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Wikipedia

Stellar corona

For other uses, see Corona (disambiguation).

A corona (Latin for 'crown', in turn derived from Ancient Greekκορώνη, korṓnē, 'garland, wreath') is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into outer space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of1000000 kelvin, much hotter than the surface of the Sun.

During a total solar eclipse, the Sun's corona and prominences are visible to the naked eye.

Light from the corona comes from three main sources, from the same volume of space:

  • The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines spreads them so greatly as to completely obscure them, giving the spectral appearance of a continuum with no absorption lines.
  • The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light.
  • The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.

Contents

In 1724, French-Italian astronomer Giacomo F. Maraldi recognized that the aura visible during a solar eclipse belongs to the Sun, not to the Moon. In 1809, Spanish astronomer José Joaquín de Ferrer coined the term 'corona'. Based in his own observations of the 1806 solar eclipse at Kinderhook (New York), de Ferrer also proposed that the corona was part of the Sun and not of the Moon. English astronomer Norman Lockyer identified the first element unknown on Earth in the Sun's chromosphere, which was called helium. French astronomer Jules Jenssen noted, after comparing his readings between the 1871 and 1878 eclipses, that the size and shape of the corona changes with the sunspot cycle. In 1930, Bernard Lyot invented the coronograph, which allows viewing the corona without a total eclipse. In 1952, American astronomer Eugene Parker proposed that the solar corona might be heated by myriad tiny 'nanoflares', miniature brightenings resembling solar flares that would occur all over the surface of the Sun.

Historical theories

The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, "coronium". Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV, or Fe13+). Bengt Edlén, following the work of Grotrian (1939), first identified the coronal spectral lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green Fe-XIV line from Fe13+ at5303Å, but also the red Fe-X line from Fe9+ at6374Å).

A drawing demonstrating the configuration of solar magnetic flux during the solar cycle

The Sun's corona is much hotter (by a factor from 150 to 450) than the visible surface of the Sun: the photosphere's average temperature is around5800kelvin compared to the corona's 1 to 3 million kelvin. The corona is 10−12 times as dense as the photosphere, and so produces about one-millionth as much visible light. The corona is separated from the photosphere by the relatively shallow chromosphere. The exact mechanism by which the corona is heated is still the subject of some debate, but likely possibilities include induction by the Sun's magnetic field and magnetohydrodynamic waves from below. The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux and hence generating the solar wind.

The corona is not always evenly distributed across the surface of the Sun. During periods of quiet, the corona is more or less confined to the equatorial regions, with coronal holes covering the polar regions. However, during the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity. The solar cycle spans approximately 11 years, from solar minimum to the following minimum. Since the solar magnetic field is continually wound up due to the faster rotation of mass at the Sun's equator (differential rotation), sunspot activity will be more pronounced at solar maximum where the magnetic field is more twisted. Associated with sunspots are coronal loops, loops of magnetic flux, upwelling from the solar interior. The magnetic flux pushes the hotter photosphere aside, exposing the cooler plasma below, thus creating the relatively dark sun spots.

Since the corona has been photographed at high resolution in the X-ray range of the spectrum by the satellite Skylab in 1973, and then later by Yohkoh and the other following space instruments, it has been seen that the structure of the corona is quite varied and complex: different zones have been immediately classified on the coronal disc. The astronomers usually distinguish several regions, as described below.

Active regions

Main article: Active region

Active regions are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere, the so-called coronal loops. They generally distribute in two zones of activity, which are parallel to the solar equator. The average temperature is between two and four million kelvin, while the density goes from 109 to 1010 particles per cm3.

Illustration depicting solar prominences and sunspots

Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights above the Sun's surface: sunspots and faculae occur in the photosphere; spicules, filaments and plages in the chromosphere; prominences in the chromosphere and transition region; and flares and coronal mass ejections (CME) happen in the corona and chromosphere. If flares are very violent, they can also perturb the photosphere and generate a Moreton wave. On the contrary, quiescent prominences are large, cool, dense structures which are observed as dark, "snake-like" Hα ribbons (appearing like filaments) on the solar disc. Their temperature is about50008000K, and so they are usually considered as chromospheric features.

In 2013, images from the High Resolution Coronal Imager revealed never-before-seen "magnetic braids" of plasma within the outer layers of these active regions.

Coronal loops

Main article: Coronal loop
TRACE 171Å coronal loops

Coronal loops are the basic structures of the magnetic solar corona. These loops are the closed-magnetic flux cousins of the open-magnetic flux that can be found in coronal holes and the solar wind. Loops of magnetic flux well up from the solar body and fill with hot solar plasma. Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and CMEs.

The solar plasma that feeds these structures is heated from under6000K to well over 106 K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one point and drain to another, called foot points (siphon flow due to a pressure difference, or asymmetric flow due to some other driver).

When the plasma rises from the foot points towards the loop top, as always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When the plasma rapidly cools and falls toward the photosphere, it is called chromospheric condensation. There may also be symmetric flow from both loop foot points, causing a build-up of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), its dark filaments obvious against the solar disk or prominences off the Sun's limb.

Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Where there is a balance in loop energy sources and sinks, coronal loops can last for long periods of time and are known as steady state or quiescent coronal loops ().

Coronal loops are very important to our understanding of the current coronal heating problem. Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as TRACE. An explanation of the coronal heating problem remains as these structures are being observed remotely, where many ambiguities are present (i.e., radiation contributions along the line-of-sight propagation). In-situ measurements are required before a definitive answer can be determined, but due to the high plasma temperatures in the corona, in-situ measurements are, at present, impossible. The next mission of the NASA Parker Solar Probe will approach the Sun very closely, allowing more direct observations.

Large-scale structures

Large-scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions.

They were first detected in the June 8, 1968, flare observation during a rocket flight.

The large-scale structure of the corona changes over the 11-year solar cycle and becomes particularly simple during the minimum period, when the magnetic field of the Sun is almost similar to a dipolar configuration (plus a quadrupolar component).

Interconnections of active regions

The interconnections of active regions are arcs connecting zones of opposite magnetic field, of different active regions. Significant variations of these structures are often seen after a flare.

Some other features of this kind are helmet streamers — large, cap-like coronal structures with long, pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered to be sources of the slow solar wind.

Filament cavities

Image taken by the Solar Dynamics Observatory on October 16, 2010. A very long filament cavity is visible across the Sun's southern hemisphere.

Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected coronal holes.

Filament cavities are cooler clouds of gases (plasma) suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in images because they are empty of hot plasma. In fact, the sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure p {\displaystyle p} can be calculated by the state equation of a perfect gas: p = n k B T {\displaystyle p=nk_{B}T} , where n {\displaystyle n} is the particle number density, k B {\displaystyle k_{B}} the Boltzmann constant and T {\displaystyle T} the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in the photosphere.

Bright points

Bright points are small active regions found on the solar disk. X-ray bright points were first detected on April 8, 1969, during a rocket flight.

The fraction of the solar surface covered by bright points varies with the solar cycle. They are associated with small bipolar regions of the magnetic field. Their average temperature ranges from 1.1 MK to 3.4 MK. The variations in temperature are often correlated with changes in the X-ray emission.

Coronal holes

Main article: Coronal hole

Coronal holes are unipolar regions which look dark in the X-rays since they do not emit much radiation. These are wide zones of the Sun where the magnetic field is unipolar and opens towards the interplanetary space. The high speed solar wind arises mainly from these regions.

In the UV images of the coronal holes, some small structures, similar to elongated bubbles, are often seen as they were suspended in the solar wind. These are the coronal plumes. More precisely, they are long thin streamers that project outward from the Sun's north and south poles.

The quiet Sun

The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun.

The equatorial region has a faster rotation speed than the polar zones. The result of the Sun's differential rotation is that the active regions always arise in two bands parallel to the equator and their extension increases during the periods of maximum of the solar cycle, while they almost disappear during each minimum. Therefore, the quiet Sun always coincides with the equatorial zone and its surface is less active during the maximum of the solar cycle. Approaching the minimum of the solar cycle (also named butterfly cycle), the extension of the quiet Sun increases until it covers the whole disk surface excluding some bright points on the hemisphere and the poles, where there are coronal holes.

