Solar Flare

Solar Flare, violent release of magnetic energy in the inner atmosphere of the Sun and other stars. Flares are most common at times of high sunspot activity; around solar maximum (the point of maximum activity in the 11-year sunspot cycle) as many as 25 may occur on the Sun’s visible hemisphere each day. The mechanisms by which flares occur remain subject to much debate, but it is generally agreed that processes of reconnection in the intense, often convoluted magnetic fields above sunspot groups are involved. This is when regions of opposed polarity are brought into contact, resulting in an explosive release of energy, particularly at X-ray and ultraviolet wavelengths. Temperatures of about 20 million K (36 million° F) have been recorded in solar flares.

Solar flares are associated with coronal mass ejections (CMEs), which throw vast amounts of material into the solar wind. Models developed in the mid-1990s suggest that flares may be a consequence of stress-release during CMEs, rather than vice versa.

Most flares are observed at the wavelength of hydrogen-alpha (656.3 nanometres; 1 nm = 10^-9 m/40 billionths of an inch); only very rarely are they detectable in visible light. They range in intensity from weak sub-flares to rare, extremely major events covering as much as 0.1 per cent of the solar disc. Typical flares last for a matter of hours.

Solar flares have important effects on the Earth’s ionosphere. Increased ionization of the D-layer (the lowest layer of the ionosphere) by X-ray radiation from flares gives rise to sudden ionospheric disturbances on the side of the earth facing the sun at the time of impact, disrupting radio communication. Heating and expansion of the upper atmosphere by repeated solar flare activity leads to increased drag on low-orbiting artificial satellites in the years around sunspot maximum, and has decreased the orbital lifetimes of several—including, ironically, the Solar Max satellite, launched to study such activity. The arrival of material released during CMEs gives rise to magnetic storms, sometimes accompanied by spectacular low-latitude aurorae.