A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometres into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. Look up corona in Wiktionary, the free dictionary. ... For other uses, see Plasma. ... Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. ... Sol redirects here. ... “km” redirects here. ... Photo taken during the 1999 eclipse. ... An example image from SOHO - NASA A coronagraph is a telescopic attachment designed specifically to block out the harsh, direct light from a star, so that nearby objects can be resolved without burning out the telescopes optics. ...

Solar eclipse of August 11, 1999, as seen from France

The high temperature of the corona gives it unusual spectral features, which led some to suggest, in the 19th century, that it contained a previously unknown element, "coronium"; however these spectral features have since been traced to highly ionized Iron (Fe(XIV)) which indicates a plasma temperature in excess of 10⁶Kelvin.^[1] Image File history File linksMetadata Download high-resolution version (3543x3489, 2027 KB) Summary Description: Solar eclipse 1999 in France view 4 Additional noise reduction performed by Diliff. ... Image File history File linksMetadata Download high-resolution version (3543x3489, 2027 KB) Summary Description: Solar eclipse 1999 in France view 4 Additional noise reduction performed by Diliff. ... Special 2. ... Animation of the dispersion of light as it travels through a triangular prism. ... Coronium was the name of a suggested chemical element, hypothesised in the 19th century. ... For other uses, see Kelvin (disambiguation). ...

Physical features

The Sun's corona is much hotter (by a factor of nearly 200) than the visible surface of the Sun: the photosphere's average temperature is 5800 kelvin compared to the corona's one to three million kelvin. The corona is 10⁻¹² as dense as the photosphere, however, 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 sonic pressure waves from below (the latter being less probable now that coronae are known to be present in early-type, highly magnetic stars). The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux generating the solar wind. The photosphere of an astronomical object is the region at which the optical depth becomes one for a photon of wavelength equal to 5000 angstroms. ... For other uses, see Temperature (disambiguation). ... For other uses, see Kelvin (disambiguation). ... The chromosphere (literally, color sphere) is a thin layer of the Suns atmosphere just above the photosphere, roughly 10,000 kilometers deep (approximating to, if a little less than, the diameter of the Earth). ... Magnetic field lines shown by iron filings Magnetostatics Electrodynamics Electrical Network Tensors in Relativity This box:      In physics, the magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. ... This article is about audible acoustic waves. ... This article is about the astronomical object. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ...

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

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 solar maximum, where the solar magnetic field is continually wound up (due to a differential rotation at the solar equator; the equator rotates quicker than the poles). Sunspot activity will be more pronounced at solar maximum where the magnetic field is twisted to a maximum. 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 dark (when compared to the solar disk) spots. Image File history File links Twistedflux. ... Image File history File links Twistedflux. ... World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe. ... A geographical pole is either of two fixed points on the surface of a spinning body or planet, at 90 degrees from the equator, based on the axis around which a body spins. ... For other uses, see Sunspot (disambiguation). ... It has been suggested that this article or section be merged with Schwabe-Wolf cycle. ... Solar minimum is the period of least solar activity in the solar cycle of the sun. ... Solar maximum or solar max is the period of greatest solar activity in the solar cycle of the sun. ... Differential rotation is seen if parts of a rotating object move with different angular velocity. ... World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses Ilhéu das Rolas, in São Tomé and Príncipe. ... Magnetic field lines shown by iron filings Magnetostatics Electrodynamics Electrical Network Tensors in Relativity This box:      In physics, the magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. ... Typical coronal loops observed by TRACE Coronal loops form the basic structure of the lower corona and transition region of the Sun. ... Magnetic flux, represented by the Greek letter Φ (phi), is a measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field. ... The photosphere of an astronomical object is the region at which the optical depth becomes one for a photon of wavelength equal to 5000 angstroms. ...

Coronal Loops

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 hole (polar) regions 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 precurser to solar flares and Coronal Mass Ejections (CMEs). Solar plasma feeding these structures are heated from under 6000K to well over 1×10⁶K from the photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one foot point and drain from the other (siphon flow due to a pressure difference, or asymmetric flow due to some other driver). This is known as chromospheric evaporation and chromosperic condensation respectively. There may also be symmetric flow from both loop foot points, causing a buildup of mass in the loop structure. The plasma may cool in this region creating dark filaments in the solar disk or prominences off the limb. Coronal loops may have lifetimes in the order of seconds (in the case of flare events), minutes, hours or days. Usually coronal loops lasting for long periods of time are known as steady state or quiescent coronal loops, where there is a balance in loop energy sources and sinks (example). Image File history File links Download high-resolution version (768x768, 355 KB) source: http://trace. ... Image File history File links Download high-resolution version (768x768, 355 KB) source: http://trace. ... Typical coronal loops observed by TRACE Coronal loops form the basic structure of the lower corona and transition region of the Sun. ... Coronal holes are areas where the Suns corona is darker, colder, and has lower-density plasma than average. ... The plasma in the solar wind meeting the heliopause The solar wind is a stream of charged particles (i. ... A solar flare is a violent explosion in the Suns atmosphere with an energy equivalent to tens of millions of hydrogen bombs. ... A composite image showing two CMEs (at 2 oclock and 8 oclock), with the sun at center. ... Not to be confused with Psiphon. ... “Vaporization” redirects here. ... For other uses, see Condensation (disambiguation). ... Symmetry is a characteristic of geometrical shapes, equations and other objects; we say that such an object is symmetric with respect to a given operation if this operation, when applied to the object, does not appear to change it. ... A filament is a fine, thinly spun thread, fiber, or wire. ... Filaments surrounding a solar flare, caused by the interaction of the plasma in the Suns atmosphere with its magnetic field. ... The limb darkened Sun - An image of the Sun in visible light showing the limb darkening effect as a drop in intensity towards the edge or limb of the solar disk. ... HELLO EVERYONE!! Steady state is a more general situation than Dynamic equilibrium. ... In cell biology, quiescence is the state of cell when it is not dividing. ...