A portrait, as diversified as the one already pointed out for the coronal features, is emphasized by the analysis of the dynamics of the main structures of the corona, which evolve at differential times. Studying coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably: from seconds to several months. The typical sizes of the regions where coronal events take place vary in the same way, as it is shown in the following table.

Coronal event Typical time-scale Typical length-scale (Mm)
Active region flare 10 to10000seconds 10–100
X-ray bright point minutes 1–10
Transient in large-scale structures from minutes to hours ~100
Transient in interconnecting arcs from minutes to hours ~100
Quiet Sun from hours to months 100–1000
Coronal hole several rotations 100–1000

Flares

Main article: Solar flares
On August 31, 2012 a long filament of solar material that had been hovering in the Sun's outer atmosphere, the corona, erupted at 4:36 p.m. EDT

Flares take place in active regions and are characterized by a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they involve several zones of the solar atmosphere and many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.

Flares are impulsive phenomena, of average duration of 15 minutes, and the most energetic events can last several hours. Flares produce a high and rapid increase of the density and temperature.

An emission in white light is only seldom observed: usually, flares are only seen at extreme UV wavelengths and into the X-rays, typical of the chromospheric and coronal emission.

In the corona, the morphology of flares, is described by observations in the UV, soft and hard X-rays, and in Hα wavelengths, and is very complex. However, two kinds of basic structures can be distinguished:

  • Compact flares, when each of the two arches where the event is happening maintains its morphology: only an increase of the emission is observed without significant structural variations. The emitted energy is of the order of 1022 – 1023 J.
  • Flares of long duration, associated with eruptions of prominences, transients in white light and two-ribbon flares: in this case the magnetic loops change their configuration during the event. The energies emitted during these flares are of such great proportion they can reach 1025 J.
Filament erupting during a solar flare, seen at EUV wavelengths (TRACE)

As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. The durations of those periods depend on the range of wavelengths used to observe the event:

  • An initial impulsive phase, whose duration is on the order of minutes, strong emissions of energy are often observed even in the microwaves, EUV wavelengths and in the hard X-ray frequencies.
  • A maximum phase
  • A decay phase, which can last several hours.

Sometimes also a phase preceding the flare can be observed, usually called as "pre-flare" phase.

Transients

Main article: Coronal mass ejection

Accompanying solar flares or large solar prominences, coronal transients (also called coronal mass ejections) are sometimes released. These are enormous loops of coronal material that travel outward from the Sun at over a million kilometers per hour, containing roughly 10 times the energy of the solar flare or prominence that accompanies them. Some larger ejections can propel hundreds of millions of tons of material into interplanetary space at roughly 1.5 million kilometers an hour.

Coronal stars are ubiquitous among the stars in the cool half of the Hertzsprung–Russell diagram. These coronae can be detected using X-ray telescopes. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. For example, FK Comae Berenices is the prototype for the FK Com class of variable star. These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (Lx ≥ 1032 erg·s−1 or 1025W) and the hottest known with dominant temperatures up to 40 MK.

The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group showed that F-, G-, K- and M-stars have chromospheres and often coronae much like our Sun. The O-B stars, which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronae, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs. Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths. Thus they appear to have neither chromospheres nor coronae.

This image, taken by Hinode on 12 January 2007, reveals the filamentary nature of the corona.

The matter in the external part of the solar atmosphere is in the state of plasma, at very high temperature (a few million kelvin) and at very low density (of the order of 1015 particles/m3). According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour.

The composition is similar to that in the Sun's interior, mainly hydrogen, but with much greater ionization than that found in the photosphere. Heavier metals, such as iron, are partially ionized and have lost most of the external electrons. The ionization state of a chemical element depends strictly on the temperature and is regulated by the Saha equation in the lowest atmosphere, but by collisional equilibrium in the optically-thin corona. Historically, the presence of the spectral lines emitted from highly ionized states of iron allowed determination of the high temperature of the coronal plasma, revealing that the corona is much hotter than the internal layers of the chromosphere.

The corona behaves like a gas which is very hot but very light at the same time: the pressure in the corona is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximately a million times higher than on the solar surface. However it is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities. Supposing that they have the same kinetic energy on average (for the equipartition theorem), electrons have a mass roughly1800 times smaller than protons, therefore they acquire more velocity. Metal ions are always slower. This fact has relevant physical consequences either on radiative processes (that are very different from the photospheric radiative processes), or on thermal conduction. Furthermore, the presence of electric charges induces the generation of electric currents and high magnetic fields. Magnetohydrodynamic waves (MHD waves) can also propagate in this plasma, even if it is not still clear how they can be transmitted or generated in the corona.

Radiation

The corona emits radiation mainly in the X-rays, observable only from space.

The plasma is transparent to its own radiation and to that one coming from below, therefore we say that it is optically-thin. The gas, in fact, is very rarefied and the photon mean free-path overcomes by far all the other length-scales, including the typical sizes of the coronal features.

Different processes of radiation take place in the emission, due to binary collisions between plasma particles, while the interactions with the photons, coming from below; are very rare. Because the emission is due to collisions between ions and electrons, the energy emitted from a unit volume in the time unit is proportional to the squared number of particles in a unit volume, or more exactly, to the product of the electron density and proton density.

Thermal conduction

A mosaic of the extreme ultraviolet images taken from STEREO on December 4, 2006. These false color images show the Sun's atmospheres at a range of different temperatures. Clockwise from top left: 1 million degrees C (171 Å—blue), 1.5 million °C (195Å—green),6000080000°C (304 Å—red), and 2.5 million °C (286 Å—yellow).
STEREO – First images as a slow animation

In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers. Responsible for the diffusion process of the heat are the electrons, which are much lighter than ions and move faster, as explained above.

When there is a magnetic field the thermal conductivity of the plasma becomes higher in the direction which is parallel to the field lines rather than in the perpendicular direction. A charged particle moving in the direction perpendicular to the magnetic field line is subject to the Lorentz force which is normal to the plane individuated by the velocity and the magnetic field. This force bends the path of the particle. In general, since particles also have a velocity component along the magnetic field line, the Lorentz force constrains them to bend and move along spirals around the field lines at the cyclotron frequency.

If collisions between the particles are very frequent, they are scattered in every direction. This happens in the photosphere, where the plasma carries the magnetic field in its motion. In the corona, on the contrary, the mean free-path of the electrons is of the order of kilometres and even more, so each electron can do a helicoidal motion long before being scattered after a collision. Therefore, the heat transfer is enhanced along the magnetic field lines and inhibited in the perpendicular direction.

In the direction longitudinal to the magnetic field, the thermal conductivity of the corona is

k = 20 ( 2 π ) 3 / 2 ( k B T ) 5 / 2 k B m e 1 / 2 e 4 ln Λ 1.8 10 10 T 5 / 2 ln Λ W m 1 K 1 {\displaystyle k=20\left({\frac {2}{\pi }}\right)^{3/2}{\frac {\left(k_{B}T\right)^{5/2}k_{B}}{m_{e}^{1/2}e^{4}\ln \Lambda }}\approx 1.8~10^{-10}~{\frac {T^{5/2}}{\ln \Lambda }}~Wm^{-1}K^{-1}}

where k B {\displaystyle k_{B}} is the Boltzmann constant, T {\displaystyle T} is the temperature in kelvin, m e {\displaystyle m_{e}} is the electron mass, e {\displaystyle e} is the electric charge of the electron,

ln Λ = ln ( 12 π n λ D 3 ) {\displaystyle \ln \Lambda =\ln \left(12\pi n\lambda _{D}^{3}\right)}

is the Coulomb logarithm, and

λ D = k B T 4 π n e 2 {\displaystyle \lambda _{D}={\sqrt {\frac {k_{B}T}{4\pi ne^{2}}}}}

is the Debye length of the plasma with particle density n {\displaystyle n} . The Coulomb logarithm ln Λ {\displaystyle \ln \Lambda } is roughly 20 in the corona, with a mean temperature of 1 MK and a density of 1015 particles/m3, and about 10 in the chromosphere, where the temperature is approximately 10kK and the particle density is of the order of 1018 particles/m3, and in practice it can be assumed constant.

