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Concept
Nanoflare
Résumé
A nanoflare is a very small episodic heating event which happens in the corona, the external atmosphere of the Sun.
The hypothesis of small impulsive heating events as a possible explanation of the coronal heating was first suggested by Thomas Gold
and then later developed and dubbed "nanoflares" 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 damped 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 , 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 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. 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.
Recent evidence from the EUNIS sounding rocket has provided some spectral evidence for non-flaring plasma at temperatures near 9 MK in active region cores.
Telescopic observations suggest that the solar magnetic field, which theoretically is "frozen" into the gas of the plasma in the photosphere, expands into roughly semicircular structures in the corona. These coronal loops, which can be seen in the EUV and X-ray images (see the figure on the left), often confine very hot plasmas, with emissions characteristic of temperature of a one to a few million degrees.
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