Solar flares are the most energetic events observed on the Sun and they are lethal to unshielded life and space hardware. Fortunately, we are substantially protected by the Earth’s magnetosphere.
Les éruptions solaires sont les événements les plus énergétiques observés sur le Soleil et elles posent une menace importante pour la vie et pour les matériaux spatiaux. Heureusement, nous en sommes bien protégés par la magnétosphere de la Terre.
Flares are short lived phenomena where an area increases in brightness, across multiple wavelengths, and high amounts of energy are released. A ’typical’ flare will last between 100 and 1000 seconds and will release within that period between 10**19 (*) to 10**25 Joules. They emit radiation (and charged atomic particles) at all wavelengths of the spectrum but are particularly strong in EUV (extreme UV) and X-ray emissions.
(*) To put this in context, the lower limit here is approx. ten thousand times greater than the total energy that would be released by the detonation of a 12-warhead Trident-II nuclear missile weapon.
The first flare to be recorded (a visible light flare) was observed on 1st September 1859 by Richard Carrington and Richard Hodgson. This event, sometimes referred to as the ‘Solar storm of 1859’ or the ‘Carrington Event’, led to significant increases in the solar wind in the radial direction of the Earth. This produced a large scale geomagnetic storm, and two days after the flare was observed, aurora displays were seen at low Earth latitudes (such as the Caribbean).
La première érruption solaire enregistrée a été observé le 1er septembre 1859 par Richard Carrington and Richard Hodgson. Cet événement quelquefois appelé ‘la tempête solaire de 1859’ a mené à l'accroissement de l'intensité du vent solaire dans le sens radial de la Terre. Cela a produit une tempête géomagnétique, et deux jours après l’observation de l’éruption on a vu des des aurores boréales à des basses latittudes (par exemple aux Caraïbes).
Whilst some flares have an associated mass emission (eruptive flares), most flares ‘only’ emit electromagnetic radiation. The latter are the higher intensity, temperature, and energy type. The image below was taken at a wavelength of 195Å (i.e. EUV) by the TRACE satellite and shows a category M1.5 flare. The flare was inherently associated with AR0633. (AR = Active Region)
M1.5 flare (August 2004; AR0633)
Flare effective temperatures can be several million Kelvin. However, because they are very localised and relatively small-scale phenomena (each occupying <0.01% of the solar disc) they are rarely seen in the optical range of the spectrum and they do not materially or noticeably affect the TSI (Total Solar Irradiance) or observed visible brightness of the Sun.
The most widely used system for the classification of flares is based upon their maximum radiative output (flux) within the range 1Å to 8Å (which is in the X-Ray part of the spectrum). Five categories are used: A, B, C, M, X, in order of increasing peak X-ray flux.
Flares have three stages of evolution. The precursor stage where soft (lower energy) X-rays are produced and emitted is followed rapidly, typically less than five minutes, by the impulsive stage.
This stage is very short lived, less than a minute, when atomic particles (predominately protons, electrons and photons) are energised up to many MeV (Million electron-Volts). Baryonic matter (protons and electrons) is accelerated to high speed (typically from 100 km/s to >600 km/s). At these speeds the plasma (as well as the high energy photons electromagnetic radiation; i.e. the X-Ray and Gamma ray in the more energetic X classes) has sufficient speed to escape from the gravitational field (attraction) of the Sun.
The decay, or sometimes referred to as the gradual phase lasts between 10 to 20 minutes with radiation emissions gradually reducing over this period; with the higher frequency (energy) radiation falling off quicker than the relatively lower frequency radiation. Loop prominences are frequently seen in the region of the flare after the flare has subsided.
The source of energy within flares is considered to be magnetic forces; specifically the conversion of magnetic force into kinetic energy. This can result from magnetic reconnection, where magnetic flux lines interact and sever and reform. The reason why the flux lines interact is due to the stresses and pressures (shear) exerted on the field lines by the convection flow of material within the convection zone. These stresses allow a build-up of magnetic energy which is released explosively during the flare.
This simplified description belies the very detailed researches currently ongoing into 3-dimensional magnetic reconnection and a comprehensive explanation for how and why particles are accelerated to such high energies and speeds is still being determined.