Thence, if we indicate with q {\displaystyle q} the heat for a volume unit, expressed in J m−3, the Fourier equation of heat transfer, to be computed only along the direction x {\displaystyle x} of the field line, becomes

q t = 0.9 10 11 2 T 7 / 2 x 2 {\displaystyle {\frac {\partial q}{\partial t}}=0.9~10^{-11}~{\frac {\partial ^{2}T^{7/2}}{\partial x^{2}}}} .

Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.

Coronal seismology

Main article: Coronal seismology

Coronal seismology is a new way of studying the plasma of the solar corona with the use of magnetohydrodynamic (MHD) waves. MHD studies the dynamics of electrically conducting fluids—in this case the fluid is the coronal plasma. Philosophically, coronal seismology is similar to the Earth's seismology, the Sun's helioseismology, and MHD spectroscopy of laboratory plasma devices. In all these approaches, waves of various kinds are used to probe a medium. The potential of coronal seismology in the estimation of the coronal magnetic field, density scale height, fine structure and heating has been demonstrated by different research groups.

Unsolved problem in physics:

Why is the Sun's corona so much hotter than the Sun's surface?

A new visualisation technique can provide clues to the coronal heating problem.

The coronal heating problem in solar physics relates to the question of why the temperature of the Sun's corona is millions of kelvins greater than that of the surface. Several theories have been proposed to explain this phenomenon but it is still challenging to determine which of these is correct. The problem first emerged when Bengt Edlen and Walter Grotrian identified Fe IX and Ca XIV lines in the solar spectrum. This led to the discovery that the emission lines seen during solar eclipses are not caused by an unknown element called "coronium" but known elements at very high stages of ionization. The comparison of the coronal and the photospheric temperatures of6000K, leads to the question of how the 200 times hotter coronal temperature can be maintained. The problem is primarily concerned with how the energy is transported up into the corona and then converted into heat within a few solar radii.

The high temperatures require energy to be carried from the solar interior to the corona by non-thermal processes, because the second law of thermodynamics prevents heat from flowing directly from the solar photosphere (surface), which is at about5800K, to the much hotter corona at about 1 to 3 MK (parts of the corona can even reach10MK).

Between the photosphere and the corona, the thin region through which the temperature increases is known as the transition region. It ranges from only tens to hundreds of kilometers thick. Energy cannot be transferred from the cooler photosphere to the corona by conventional heat transfer as this would violate the second law of thermodynamics. An analogy of this would be a light bulb raising the temperature of the air surrounding it to something greater than its glass surface. Hence, some other manner of energy transfer must be involved in the heating of the corona.

The amount of power required to heat the solar corona can easily be calculated as the difference between coronal radiative losses and heating by thermal conduction toward the chromosphere through the transition region. It is about 1 kilowatt for every square meter of surface area on the Sun's chromosphere, or 1/40000 of the amount of light energy that escapes the Sun.

Many coronal heating theories have been proposed, but two theories have remained as the most likely candidates: wave heating and magnetic reconnection (or nanoflares). Through most of the past 50 years, neither theory has been able to account for the extreme coronal temperatures.

In 2012, high resolution (<0.2″) soft X-ray imaging with the High Resolution Coronal Imager aboard a sounding rocket revealed tightly wound braids in the corona. It is hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the active solar corona to temperatures of up to 4 million kelvin. The main heat source in the quiescent corona (about 1.5 million kelvin) is assumed to originate from MHD waves.

NASA's Parker Solar Probe is intended to approach the Sun to a distance of approximately 9.5 solar radii to investigate coronal heating and the origin of the solar wind. It was successfully launched on August 12, 2018 and has completed the first few of the more than 20 planned close approaches to the Sun.

Competing heating mechanisms
Heating Models
Hydrodynamic Magnetic
  • No magnetic field
  • Slow rotating stars
DC (reconnection) AC (waves)
  • B-field stresses
  • Reconnection events
  • Flares-nanoflares
  • Uniform heating rates
  • Photospheric foot point shuffling
  • MHD wave propagation
  • High Alfvén wave flux
  • Non-uniform heating rates
Competing theories

Wave heating theory

The wave heating theory, proposed in 1949 by Évry Schatzman, proposes that waves carry energy from the solar interior to the solar chromosphere and corona. The Sun is made of plasma rather than ordinary gas, so it supports several types of waves analogous to sound waves in air. The most important types of wave are magneto-acoustic waves and Alfvén waves. Magneto-acoustic waves are sound waves that have been modified by the presence of a magnetic field, and Alfvén waves are similar to ultra low frequency radio waves that have been modified by interaction with matter in the plasma. Both types of waves can be launched by the turbulence of granulation and super granulation at the solar photosphere, and both types of waves can carry energy for some distance through the solar atmosphere before turning into shock waves that dissipate their energy as heat.

One problem with wave heating is delivery of the heat to the appropriate place. Magneto-acoustic waves cannot carry sufficient energy upward through the chromosphere to the corona, both because of the low pressure present in the chromosphere and because they tend to be reflected back to the photosphere. Alfvén waves can carry enough energy, but do not dissipate that energy rapidly enough once they enter the corona. Waves in plasmas are notoriously difficult to understand and describe analytically, but computer simulations, carried out by Thomas Bogdan and colleagues in 2003, seem to show that Alfvén waves can transmute into other wave modes at the base of the corona, providing a pathway that can carry large amounts of energy from the photosphere through the chromosphere and transition region and finally into the corona where it dissipates it as heat.

Another problem with wave heating has been the complete absence, until the late 1990s, of any direct evidence of waves propagating through the solar corona. The first direct observation of waves propagating into and through the solar corona was made in 1997 with the Solar and Heliospheric Observatory space-borne solar observatory, the first platform capable of observing the Sun in the extreme ultraviolet (EUV) for long periods of time with stable photometry. Those were magneto-acoustic waves with a frequency of about 1 millihertz (mHz, corresponding to a1000second wave period), that carry only about 10% of the energy required to heat the corona. Many observations exist of localized wave phenomena, such as Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat.

It is not yet known exactly how much wave energy is available to heat the corona. Results published in 2004 using data from the TRACE spacecraft seem to indicate that there are waves in the solar atmosphere at frequencies as high as100mHz (10 second period). Measurements of the temperature of different ions in the solar wind with the UVCS instrument aboard SOHO give strong indirect evidence that there are waves at frequencies as high as200Hz, well into the range of human hearing. These waves are very difficult to detect under normal circumstances, but evidence collected during solar eclipses by teams from Williams College suggest the presences of such waves in the 1–10Hz range.

Recently, Alfvénic motions have been found in the lower solar atmosphere and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the Solar Dynamics Observatory. These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the Hinode spacecraft.

Solar wind observations with the Wind spacecraft have recently shown evidence to support theories of Alfvén-cyclotron dissipation, leading to local ion heating.

Magnetic reconnection theory

Main article: Magnetic reconnection
Arcing active region by Solar Dynamics Observatory

The magnetic reconnection theory relies on the solar magnetic field to induce electric currents in the solar corona. The currents then collapse suddenly, releasing energy as heat and wave energy in the corona. This process is called "reconnection" because of the peculiar way that magnetic fields behave in plasma (or any electrically conductive fluid such as mercury or seawater). In a plasma, magnetic field lines are normally tied to individual pieces of matter, so that the topology of the magnetic field remains the same: if a particular north and south magnetic pole are connected by a single field line, then even if the plasma is stirred or if the magnets are moved around, that field line will continue to connect those particular poles. The connection is maintained by electric currents that are induced in the plasma. Under certain conditions, the electric currents can collapse, allowing the magnetic field to "reconnect" to other magnetic poles and release heat and wave energy in the process.

