|
A solar flare is a sudden flash of brightness observed near the Sun's surface. It involves a very broad spectrum of emissions, requiring an energy release of up to 6 × 1025 joules of energy (roughly the equivalent of 160,000,000,000 megatons of TNT, over 25,000 times more energy than released from the impact of Comet Shoemaker–Levy 9 with Jupiter). Flares are often, but not always, accompanied by a spectacular coronal mass ejection. The flare ejects clouds of electrons, ions, and atoms through the corona of the sun into space. These clouds typically reach Earth a day or two after the event.〔Menzel, Whipple, and de Vaucouleurs, "Survey of the Universe", 1970〕 The term is also used to refer to similar phenomena in other stars, where the term stellar flare applies. Solar flares affect all layers of the solar atmosphere (photosphere, chromosphere, and corona), when the plasma medium is heated to tens of millions of Kelvin, while the cosmic-ray-like electrons, protons, and heavier ions are accelerated to near the speed of light. They produce radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays, although most of the energy is spread over frequencies outside the visual range and for this reason the majority of the flares are not visible to the naked eye and must be observed with special instruments. Flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior. Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. The same energy releases may produce coronal mass ejections (CME), although the relation between CMEs and flares is still not well established. X-rays and UV radiation emitted by solar flares can affect Earth's ionosphere and disrupt long-range radio communications. Direct radio emission at decimetric wavelengths may disturb the operation of radars and other devices that use those frequencies. Solar flares were first observed on the Sun by Richard Christopher Carrington and independently by Richard Hodgson in 1859〔"(Description of a Singular Appearance seen in the Sun on September 1, 1859 )", ''Monthly Notices of the Royal Astronomical Society'', v20, pp13+, 1859〕 as localized visible brightenings of small areas within a sunspot group. Stellar flares can be inferred by looking at the lightcurves produced from the telescope or satellite data of variety of other stars. The frequency of occurrence of solar flares varies, from several per day when the Sun is particularly "active" to less than one every week when the Sun is "quiet", following the 11-year cycle (the solar cycle). Large flares are less frequent than smaller ones. On July 23, 2012, a massive, and potentially damaging, solar superstorm (solar flare, coronal mass ejection, solar EMP) barely missed Earth, according to NASA. There is an estimated 12% chance of a similar event occurring between 2012 and 2022.〔 == Cause == Flares occur when speed up charged particles, mainly electrons, interact with the plasma medium. Scientific research suggests that the phenomenon of magnetic reconnection leads to this copious acceleration of charged particles. On the Sun, magnetic reconnection may happen on solar arcades – a series of closely occurring loops of magnetic lines of force. These lines of force quickly reconnect into a low arcade of loops leaving a helix of magnetic field unconnected to the rest of the arcade. The sudden release of energy in this reconnection is the origin of the particle acceleration. The unconnected magnetic helical field and the material that it contains may violently expand outwards forming a coronal mass ejection.〔"(The Mysterious Origins of Solar Flares )", ''Scientific American'', April 2006〕 This also explains why solar flares typically erupt from what are known as the active regions on the Sun where magnetic fields are much stronger on average. Although there is a general agreement on the flares' causes, the details are still not well known. It is not clear how the magnetic energy is transformed into the particle kinetic energy, nor is it known how the particles are accelerated to energies as high as 10 MeV (mega electron volt) and beyond. There are also some inconsistencies regarding the total number of accelerated particles, which sometimes seems to be greater than the total number in the coronal loop. Scientists are unable to forecast flares, even to this day. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「solar flare」の詳細全文を読む スポンサード リンク
|