Nanoflare
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 fluid plasma motion occurs at length-scales so small that it is soon damped by turbulence and then by viscosity. Damping quickly converts energy into heat, which is conducted by free electrons along the magnetic field lines closest to the place where the nanoflare switches on. In order to heat a region of very high X-ray emission, over an area of one square arcsec on the Sun, a nanoflare of 1017 J should happen every 20 seconds, and 1000 nanoflares per second should occur in a large active region of 105 × 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 was proposed long before sensors were able to confirm it empirically. Simulations predict that nanoflares produce a faint, hot (~10 MK) component of the emission measure. As of 2012, then-current instruments, such as the Extreme-Ultraviolet Imaging Spectrometer on board Hinode, were not adequately sensitive to the range in which this faint emission occurs, making a confident detection impossible. Evidence published in 2014 from the EUNIS sounding rocket provided some spectral evidence for non-flaring plasma at temperatures near 9 MK in active region cores. Observation of the full life-cycle of a nanoflare was first reported in 2020.