Magnetic reconnection is hypothesized to be the mechanism behind solar flares, the largest explosions in the Solar System. Furthermore, the surface of the Sun is covered with millions of small magnetized regions 50–1000km across. These small magnetic poles are buffeted and churned by the constant granulation. The magnetic field in the solar corona must undergo nearly constant reconnection to match the motion of this "magnetic carpet", so the energy released by the reconnection is a natural candidate for the coronal heat, perhaps as a series of "microflares" that individually provide very little energy but together account for the required energy.

The idea that nanoflares might heat the corona was proposed by Eugene Parker in the 1980s but is still controversial. In particular, ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light, but there seem to be too few of these small events to account for the energy released into the corona. The additional energy not accounted for could be made up by wave energy, or by gradual magnetic reconnection that releases energy more smoothly than micro-flares and therefore doesn't appear well in the TRACE data. Variations on the micro-flare hypothesis use other mechanisms to stress the magnetic field or to release the energy, and are a subject of active research in 2005.

Spicules (type II)

For decades, researchers believed spicules could send heat into the corona. However, following observational research in the 1980s, it was found that spicule plasma did not reach coronal temperatures, and so the theory was discounted.

As per studies performed in 2010 at the National Center for Atmospheric Research in Colorado, in collaboration with the Lockheed Martin's Solar and Astrophysics Laboratory (LMSAL) and the Institute of Theoretical Astrophysics of the University of Oslo, a new class of spicules (TYPE II) discovered in 2007, which travel faster (up to 100 km/s) and have shorter lifespans, can account for the problem. These jets insert heated plasma into the Sun's outer atmosphere.

Thus, a much greater understanding of the Corona and improvement in the knowledge of the Sun's subtle influence on the Earth's upper atmosphere can be expected henceforth. The Atmospheric Imaging Assembly on NASA's recently launched Solar Dynamics Observatory and NASA's Focal Plane Package for the Solar Optical Telescope on the Japanese Hinode satellite which was used to test this hypothesis. The high spatial and temporal resolutions of the newer instruments reveal this coronal mass supply.

These observations reveal a one-to-one connection between plasma that is heated to millions of degrees and the spicules that insert this plasma into the corona.

