Nanoflares
Encyclopedia
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 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 micro-nanoflares, not observable individually.
in the photosphere
, opens into semicirculal structures in the corona. These coronal loops, which can be seen in the EUV and X-ray images (see the figure on the left), confine very hot plasma, emitting as it were at a temperature of a few million degrees.
Many flux tubes are stable for several days on the solar corona in the 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 coronal heating is released by current dissipation or Joule effect.
However, at first, astronomers believed that a single event of magnetic reconnection
was responsible for very dynamic processes like flares and other phenomena linked to coronal activity.
On the other hand, the theory of nanoflares supposes that these events of magnetic reconnection, occurring at the same time on small length-scales wherever in the corona, are very numerous and give only a small fraction of energy. These nanoflares might be very tiny flares, so close one to each other, both in time and in space, to heat the corona and to cause all the phenomena due to solar activity.
The sudden light bursts periodically observed in active regions as well 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 supporting this theory is the fact that the distribution of the number of flares observed in the hard X-rays is a function of the energy, following a power law with negative spectral index 1.8
.
If this distribution would have the same spectral index also at lower energies, flares, micro-flares and nanoflares might provide a considerable part of coronal heating.
Actually a negative spectral index of the order of 2 is required in order to maintain the solar corona.
The amount of energy stored in the solar magnetic field can account for the coronal heating necessary to maintain the plasma at this temperature and to balance coronal radiative losses
.
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 irradiated externally,
is conducted back towards the chromosphere
along the arcs.
In the transition region where the temperature is about 104 -105 K, radiative losses are too high to be balanced by whatever 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, principally due to the numerous emission lines, which are formed from ionized atoms (oxygen, carbon, iron and so on).
The solar convection can supply for 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.
And furthermore, where does it occur exactly: in the low corona
or mostly higher, where the magnetic field lines open into the space heliosphere
, blowing the solar wind
into the solar system
?
By now the importance of the magnetic field is recognized by all the scientists:
there is a strict correspondence between the active regions, 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 wherever and so uniformly 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 generated by convective motions in the photosphere
can go through the chromosphere
and transition region, carrying an energy flux comparable to that required to sustain the corona
.
Anyway wave train 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 elapsed by alfvén
waves to cross a typical coronal loop. This means that most of the dissipative mechanisms might 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
in coronal holes
.
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 transformed into electric one and then into heat for Joule effect.
The braiding of the field lines of the coronal magnetic flux tubes provokes events of magnetic reconnection
with a consequent change of the magnetic field
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 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
has been invoked as the main power source of solar flares. 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.
Solar flare
A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...
which happens in the corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
, the external atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
.
The hypothesis of "microflares" as a possible explanation of the coronal heating was first suggested by Gold
and then later developed by Eugene Parker
Eugene Parker
Eugene N. Parker is an American solar astrophysicist who received his B.S. degree in physics from Michigan State University in 1948 and Ph.D. from Caltech in 1951. In the mid 1950s Parker developed the theory on the supersonic solar wind and predicted the Parker spiral shape of the solar magnetic...
.
According to Parker a nanoflare arises from an event of magnetic reconnection
Magnetic reconnection
Magnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration...
which converts the energy stored in the solar magnetic field into the motion of the plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
.
The plasma motion (thought as fluid motion) occurs at length-scales so small that it is soon dumped by the turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
and then by the viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
. In such a way the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
is quickly converted into heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
, 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
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
emission, over an area 1" x 1", 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 micro-nanoflares, not observable individually.
Nanoflares and coronal activity
The observations show that the solar magnetic field, which is frozen into the motion of the plasmaPlasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
in the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
, opens into semicirculal structures in the corona. These coronal loops, which can be seen in the EUV and X-ray images (see the figure on the left), confine very hot plasma, emitting as it were at a temperature of a few million degrees.
Many flux tubes are stable for several days on the solar corona in the 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 coronal heating is released by current dissipation or Joule effect.
However, at first, astronomers believed that a single event of magnetic reconnection
Magnetic reconnection
Magnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration...
was responsible for very dynamic processes like flares and other phenomena linked to coronal activity.
