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A nanoflare is a very small solar flare which happens in the corona, the external atmosphere of the Sun. The hypothesis of "microflares" as a possible explanation of the coronal heating was first suggested by Gold and then later developed by Eugene Parker. According to Parker a nanoflare arises from an event of magnetic reconnection which converts the energy stored in the solar magnetic field into the motion of the plasma. The plasma motion (thought as fluid motion) occurs at length-scales so small that it is soon dumped by the turbulence and then by the viscosity. In such a way the energy is quickly converted into heat, and conducted by the free electrons along the magnetic field lines closer to the place where the nanoflare switches on. In order to heat a region of very high X-ray emission, over an area 1" x 1", a nanoflare of 1017 J should happen every 20 seconds, and 1000 nanoflares per second should occur in a large active region of 105 x 105 km2. On the basis of this theory, the emission coming from a big flare could be caused by a series of micro-nanoflares, not observable individually. The nanoflare model has long suffered from a lack of observational evidence. Simulations predict that nanoflares produce a faint, hot (~10 MK) component of the emission measure. Unfortunately, current instruments, such as the Extreme-Ultraviolet Imaging Spectrometer on board Hinode, are not adequately sensitive to the range in which this faint emission occurs, making a confident detection impossible. However, recent evidence from the EUNIS sounding rocket has provided promising spectral evidence for non-flaring plasma at temperatures near 9 MK in active region cores. == Nanoflares and coronal activity == The observations show that the solar magnetic field, which is frozen into the motion of the plasma in the photosphere, opens into semicirculal structures in the corona. These coronal loops, which can be seen in the EUV and X-ray images (see the figure on the left), confine very hot plasma, emitting as it were at a temperature of a few million degrees. Many flux tubes are stable for several days on the solar corona in the X-ray images, emitting at steady rate. However flickerings, brightenings, small explosions, bright points, flares and mass eruptions are observed very frequently, especially in active regions. These macroscopic signs of solar activity are considered by astrophysicists as the phenomenology related to events of relaxation of stressed magnetic fields, during which part of the coronal heating is released by current dissipation or Joule effect. However, at first, astronomers believed that a single event of magnetic reconnection was responsible for very dynamic processes like flares and other phenomena linked to coronal activity. On the other hand, the theory of nanoflares supposes that these events of magnetic reconnection, occurring at the same time on small length-scales wherever in the corona, are very numerous and give only a small fraction of energy. These nanoflares might be very tiny flares, so close one to each other, both in time and in space, to heat the corona and to cause all the phenomena due to solar activity. The sudden light bursts periodically observed in active regions as well as flares and coronal mass ejections could be provoked by cascade effects, similar to those described by the mathematical theories of catastrophes. In the hypothesis that the solar corona is in a state of self-organized criticality, the stressing of the magnetic field should be enhanced until a small perturbation switches on many small instabilities, happening together as it occurs in avalanches. One of the experimental results supporting this theory is the fact that the distribution of the number of flares observed in the hard X-rays is a function of the energy, following a power law with negative spectral index 1.8 . Actually, though, a negative spectral index larger than 2 is required in order to maintain the solar corona via the nanoflare hypothesis . 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「nanoflares」の詳細全文を読む スポンサード リンク
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