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Stellar corona
Stellar corona Language Watch Edit For other uses see Corona disambiguation A corona Latin for crown in turn derived from Ancient Greek korwnh korṓne garland wreath is an aura of plasma that surrounds the Sun and other stars The Sun s corona extends millions of kilometres into outer space and is most easily seen during a total solar eclipse but it is also observable with a coronagraph Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of 1000 000 kelvin 1 much hotter than the surface of the Sun During a total solar eclipse the Sun s corona and prominences are visible to the naked eye Light from the corona comes from three main sources from the same volume of space The K corona K for kontinuierlich continuous in German is created by sunlight scattering off free electrons Doppler broadening of the reflected photospheric absorption lines spreads them so greatly as to completely obscure them giving the spectral appearance of a continuum with no absorption lines The F corona F for Fraunhofer is created by sunlight bouncing off dust particles and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight the F corona extends to very high elongation angles from the Sun where it is called the zodiacal light The E corona E for emission is due to spectral emission lines produced by ions that are present in the coronal plasma it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona s composition 2 Contents 1 History 1 1 Historical theories 2 Physical features 2 1 Active regions 2 1 1 Coronal loops 2 1 2 Large scale structures 2 1 3 Interconnections of active regions 2 1 4 Filament cavities 2 1 5 Bright points 2 2 Coronal holes 2 3 The quiet Sun 3 Variability of the corona 3 1 Flares 3 2 Transients 4 Stellar coronae 5 Physics of the corona 5 1 Radiation 5 2 Thermal conduction 5 3 Coronal seismology 6 Coronal heating problem 6 1 Wave heating theory 6 2 Magnetic reconnection theory 6 3 Spicules type II 7 See also 8 References 9 Further reading 10 External linksHistory Edit Corona sketched by Jose Joaquin de Ferrer during the solar eclipse of June 16 1806 in Kinderhook New York In 1724 French Italian astronomer Giacomo F Maraldi recognized that the aura visible during a solar eclipse belongs to the Sun not to the Moon 3 In 1809 Spanish astronomer Jose Joaquin de Ferrer coined the term corona 4 Based in his own observations of the 1806 solar eclipse at Kinderhook New York de Ferrer also proposed that the corona was part of the Sun and not of the Moon English astronomer Norman Lockyer identified the first element unknown on Earth in the Sun s chromosphere which was called helium French astronomer Jules Jenssen noted after comparing his readings between the 1871 and 1878 eclipses that the size and shape of the corona changes with the sunspot cycle 5 In 1930 Bernard Lyot invented the coronograph which allows viewing the corona without a total eclipse In 1952 American astronomer Eugene Parker proposed that the solar corona might be heated by myriad tiny nanoflares miniature brightenings resembling solar flares that would occur all over the surface of the Sun Historical theories Edit The high temperature of the Sun s corona gives it unusual spectral features which led some in the 19th century to suggest that it contained a previously unknown element coronium Instead these spectral features have since been explained by highly ionized iron Fe XIV or Fe13 Bengt Edlen following the work of Grotrian 1939 first identified the coronal spectral lines in 1940 observed since 1869 as transitions from low lying metastable levels of the ground configuration of highly ionised metals the green Fe XIV line from Fe13 at 5303 A but also the red Fe X line from Fe9 at 6374 A 1 Physical features Edit A drawing demonstrating the configuration of solar magnetic flux during the solar cycle The Sun s corona is much hotter by a factor from 150 to 450 than the visible surface of the Sun the photosphere s average temperature is around 5800 kelvin compared to the corona s 1 to 3 million kelvin The corona is 10 12 times as dense as the photosphere and so produces about one millionth as much visible light The corona is separated from the photosphere by the relatively shallow chromosphere The exact mechanism by which the corona is heated is still the subject of some debate but likely possibilities include induction by the Sun s magnetic field and magnetohydrodynamic waves from below The outer edges of the Sun s corona are constantly being transported away due to open magnetic flux and hence generating the solar wind The corona is not always evenly distributed across the surface of the Sun During periods of quiet the corona is more or less confined to the equatorial regions with coronal holes covering the polar regions However during the Sun s active periods the corona is evenly distributed over the equatorial and polar regions though it is most prominent in areas with sunspot activity The solar cycle spans approximately 11 years from solar minimum to the following minimum Since the solar magnetic field is continually wound up due to the faster rotation of mass at the Sun s equator differential rotation sunspot activity will be more pronounced at solar maximum where the magnetic field is more twisted Associated with sunspots are coronal loops loops of magnetic flux upwelling from the solar interior The magnetic flux pushes the hotter photosphere aside exposing the cooler plasma below thus creating the relatively dark sun spots Since the corona has been photographed at high resolution in the X ray range of the spectrum by the satellite Skylab in 1973 and then later by Yohkoh and the other following space instruments it has been seen that the structure of the corona is quite varied and complex different zones have been immediately classified on the coronal disc 6 7 8 The astronomers usually distinguish several regions 9 as described below Active regions Edit Main article Active region Active regions are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere the so called coronal loops They generally distribute in two zones of activity which are parallel to the solar equator The average temperature is between two and four million kelvin while the density goes from 109 to 1010 particles per cm3 Illustration depicting solar prominences and sunspots Active regions involve all the phenomena directly linked to the magnetic field which occur at different heights above the Sun s surface 9 sunspots and faculae occur in the photosphere spicules Ha filaments and plages in the chromosphere prominences in the chromosphere and transition region and flares and coronal mass ejections CME happen in the corona and chromosphere If flares are very violent they can also perturb the photosphere and generate a Moreton wave On the contrary quiescent prominences are large cool dense structures which are observed as dark snake like Ha ribbons appearing like filaments on the solar disc Their temperature is about 5000 8000 K and so they are usually considered as chromospheric features In 2013 images from the High Resolution Coronal Imager revealed never before seen magnetic braids of plasma within the outer layers of these active regions 10 Coronal loops Edit Main article Coronal loop TRACE 171A coronal loops Coronal loops are the basic structures of the magnetic solar corona These loops are the closed magnetic flux cousins of the open magnetic flux that can be found in coronal holes and the solar wind Loops of magnetic flux well up from the solar body and fill with hot solar plasma 11 Due to the heightened magnetic activity in these coronal loop regions coronal loops can often be the precursor to solar flares and CMEs The solar plasma that feeds these structures is heated from under 6000 K to well over 106 K from the photosphere through the transition region and into the corona Often the solar plasma will fill these loops from one point and drain to another called foot points siphon flow due to a pressure difference 12 or asymmetric flow due to some other driver When the plasma rises from the foot points towards the loop top as always occurs during the initial phase of a compact flare it is defined as chromospheric evaporation When the plasma rapidly cools and falls toward the photosphere it is called chromospheric condensation There may also be symmetric flow from both loop foot points causing a build up of mass in the loop structure The plasma may cool rapidly in this region for a thermal instability its dark filaments obvious against the solar disk or prominences off the Sun s limb Coronal loops may have lifetimes in the order of seconds in the case of flare events minutes hours or days Where there is a balance in loop energy sources and sinks coronal loops can last for long periods of time and are known as steady state or quiescent coronal loops example Coronal loops are very important to our understanding of the current coronal heating problem Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as TRACE An explanation of the coronal heating problem remains as these structures are being observed remotely where many ambiguities are present i e radiation contributions along the line of sight propagation In situ measurements are required before a definitive answer can be determined but due to the high plasma temperatures in the corona in situ measurements are at present impossible The next mission of the NASA Parker Solar Probe will approach the Sun very closely allowing more direct observations Large scale structures Edit Large scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions They were first detected in the June 8 1968 flare observation during a rocket flight 13 The large scale structure of the corona changes over the 11 year solar cycle and becomes particularly simple during the minimum period when the magnetic field of the Sun is almost similar to a dipolar configuration plus a quadrupolar component Interconnections of active regions Edit The interconnections of active regions are arcs connecting zones of opposite magnetic field of different active regions Significant variations of these structures are often seen after a flare 14 Some other features of this kind are helmet streamers large cap like coronal structures with long pointed peaks that usually overlie sunspots and active regions Coronal streamers are considered to be sources of the slow solar wind 14 Filament cavities Edit Image taken by the Solar Dynamics Observatory on October 16 2010 A very long filament cavity is visible across the Sun s southern hemisphere Filament cavities are zones which look dark in the X rays and are above the regions where Ha filaments are observed in the chromosphere They were first observed in the two 1970 rocket flights which also detected coronal holes 13 Filament cavities are cooler clouds of gases plasma suspended above the Sun s surface by magnetic forces The regions of intense magnetic field look dark in images because