On the other hand, the theory of nanoflares supposes that these events of magnetic reconnection, occurring at the same time on small length-scales wherever in the corona, are very numerous and give only a small fraction of energy. These nanoflares might be very tiny flares, so close one to each other, both in time and in space, to heat the corona and to cause all the phenomena due to solar activity.
The sudden light bursts periodically observed in active regions as well 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
Self-organized criticality
In physics, self-organized criticality is a property of dynamical systems which have a critical point as an attractor. Their macroscopic behaviour thus displays the spatial and/or temporal scale-invariance characteristic of the critical point of a phase transition, but without the need to tune...
, 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 supporting this theory is the fact that the distribution of the number of flares observed in the hard X-rays is a function of the energy, following a power law with negative spectral index 1.8
.
If this distribution would have the same spectral index also at lower energies, flares, micro-flares and nanoflares might provide a considerable part of coronal heating.
Actually a negative spectral index of the order of 2 is required in order to maintain the solar corona.
Nanoflares and coronal heating
The problem of coronal heating is still unsolved, although many steps ahead have been done in this direction and other evidences of nanoflares have been found in the solar corona.The amount of energy stored in the solar magnetic field can account for the coronal heating necessary to maintain the plasma at this temperature and to balance coronal radiative losses
Coronal radiative losses
In astronomy and in astrophysics, for radiative losses of the solar corona, it is meant the energy flux irradiated from the external atmosphere of the Sun , and, in particular, the processes of production of the radiation coming from the solar corona and transition region, where the plasma is...
.
The radiation is not the only mechanism of energy loss in the corona: since the plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
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 irradiated externally,
is conducted back towards the chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
along the arcs.
In the transition region where the temperature is about 104 -105 K, radiative losses are too high to be balanced by whatever 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, principally due to the numerous emission lines, which are formed from ionized atoms (oxygen, carbon, iron and so on).
The solar convection can supply for 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.
And furthermore, where does it occur exactly: in the low corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
or mostly higher, where the magnetic field lines open into the space heliosphere
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
, blowing the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
into the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
?
By now the importance of the magnetic field is recognized by all the scientists:
there is a strict correspondence between the active regions, 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 wherever and so uniformly 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 generated by convective motions in the photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
can go through the chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
and transition region, carrying an energy flux comparable to that required to sustain the corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
.
Anyway wave train 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 elapsed by alfvén
waves to cross a typical coronal loop. This means that most of the dissipative mechanisms might 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
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
in coronal holes
Coronal holes
Coronal holes are areas where the Sun's corona is darker, colder, and has lower-density plasma than average. These were found when X-ray telescopes in the Skylab mission were flown above the Earth's atmosphere to reveal the structure of the corona. Coronal holes are linked to unipolar...
.
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 transformed into electric one and then into heat for Joule effect.
The braiding of the field lines of the coronal magnetic flux tubes provokes events of magnetic reconnection
Magnetic reconnection
Magnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration...
with a consequent change of the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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 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
Magnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration...
has been invoked as the main power source of solar flares. 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
- SunSunThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
- CoronaCoronaA corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
- Solar flareSolar flareA solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...
- Coronal mass ejectionCoronal mass ejectionA coronal mass ejection is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space....
- Coronal radiative lossesCoronal radiative lossesIn astronomy and in astrophysics, for radiative losses of the solar corona, it is meant the energy flux irradiated from the external atmosphere of the Sun , and, in particular, the processes of production of the radiation coming from the solar corona and transition region, where the plasma is...
- Coronal loops
- Solar windSolar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
- Coronal cloud
- PhotospherePhotosphereThe photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
- ChromosphereChromosphereThe chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
- Transition region
- Stellar active region
- Magnetic reconnectionMagnetic reconnectionMagnetic reconnection is a physical process in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration...
- Current sheetCurrent sheetA current sheet is an electric current that is confined to a surface, rather than being spread through a volume of space. Current sheets feature in magnetohydrodynamics , the study of the behavior of electrically conductive fluids: if there is an electric current through part of the volume of such...
- Plasma physics
- X-ray astronomyX-ray astronomyX-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and...
External links
- Nasa news Tiny Flares Responsible for Outsized Heat of Sun's Atmosphere.