Solar Flares: The Ultimate Guide to These Cosmic Blasts

Solar flares are believed to result from the acceleration of charged particles in the plasma of the Sun's atmosphere due to stored magnetic energy. This process releases electromagnetic radiation across the spectrum. The most intense ultraviolet and X-ray radiation from these flares is absorbed by the Earth's upper atmosphere, particularly the ionosphere, impacting short-wave radio communications temporarily by increasing ionization levels.

Characteristics and Impact

Solar flares affect all layers of the solar atmosphere—the photosphere, chromosphere, and corona. During these events, the plasma can heat to over 10 million kelvin, and particles like electrons and protons accelerate to nearly the speed of light. This results in emissions ranging from radio waves to gamma rays.

Typically occurring around sunspots where magnetic fields are most concentrated, solar flares release stored magnetic energy over minutes to tens of minutes. This rapid release can also trigger CMEs, although the exact relationship between the two phenomena remains unclear. Flares are also associated with flare sprays, which eject material at speeds between 20 and 2000 kilometers per second.

Frequency and Patterns

The frequency of solar flares varies throughout the solar cycle, ranging from several per day at solar maximum to fewer than one per week at solar minimum. Larger flares are less common than smaller ones, with severe X10-class flares occurring around eight times per cycle compared to minor M1-class flares, which can occur up to 2000 times per cycle.

A notable discovery by Erich Rieger and colleagues in 1984 revealed a 154-day periodicity in the occurrence of gamma-ray emitting solar flares, known as the Rieger period. This period has been confirmed through various heliophysics data and appears in the interplanetary magnetic field.

Duration and Post-Eruption Phenomena

The duration of a solar flare can vary depending on the wavelength used for measurement. For example, the full width at half maximum (FWHM) time of soft X-ray flux, as observed by the GOES spacecraft, ranges from tens of seconds to several hours. Median durations are about 6 and 11 minutes for specific wavelength bands.

Following a flare, post-eruption loops of hot plasma form along the neutral line separating regions of opposite magnetic polarity. These loops can evolve into a post-eruption arcade, lasting from hours to days. In some cases, supra-arcade downflows, which are dark plasma voids, may form above these arcades.

The Mystery of Magnetic Reconnection

The primary mechanism behind solar flares is believed to be magnetic reconnection, where magnetic energy is converted into kinetic energy, accelerating particles. This process often occurs in solar arcades, where magnetic lines reconnect, releasing energy and potentially causing CMEs. However, the detailed mechanisms of particle acceleration and energy transformation remain partially understood.