they are empty of hot plasma In fact the sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration where the magnetic field is higher the plasma must be cooler or less dense The plasma pressure p displaystyle p can be calculated by the state equation of a perfect gas p n k B T displaystyle p nk B T where n displaystyle n is the particle number density k B displaystyle k B the Boltzmann constant and T displaystyle T the plasma temperature It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties The same physical effect renders sunspots apparently dark in the photosphere Bright points Edit Bright points are small active regions found on the solar disk X ray bright points were first detected on April 8 1969 during a rocket flight 13 The fraction of the solar surface covered by bright points varies with the solar cycle They are associated with small bipolar regions of the magnetic field Their average temperature ranges from 1 1 MK to 3 4 MK The variations in temperature are often correlated with changes in the X ray emission 15 Coronal holes Edit Main article Coronal hole Coronal holes are unipolar regions which look dark in the X rays since they do not emit much radiation 16 These are wide zones of the Sun where the magnetic field is unipolar and opens towards the interplanetary space The high speed solar wind arises mainly from these regions In the UV images of the coronal holes some small structures similar to elongated bubbles are often seen as they were suspended in the solar wind These are the coronal plumes More precisely they are long thin streamers that project outward from the Sun s north and south poles 17 The quiet Sun Edit The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun The equatorial region has a faster rotation speed than the polar zones The result of the Sun s differential rotation is that the active regions always arise in two bands parallel to the equator and their extension increases during the periods of maximum of the solar cycle while they almost disappear during each minimum Therefore the quiet Sun always coincides with the equatorial zone and its surface is less active during the maximum of the solar cycle Approaching the minimum of the solar cycle also named butterfly cycle the extension of the quiet Sun increases until it covers the whole disk surface excluding some bright points on the hemisphere and the poles where there are coronal holes Variability of the corona EditA portrait as diversified as the one already pointed out for the coronal features is emphasized by the analysis of the dynamics of the main structures of the corona which evolve at differential times Studying coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably from seconds to several months The typical sizes of the regions where coronal events take place vary in the same way as it is shown in the following table Coronal event Typical time scale Typical length scale Mm Active region flare 10 to 10000 seconds 10 100X ray bright point minutes 1 10Transient in large scale structures from minutes to hours 100Transient in interconnecting arcs from minutes to hours 100Quiet Sun from hours to months 100 1000Coronal hole several rotations 100 1000Flares Edit Main article Solar flares On August 31 2012 a long filament of solar material that had been hovering in the Sun s outer atmosphere the corona erupted at 4 36 p m EDT Flares take place in active regions and are characterized by a sudden increase of the radiative flux emitted from small regions of the corona They are very complex phenomena visible at different wavelengths they involve several zones of the solar atmosphere and many physical effects thermal and not thermal and sometimes wide reconnections of the magnetic field lines with material expulsion Flares are impulsive phenomena of average duration of 15 minutes and the most energetic events can last several hours Flares produce a high and rapid increase of the density and temperature An emission in white light is only seldom observed usually flares are only seen at extreme UV wavelengths and into the X rays typical of the chromospheric and coronal emission In the corona the morphology of flares is described by observations in the UV soft and hard X rays and in Ha wavelengths and is very complex However two kinds of basic structures can be distinguished 18 Compact flares when each of the two arches where the event is happening maintains its morphology only an increase of the emission is observed without significant structural variations The emitted energy is of the order of 1022 1023 J Flares of long duration associated with eruptions of prominences transients in white light and two ribbon flares 19 in this case the magnetic loops change their configuration during the event The energies emitted during these flares are of such great proportion they can reach 1025 J Filament erupting during a solar flare seen at EUV wavelengths TRACE As for temporal dynamics three different phases are generally distinguished whose duration are not comparable The durations of those periods depend on the range of wavelengths used to observe the event An initial impulsive phase whose duration is on the order of minutes strong emissions of energy are often observed even in the microwaves EUV wavelengths and in the hard X ray frequencies A maximum phase A decay phase which can last several hours Sometimes also a phase preceding the flare can be observed usually called as pre flare phase Transients Edit Main article Coronal mass ejection Accompanying solar flares or large solar prominences coronal transients also called coronal mass ejections are sometimes released These are enormous loops of coronal material that travel outward from the Sun at over a million kilometers per hour containing roughly 10 times the energy of the solar flare or prominence that accompanies them Some larger ejections can propel hundreds of millions of tons of material into interplanetary space at roughly 1 5 million kilometers an hour Stellar coronae EditCoronal stars are ubiquitous among the stars in the cool half of the Hertzsprung Russell diagram 20 These coronae can be detected using X ray telescopes Some stellar coronae particularly in young stars are much more luminous than the Sun s For example FK Comae Berenices is the prototype for the FK Com class of variable star These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity Their X ray coronae are among the most luminous Lx 1032 erg s 1 or 1025W and the hottest known with dominant temperatures up to 40 MK 20 The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group 21 showed that F G K and M stars have chromospheres and often coronae much like our Sun The O B stars which do not have surface convection zones have a strong X ray emission However these stars do not have coronae but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs Also A stars do not have convection zones but they do not emit at the UV and X ray wavelengths Thus they appear to have neither chromospheres nor coronae Physics of the corona Edit This image taken by Hinode on 12 January 2007 reveals the filamentary nature of the corona The matter in the external part of the solar atmosphere is in the state of plasma at very high temperature a few million kelvin and at very low density of the order of 1015 particles m3 According to the definition of plasma it is a quasi neutral ensemble of particles which exhibits a collective behaviour The composition is similar to that in the Sun s interior mainly hydrogen but with much greater ionization than that found in the photosphere Heavier metals such as iron are partially ionized and have lost most of the external electrons The ionization state of a chemical element depends strictly on the temperature and is regulated by the Saha equation in the lowest atmosphere but by collisional equilibrium in the optically thin corona Historically the presence of the spectral lines emitted from highly ionized states of iron allowed determination of the high temperature of the coronal plasma revealing that the corona is much hotter than the internal layers of the chromosphere The corona behaves like a gas which is very hot but very light at the same time the pressure in the corona is usually only 0 1 to 0 6 Pa in active regions while on the Earth the atmospheric pressure is about 100 kPa approximately a million times higher than on the solar surface However it is not properly a gas because it is made of charged particles basically protons and electrons moving at different velocities Supposing that they have the same kinetic energy on average for the equipartition theorem electrons have a mass roughly 1800 times smaller than protons therefore they acquire more velocity Metal ions are always slower This fact has relevant physical consequences either on radiative processes that are very different from the photospheric radiative processes or on thermal conduction Furthermore the presence of electric charges induces the generation of electric currents and high magnetic fields Magnetohydrodynamic waves MHD waves can also propagate in this plasma 22 even if it is not still clear how they can be transmitted or generated in the corona Radiation Edit Main article Coronal radiative losses The corona emits radiation mainly in the X rays observable only from space The plasma is transparent to its own radiation and to that one coming from below therefore we say that it is optically thin The gas in fact is very rarefied and the photon mean free path overcomes by far all the other length scales including the typical sizes of the coronal features Different processes of radiation take place in the emission due to binary collisions between plasma particles while the interactions with the photons coming from below are very rare Because the emission is due to collisions between ions and electrons the energy emitted from a unit volume in the time unit is proportional to the squared number of particles in a unit volume or more exactly to the product of the electron density and proton density 23 Thermal conduction Edit A mosaic of the extreme ultraviolet images taken from STEREO on December 4 2006 These false color images show the Sun s atmospheres at a range of different temperatures Clockwise from top left 1 million degrees C 171 A blue 1 5 million C 195A green 60000 80000 C 304 A red and 2 5 million C 286 A yellow STEREO First images as a slow animation In the corona thermal conduction occurs from the external hotter atmosphere towards the inner cooler layers Responsible for the diffusion process of the heat are the electrons which are much lighter than ions and move faster as explained above When there is a magnetic field the thermal conductivity of the plasma becomes higher in the direction which is parallel to the field lines rather than in the perpendicular direction 24 A charged particle moving in the direction perpendicular to the magnetic field line is subject to the Lorentz force which is normal to the plane