over time. This variation follows the 11-year solar cycle. During solar maxima, there may be several flares per day. During solar minima, there may be fewer than one per week. Powerful flares are much less common than weaker ones. For example, M1-class flares occur about 2,000 times per cycle. However, X10-class flares occur only about eight times per cycle.

In 1984, Erich Rieger discovered a 154-day period in gamma-ray flares. This is now known as the Rieger period.

Solar flares can have significant effects on Earth. The X-ray and extreme ultraviolet (XUV) radiation hits our upper atmosphere. This layer is known as the ionosphere.

This radiation causes photoionization, which frees electrons in the atmosphere. This process can create sudden ionospheric disturbances. These disturbances can interfere with short-wave radio communication. They can also disrupt global navigation satellite systems (GNSS), such as GPS. Furthermore, the heated atmosphere expands. This expansion increases drag on satellites in low Earth orbit. This drag can cause their orbits to decay over time.

After a flare, new structures can form in the Sun's atmosphere. These are called post-eruption loops. They are made of hot plasma and form across the neutral line. These loops extend from the photosphere up into the corona. In powerful flares, these loops may form a post-eruption arcade. These large, arch-like structures can last for hours or even days.

These events show how deeply connected magnetic energy is to the entire solar system.