What is "nanoflares"

n. (plural of nanoflare English)

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 10 J should happen every 20 seconds, and 1000 nanoflares per second should occur in a large active region of 10 x 10 km. 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.