individuated by the velocity and the magnetic field This force bends the path of the particle In general since particles also have a velocity component along the magnetic field line the Lorentz force constrains them to bend and move along spirals around the field lines at the cyclotron frequency If collisions between the particles are very frequent they are scattered in every direction This happens in the photosphere where the plasma carries the magnetic field in its motion In the corona on the contrary the mean free path of the electrons is of the order of kilometres and even more so each electron can do a helicoidal motion long before being scattered after a collision Therefore the heat transfer is enhanced along the magnetic field lines and inhibited in the perpendicular direction In the direction longitudinal to the magnetic field the thermal conductivity of the corona is 24 k 20 2 p 3 2 k B T 5 2 k B m e 1 2 e 4 ln L 1 8 10 10 T 5 2 ln L W m 1 K 1 displaystyle k 20 left frac 2 pi right 3 2 frac left k B T right 5 2 k B m e 1 2 e 4 ln Lambda approx 1 8 10 10 frac T 5 2 ln Lambda Wm 1 K 1 where k B displaystyle k B is the Boltzmann constant T displaystyle T is the temperature in kelvin m e displaystyle m e is the electron mass e displaystyle e is the electric charge of the electron ln L ln 12 p n l D 3 displaystyle ln Lambda ln left 12 pi n lambda D 3 right is the Coulomb logarithm and l D k B T 4 p n e 2 displaystyle lambda D sqrt frac k B T 4 pi ne 2 is the Debye length of the plasma with particle density n displaystyle n The Coulomb logarithm ln L displaystyle ln Lambda is roughly 20 in the corona with a mean temperature of 1 MK and a density of 1015 particles m3 and about 10 in the chromosphere where the temperature is approximately 10kK and the particle density is of the order of 1018 particles m3 and in practice it can be assumed constant Thence if we indicate with q displaystyle q the heat for a volume unit expressed in J m 3 the Fourier equation of heat transfer to be computed only along the direction x displaystyle x of the field line becomes q t 0 9 10 11 2 T 7 2 x 2 displaystyle frac partial q partial t 0 9 10 11 frac partial 2 T 7 2 partial x 2 Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper Coronal seismology Edit Main article Coronal seismology Coronal seismology is a new way of studying the plasma of the solar corona with the use of magnetohydrodynamic MHD waves MHD studies the dynamics of electrically conducting fluids in this case the fluid is the coronal plasma Philosophically coronal seismology is similar to the Earth s seismology the Sun s helioseismology and MHD spectroscopy of laboratory plasma devices In all these approaches waves of various kinds are used to probe a medium The potential of coronal seismology in the estimation of the coronal magnetic field density scale height fine structure and heating has been demonstrated by different research groups Coronal heating problem EditUnsolved problem in physics Why is the Sun s corona so much hotter than the Sun s surface more unsolved problems in physics Play media A new visualisation technique can provide clues to the coronal heating problem The coronal heating problem in solar physics relates to the question of why the temperature of the Sun s corona is millions of kelvins greater than that of the surface Several theories have been proposed to explain this phenomenon but it is still challenging to determine which of these is correct 25 The problem first emerged when Bengt Edlen and Walter Grotrian identified Fe IX and Ca XIV lines in the solar spectrum 26 This led to the discovery that the emission lines seen during solar eclipses are not caused by an unknown element called coronium but known elements at very high stages of ionization 25 The comparison of the coronal and the photospheric temperatures of 6000 K leads to the question of how the 200 times hotter coronal temperature can be maintained 26 The problem is primarily concerned with how the energy is transported up into the corona and then converted into heat within a few solar radii 27 The high temperatures require energy to be carried from the solar interior to the corona by non thermal processes because the second law of thermodynamics prevents heat from flowing directly from the solar photosphere surface which is at about 5800 K to the much hotter corona at about 1 to 3 MK parts of the corona can even reach 10MK Between the photosphere and the corona the thin region through which the temperature increases is known as the transition region It ranges from only tens to hundreds of kilometers thick Energy cannot be transferred from the cooler photosphere to the corona by conventional heat transfer as this would violate the second law of thermodynamics An analogy of this would be a light bulb raising the temperature of the air surrounding it to something greater than its glass surface Hence some other manner of energy transfer must be involved in the heating of the corona The amount of power required to heat the solar corona can easily be calculated as the difference between coronal radiative losses and heating by thermal conduction toward the chromosphere through the transition region It is about 1 kilowatt for every square meter of surface area on the Sun s chromosphere or 1 40000 of the amount of light energy that escapes the Sun Many coronal heating theories have been proposed 28 but two theories have remained as the most likely candidates wave heating and magnetic reconnection or nanoflares 29 Through most of the past 50 years neither theory has been able to account for the extreme coronal temperatures In 2012 high resolution lt 0 2 soft X ray imaging with the High Resolution Coronal Imager aboard a sounding rocket revealed tightly wound braids in the corona It is hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the active solar corona to temperatures of up to 4 million kelvin The main heat source in the quiescent corona about 1 5 million kelvin is assumed to originate from MHD waves 30 NASA s Parker Solar Probe is intended to approach the Sun to a distance of approximately 9 5 solar radii to investigate coronal heating and the origin of the solar wind It was successfully launched on August 12 2018 31 and has completed the first few of the more than 20 planned close approaches to the Sun 32 Competing heating mechanisms Heating ModelsHydrodynamic MagneticNo magnetic field Slow rotating stars DC reconnection AC waves B field stresses Reconnection events Flares nanoflares Uniform heating rates Photospheric foot point shuffling MHD wave propagation High Alfven wave flux Non uniform heating ratesCompeting theoriesWave heating theory Edit The wave heating theory proposed in 1949 by Evry Schatzman proposes that waves carry energy from the solar interior to the solar chromosphere and corona The Sun is made of plasma rather than ordinary gas so it supports several types of waves analogous to sound waves in air The most important types of wave are magneto acoustic waves and Alfven waves 33 Magneto acoustic waves are sound waves that have been modified by the presence of a magnetic field and Alfven waves are similar to ultra low frequency radio waves that have been modified by interaction with matter in the plasma Both types of waves can be launched by the turbulence of granulation and super granulation at the solar photosphere and both types of waves can carry energy for some distance through the solar atmosphere before turning into shock waves that dissipate their energy as heat One problem with wave heating is delivery of the heat to the appropriate place Magneto acoustic waves cannot carry sufficient energy upward through the chromosphere to the corona both because of the low pressure present in the chromosphere and because they tend to be reflected back to the photosphere Alfven waves can carry enough energy but do not dissipate that energy rapidly enough once they enter the corona Waves in plasmas are notoriously difficult to understand and describe analytically but computer simulations carried out by Thomas Bogdan and colleagues in 2003 seem to show that Alfven waves can transmute into other wave modes at the base of the corona providing a pathway that can carry large amounts of energy from the photosphere through the chromosphere and transition region and finally into the corona where it dissipates it as heat Another problem with wave heating has been the complete absence until the late 1990s of any direct evidence of waves propagating through the solar corona The first direct observation of waves propagating into and through the solar corona was made in 1997 with the Solar and Heliospheric Observatory space borne solar observatory the first platform capable of observing the Sun in the extreme ultraviolet EUV for long periods of time with stable photometry Those were magneto acoustic waves with a frequency of about 1 millihertz mHz corresponding to a 1000 second wave period that carry only about 10 of the energy required to heat the corona Many observations exist of localized wave phenomena such as Alfven waves launched by solar flares but those events are transient and cannot explain the uniform coronal heat It is not yet known exactly how much wave energy is available to heat the corona Results published in 2004 using data from the TRACE spacecraft seem to indicate that there are waves in the solar atmosphere at frequencies as high as 100mHz 10 second period Measurements of the temperature of different ions in the solar wind with the UVCS instrument aboard SOHO give strong indirect evidence that there are waves at frequencies as high as 200Hz well into the range of human hearing These waves are very difficult to detect under normal circumstances but evidence collected during solar eclipses by teams from Williams College suggest the presences of such waves in the 1 10Hz range Recently Alfvenic motions have been found in the lower solar atmosphere 34 35 and also in the quiet Sun in coronal holes and in active regions using observations with AIA on board the Solar Dynamics Observatory 36 These Alfvenic oscillations have significant power and seem to be connected to the chromospheric Alfvenic oscillations previously reported with the Hinode spacecraft 37 Solar wind observations with the Wind spacecraft have recently shown evidence to support theories of Alfven cyclotron dissipation leading to local ion heating 38 Magnetic reconnection theory Edit Main article Magnetic reconnection Arcing active region by Solar Dynamics Observatory The magnetic reconnection theory relies on the solar magnetic field to induce electric currents in the solar corona 39 The currents then collapse suddenly releasing energy as heat and wave energy in the corona This process is called reconnection because of the peculiar way that magnetic fields behave in plasma or any electrically conductive fluid such as mercury or seawater In a plasma magnetic field lines are normally tied to individual pieces