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A nanoflare is a very small episodic heating event which happens in the
corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
, the external
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
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 Eugene Newman Parker (June 10, 1927 – March 15, 2022) was an American solar and plasma physicist. In the 1950s he proposed the existence of the solar wind and that the magnetic field in the outer Solar System would be in the shape of a Pa ...
. According to Parker a nanoflare arises from an event of
magnetic reconnection Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnecti ...
which converts the energy stored in the
solar magnetic field The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
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 In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
and then by the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the int ...
. In such a way the
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
is quickly converted into
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
, 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 An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
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. 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. 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.


Nanoflares and coronal activity

Telescopic observations suggest that the
solar magnetic field The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, which theoretically is "frozen" into the gas of the plasma in the
photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
, expands into roughly semicircular structures in the corona. These
coronal loops In solar physics, a coronal loop is a well-defined arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes. Coronal loops begin and end at two foo ...
, 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 An active region is a temporary region in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and sola ...
. 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 Joule effect and Joule's law are any of several different physical effects discovered or characterized by English physicist James Prescott Joule. These physical effects are not the same, but all are frequently or occasionally referred to in the lit ...
, or any of several non-thermal plasma effects. Theoretical work often appeals to the concept of
magnetic reconnection Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnecti ...
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 An active region is a temporary region in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and sola ...
, including major events such as
flares A flare, also sometimes called a fusée, fusee, or bengala in some Latin-speaking countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illumination, ...
and
coronal mass ejections A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly acce ...
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 A corona ( coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma. The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total solar ...
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

The problem of coronal heating is still unsolved, although research is ongoing and other evidence of nanoflares has been found in the solar corona. The amount of energy stored in the
solar magnetic field The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
can account for the coronal heating necessary to maintain the plasma at this temperature and to balance
coronal radiative losses In astronomy and in astrophysics, for radiative losses of the solar corona, it is meant the energy flux radiated from the external atmosphere of the Sun (traditionally divided into chromosphere, transition region and corona), and, in particular, ...
. The radiation is not the only mechanism of energy loss in the corona: since the plasma is highly ionized and the magnetic field is well organized, the thermal conduction is a competitive process. The energy losses due to the thermal conduction are of the same order of coronal radiative losses. The energy released in the corona which is not radiated externally is conducted back towards the
chromosphere A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively. In the ...
along the arcs. In the transition region where the temperature is about 104 -105 K, radiative losses are too high to be balanced by any form of mechanical heating. The very high temperature gradient observed in this range of temperatures increases the conductive flux in order to supply for the irradiated power. In other words, the transition region is so steep (the temperature increases from 10kK to 1MK in a distance of the order of 100 km) because the thermal conduction from the superior hotter atmosphere must balance the high radiative losses, as indicated to the numerous
emission lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...
, which are formed from ionized atoms (oxygen, carbon, iron and so on). The solar convection can supply the required heating, but in a way not yet known in detail. Actually, it is still unclear how this energy is transmitted from the chromosphere(where it could be absorbed or reflected), and then dissipated into the corona instead of dispersing into the solar wind. Furthermore, where does it occur exactly? In the low
corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
or mainly in the higher corona, where the magnetic field lines open into the space
heliosphere The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
, driving the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
into the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
. The importance of the magnetic field is recognized by all the scientists: there is a strict correspondence between the
active regions An active region is a temporary region in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and sola ...
, where the irradiated flux is higher (especially in the X-rays), and the regions of intense magnetic field. The problem of coronal heating is complicated by the fact that different coronal features require very different amounts of energy. It is difficult to believe that very dynamic and energetic phenomena such as flares and coronal mass ejections share the same source of energy with stable structures covering very large areas on the Sun: if nanoflares would have heated the whole corona, then they should be distributed so uniformly so as to look like a steady heating. Flares themselves – and microflares, which when studied in detail seem to have the same physics – are highly intermittent in space and time, and would not therefore be relevant to any requirement for continuous heating. On the other hand, in order to explain very rapid and energetic phenomena such as solar flares, the magnetic field should be structured on distances of the order of the metre. The
Alfvén waves Alfvén may refer to: People * Hannes Alfvén (1908–1995), Swedish plasma physicist and Nobel Prize in Physics laureate * Hugo Alfvén (1872–1960), Swedish composer, conductor, violinist, and painter * Marie Triepcke Krøyer Alfvén (1867–19 ...
generated by convective motions in the
photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
can go through the
chromosphere A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively. In the ...
and transition region, carrying an energy flux comparable to that required to sustain the
corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
. Anyway, wavetrain periods observed in the high chromosphere and in the lower transition region are of the order of 3-5 min. These times are longer than the time taken by Alfvén waves to cross a typical coronal loop. This means that most of the dissipative mechanisms may provide enough energy only at distances further from the solar corona. More probably, the Alfvén waves are responsible for the acceleration of the
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
in
coronal holes A coronal hole is a temporary region of relatively cool, less dense plasma in the solar corona where the Sun's magnetic field extends into interplanetary space as an open field.Freedman, Roger A., and William J. Kaufmann III. "Our Star, the Sun." ...
. The theory initially developed by Parker of micro-nanoflares is one of those explaining the heating of the corona as the dissipation of electric currents generated by a spontaneous relaxation of the magnetic field towards a configuration of lower energy. The magnetic energy is thus transformed into
Joule heating Joule heating, also known as resistive, resistance, or Ohmic heating, is the process by which the passage of an electric current through a conductor produces heat. Joule's first law (also just Joule's law), also known in countries of former US ...
. The braiding of the field lines of the coronal magnetic flux tubes provokes events of
magnetic reconnection Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnecti ...
with a consequent change of the
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
at small length-scales without a simultaneous alteration of the magnetic field lines at large length-scales. In this way it can be explained why
coronal loops In solar physics, a coronal loop is a well-defined arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes. Coronal loops begin and end at two foo ...
are stable and so hot at the same time. The Ohmic dissipation by currents could be a valid alternative to explain the coronal activity. For many years the
magnetic reconnection Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnecti ...
has been invoked as the main power source of
solar flares A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other sola ...
. However this heating mechanism is not very efficient in large current sheets, while more energy is released in turbulent regimes when nanoflares happen at much smaller scale-lengths, where non-linear effects are not negligible.


See also


References


External links


Nasa news
Tiny Flares Responsible for Outsized Heat of Sun's Atmosphere. {{The Sun Solar phenomena Sun