Nanoflare

A nanoflare is a very small episodic heating event which could be prolific in the corona, the external atmosphere of the Sun. These would blend together to give the appearance of continuous heating. 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.

Nanoflares and coronal activity

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. Many flux tubes are relatively stable as seen in soft 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 energy they have stored is released ultimately into particle kinetic energy (heating); this could be via current dissipation, Joule effect, or any of several non-thermal plasma effects. Theoretical work often appeals to the concept of magnetic reconnection to explain these outbursts. Rather than a single large-scale episode of such a process, though, modern thinking suggests that a multitude of small-scale versions reconnection, cascading together, might be a better description. The theory of nanoflares then supposes that these events of magnetic reconnection, occurring at nearly the same time on small length-scales wherever in the corona, are very numerous, each providing an imperceptibly small fraction of the total energy required in a macroscopic event. These nanoflares might themselves resemble very tiny flares, close one to each other, both in time and in space, effectively heating the corona and underlying many of the phenomena of solar magnetic activity. Episodic heating often observed in active regions, including major events such 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 often cited in supporting the nanoflare theory is the fact that the distribution of the number of flares observed in the hard X-rays is a function of their energy, following a power law with negative spectral index. A sufficiently large power-law index would allow the smallest events to dominate the total energy. In the energy range of normal flares, the index has a value of approximately -1.8. This falls short of the power-law index which would be required order to maintain the heating of the solar corona via the nanoflare hypothesis. A power-law index greater than -2 is required to maintain the temperature observed in the corona.

Nanoflares and coronal heating

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