of matter so that the topology of the magnetic field remains the same if a particular north and south magnetic pole are connected by a single field line then even if the plasma is stirred or if the magnets are moved around that field line will continue to connect those particular poles The connection is maintained by electric currents that are induced in the plasma Under certain conditions the electric currents can collapse allowing the magnetic field to reconnect to other magnetic poles and release heat and wave energy in the process Magnetic reconnection is hypothesized to be the mechanism behind solar flares the largest explosions in the Solar System Furthermore the surface of the Sun is covered with millions of small magnetized regions 50 1000 km across These small magnetic poles are buffeted and churned by the constant granulation The magnetic field in the solar corona must undergo nearly constant reconnection to match the motion of this magnetic carpet so the energy released by the reconnection is a natural candidate for the coronal heat perhaps as a series of microflares that individually provide very little energy but together account for the required energy The idea that nanoflares might heat the corona was proposed by Eugene Parker in the 1980s but is still controversial In particular ultraviolet telescopes such as TRACE and SOHO EIT can observe individual micro flares as small brightenings in extreme ultraviolet light 40 but there seem to be too few of these small events to account for the energy released into the corona The additional energy not accounted for could be made up by wave energy or by gradual magnetic reconnection that releases energy more smoothly than micro flares and therefore doesn t appear well in the TRACE data Variations on the micro flare hypothesis use other mechanisms to stress the magnetic field or to release the energy and are a subject of active research in 2005 Spicules type II Edit For decades researchers believed spicules could send heat into the corona However following observational research in the 1980s it was found that spicule plasma did not reach coronal temperatures and so the theory was discounted As per studies performed in 2010 at the National Center for Atmospheric Research in Colorado in collaboration with the Lockheed Martin s Solar and Astrophysics Laboratory LMSAL and the Institute of Theoretical Astrophysics of the University of Oslo a new class of spicules TYPE II discovered in 2007 which travel faster up to 100 km s and have shorter lifespans can account for the problem 41 These jets insert heated plasma into the Sun s outer atmosphere Thus a much greater understanding of the Corona and improvement in the knowledge of the Sun s subtle influence on the Earth s upper atmosphere can be expected henceforth The Atmospheric Imaging Assembly on NASA s recently launched Solar Dynamics Observatory and NASA s Focal Plane Package for the Solar Optical Telescope on the Japanese Hinode satellite which was used to test this hypothesis The high spatial and temporal resolutions of the newer instruments reveal this coronal mass supply These observations reveal a one to one connection between plasma that is heated to millions of degrees and the spicules that insert this plasma into the corona 42 See also EditAdvanced Composition Explorer Geocorona Supra arcade downflows X ray astronomyReferences Edit a b Aschwanden Markus J 2005 Physics of the Solar Corona An Introduction with Problems and Solutions Chichester UK Praxis Publishing ISBN 978 3 540 22321 4 Corfield Richard 2007 Lives of the Planets Basic Books ISBN 978 0 465 01403 3 Hall Graham et al 2007 Maraldi Giacomo Filippo Biographical Encyclopedia of Astronomers New York Springer p 736 doi 10 1007 978 0 387 30400 7 899 ISBN 978 0 387 31022 0 Retrieved 31 October 2021 de Ferrer Jose Joaquin 1809 Observations of the eclipse of the sun June 16th 1806 made at Kinderhook in the State of New York Transactions of the American Philosophical Society 6 264 275 doi 10 2307 1004801 JSTOR 1004801 Espenak Fred Chronology of Discoveries about the Sun Mr Eclipse Archived from the original on 19 October 2020 Retrieved 6 November 2020 Vaiana G S Krieger A S Timothy A F 1973 Identification and analysis of structures in the corona from X ray photography Solar Physics 32 1 81 116 Bibcode 1973SoPh 32 81V doi 10 1007 BF00152731 S2CID 121940724 Vaiana G S Tucker W H 1974 R Giacconi H Gunsky eds Solar X Ray Emission in X Ray Astronomy 169 Cite journal requires journal help Vaiana G S Rosner R 1978 Recent advances in Coronae Physics Annu Rev Astron Astrophys 16 393 428 Bibcode 1978ARA amp A 16 393V doi 10 1146 annurev aa 16 090178 002141 a b Gibson E G 1973 The Quiet Sun National Aeronautics and Space Administration Washington D C How NASA Revealed Sun s Hottest Secret in 5 Minute Spaceflight Space com 23 January 2013 Archived from the original on 2013 01 24 Katsukawa Yukio Tsuneta Saku 2005 Magnetic Properties at Footpoints of Hot and Cool Loops The Astrophysical Journal 621 1 498 511 Bibcode 2005ApJ 621 498K doi 10 1086 427488 Betta Rita Orlando Salvatore Peres Giovanni Serio Salvatore 1999 On the Stability of Siphon Flows in Coronal Loops Space Science Reviews 87 133 136 Bibcode 1999SSRv 87 133B doi 10 1023 A 1005182503751 S2CID 117127214 a b c Giacconi Riccardo 1992 J F Linsky and S Serio ed G S Vaiana memorial lecture inProceedinds of Physics of Solar and Stellar Coronae G S Vaiana Memorial Symposium Kluwer Academic Publishers Printed in the Netherlands pp 3 19 ISBN 978 0 7923 2346 4 a b Ofman Leon 2000 Source regions of the slow solar wind in coronal streamers Geophysical Research Letters 27 18 2885 2888 Bibcode 2000GeoRL 27 2885O doi 10 1029 2000GL000097 Kariyappa R Deluca E E Saar S H Golub L Dame L Pevtsov A A Varghese B A 2011 Temperature variability in X ray bright points observed with Hinode XRT Astronomy amp Astrophysics 526 A78 Bibcode 2011A 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stellar coronae PDF The Astronomy and Astrophysics Review 12 2 3 71 237 arXiv astro ph 0406661 Bibcode 2004A amp ARv 12 71G doi 10 1007 s00159 004 0023 2 S2CID 119509015 Archived from the original PDF on 2011 08 11 Vaiana G S et al 1981 Results from an extensive Einstein stellar survey The Astrophysical Journal 245 163 Bibcode 1981ApJ 245 163V doi 10 1086 158797 Jeffrey Alan 1969 Magneto hydrodynamics UNIVERSITY MATHEMATICAL TEXTS Mewe R 1991 X ray spectroscopy of stellar coronae The Astronomy and Astrophysics Review 3 2 127 Bibcode 1991A amp ARv 3 127M doi 10 1007 BF00873539 S2CID 55255606 a b Spitzer L 1962 Physics of fully ionized gas Interscience tracts of physics and astronomy a b 2004ESASP 575 2K Page 2 adsbit harvard edu Retrieved 2019 02 28 a b Aschwanden Markus 2006 Physics of the Solar Corona An Introduction with Problems and Solutions Berlin Springer Science amp Business Media pp 355 ISBN 978 3540307655 Falgarone Edith Passot Thierry 2003 Turbulence and Magnetic Fields in Astrophysics Berlin Springer Science amp Business Media pp 28 ISBN 978 3540002741 Ulmshneider Peter 1997 J C Vial K Bocchialini P Boumier eds Heating of Chromospheres and Coronae inSpace Solar Physics Proceedings Orsay France Springer pp 77 106 ISBN 978 3 540 64307 4 Malara F Velli M 2001 Pal Brekke Bernhard Fleck Joseph B Gurman eds Observations and Models of Coronal Heating inRecent Insights into the Physics of the Sun and Heliosphere Highlights from SOHO and Other Space Missions Proceedings of IAU Symposium 203 Astronomical Society of the Pacific pp 456 466 ISBN 978 1 58381 069 9 Cirtain J W Golub L Winebarger A R De Pontieu B Kobayashi K Moore R L Walsh R W Korreck K E Weber M McCauley P Title A Kuzin S Deforest C E 2013 Energy release in the solar corona from spatially resolved magnetic braids Nature 493 7433 501 503 Bibcode 2013Natur 493 501C doi 10 1038 nature11772 PMID 23344359 S2CID 205232074 http parkersolarprobe jhuapl edu The Mission index php Journey to the Sun Archived 2017 08 22 at the Wayback Machine Parker Solar Probe Completes Third Close Approach of the Sun blogs nasa gov Retrieved 2019 12 06 Alfven Hannes 1947 Magneto hydrodynamic waves and the heating of the solar corona MNRAS 107 2 211 219 Bibcode 1947MNRAS 107 211A doi 10 1093 mnras 107 2 211 Alfven Waves Our Sun Is Doing The Magnetic Twist read on Jan 6 2011 Archived from the original on 2011 07 23 Jess DB Mathioudakis M Erdelyi R Crockett PJ Keenan FP Christian DJ 2009 Alfven Waves in the Lower Solar Atmosphere Science 323 5921 1582 1585 arXiv 0903 3546 Bibcode 2009Sci 323 1582J doi 10 1126 science 1168680 hdl 10211 3 172550 PMID 19299614 S2CID 14522616 McIntosh S W de Pontieu B Carlsson M Hansteen V H The Sdo Aia Mission Team 2010 Ubiquitous Alfvenic Motions in Quiet Sun Coronal Hole and Active Region Corona American Geophysical Union Fall Meeting abstract SH14A 01 SH14A 01 Bibcode 2010AGUFMSH14A 01M Sun s Magnetic Secret Revealed read on Jan 6 2011 Archived from the original on 2010 12 24 Kasper J C et al December 2008 Hot Solar Wind Helium Direct Evidence for Local Heating by Alfven Cyclotron Dissipation Phys Rev Lett 101 26 261103 Bibcode 2008PhRvL 101z1103K doi 10 1103 PhysRevLett 101 261103 PMID 19113766 Priest Eric 1982 Solar Magneto hydrodynamics D Reidel Publishing Company Dordrecht Holland ISBN 978 90 277 1833 4 Patsourakos S Vial J C 2002 Intermittent behavior in the transition region and the low corona of the quiet Sun Astronomy and Astrophysics 385 3 1073 1077 Bibcode 2002A amp A 385 1073P doi 10 1051 0004 6361 20020151 Mystery of Sun s hot outer atmosphere solved Rediff com News Rediff com 2011 01 07 Archived from the original on 2012 04 15 Retrieved 2012 05 21 De Pontieu B McIntosh SW Carlsson M Hansteen VH Tarbell TD Boerner P Martinez Sykora J Schrijver CJ Title AM 2011 The Origins of Hot Plasma in the Solar Corona Science 331 6013 55 58 Bibcode 2011Sci 331 55D doi 10 1126 science 1197738 PMID 21212351 S2CID 42068767 Further reading EditThorsten Dambeck Seething Cauldron in the Suns s Furnace MaxPlanckResearch 2 2008 p 28 33 B N Dwivedi and A K Srivastava Coronal heating by Alfven waves CURRENT 296 SCIENCE VOL 98 NO 3 10 FEBRUARY 2010 pp 295 296External links EditWikimedia Commons has media related to Solar corona NASA description of the solar corona Coronal heating problem at Innovation Reports NASA GSFC description of the coronal heating problem FAQ about coronal heating Solar and Heliospheric Observatory including near real time images of the solar corona Coronal x ray images from the Hinode XRT nasa gov Astronomy Picture of the Day July 26 2009 a combination of thirty three photographs of the Sun s corona that were digitally processed to highlight faint features of a total eclipse that occurred in March 2006 Animated explanation of the core of the Sun University of South Wales Alfven waves may heat the Sun s corona Solar Interface Region Bart de Pontieu SETI Talks Video Retrieved from https en wikipedia org w index php 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