Coronal loops have become very important when trying to understand the current coronal heating problem. Coronal loops are highly radiating sources of plasma and therefore easy to observe by instruments such as TRACE, they are highly observable laboratories to study phenomena such as solar oscillations, wave activity and nanoflares. However, it remains difficult to find a solution to the coronal heating problem as these structures are being observed remotely, where many ambiguities are present (i.e. radiation contributions along the LOS). In-situ measurements are required before a definitive answer can be arrived at, but due to the high plasma temperatures in the corona, in-situ measurements are impossible (at least for the time-being). Look up Trace in Wiktionary, the free dictionary. ... When viewing a scene, as in optics, photography, or even hunting, the line of sight is the straight line between the observer and the target. ... In situ is a Latin phrase meaning in the place. ...

Transients

Generated by solar flares or large solar prominences, "coronal transients" (also called coronal mass ejections) are sometimes released. These are enormous loops of coronal material traveling outward from the Sun at over a million kilometers per hour, containing roughly 10 times the energy of the solar flare or prominence that triggered them. Some larger ejections can propel hundreds of millions of tons of material in to space at roughly a million miles an hour. A solar flare observed by Hinode in the G-band. ... Filaments surrounding a solar flare, caused by the interaction of the plasma in the Suns atmosphere with its magnetic field. ... A composite image showing two CMEs (at 2 oclock and 8 oclock), with the sun at center. ... This article is about the idea of space. ...

Other stars

Stars other than the Sun have coronae, which can be detected using X-ray telescopes. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... This article does not cite any references or sources. ...

Coronal heating problem

The coronal heating problem in solar physics relates to the question of why the temperature of the Suns corona is millions of Kelvin higher than that of the surface. 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, or surface, at about 5800 Kelvin, to the much hotter corona at about 1 to 3 MK (parts of the corona can even reach 10 MK). The amount of power required to heat the solar corona can easily be calculated. It is about 1 kilowatt for every square meter of surface area on the Sun, or 1/40000 of the amount of light energy that escapes the Sun. The Sun is the star at the centre of our Solar system. ... The second law of thermodynamics is an expression of the universal law of increasing entropy. ... An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol) to form a decimal multiple or submultiple. ...

Unsolved problems in physics: Why is the Sun's Corona so much hotter than the Sun's surface?

This thin region of temperature increase from the chromosphere to the corona is known as the transition region and can range from tens to hundreds of kilometers thick. An analogy of this would be a light bulb heating the air surrounding it hotter than its glass surface. The second law of thermodynamics would be broken. So, what mechanism is heating the tenuous coronal plasma to these temperatures? Image File history File links No higher resolution available. ... This is a list of some of the unsolved problems in physics. ... TRACE 19. ... The second law of thermodynamics is an expression of the universal law of increasing entropy. ...

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. Most solar physicists now believe that some combination of the two theories can probably explain coronal heating, although the details are not yet complete. The Sun is the star at the centre of our Solar system. ...

Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... City lights viewed in a motion blurred exposure. ... A solar flare observed by Hinode in the G-band. ...

Wave heating theory

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 Alfvén waves.^[2] Magneto-acoustic waves are sound waves that have been modified by the presence of a magnetic field, and Alfvén waves are similar to ULF 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. Évry Léon Schatzman (born September 16, 1920 in Neuilly-sur-Seine, France) is a French astrophysicist. ... A Plasma lamp In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase of matter. ... This article is about compression waves. ... A magnetosonic wave is a longitudinal wave[1] of ions (and electrons) in a magnetized plasma propagating perpendicular to the stationary magnetic field. ... A cluster of double layers forming in an Alfvén wave, about a sixth of the distance from the left. ... ULF can refer to: Ultra low frequency, the radio frequency band from 300 Hz to 3 kHz Ultra Large Format, used for cameras producing negatives larger than 8 x 10 Ultra Low Floor, is a type of tram United Labour Front, a defunct political party in Trinidad and Tobago Ulf... Radio waves are electromagnetic waves occurring on the radio frequency portion of the electromagnetic spectrum. ... This article is about matter in physics and chemistry. ... Granulation can refer to: Granulation tissue, a product of healing in major wounds; Granular synthesis, a sound synthesis method; Solar granulation, a sign of turbulent convection on the Sun; or a process used by goldsmiths or silversmiths to decorate precious metals with small spheres fused to a base piece. ... Supergranulation is a particular pattern on the Sun surface. ... Shock Waves is a horror movie from 1977 directed by Ken Wiederhorn. ...

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 into the corona and then dissipate it as heat. The reflection of sunlight on water Reflection is the abrupt change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated. ...

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 SOHO space-borne solar observatory, the first platform capable of observing the Sun in the extreme ultraviolet for long periods of time with stable photometry. Those were magneto-acoustic waves with a frequency of about 1 millihertz (MHz, corresponding to a 1,000 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 Alfvén waves launched by solar flares, but those events are transient and cannot explain the uniform coronal heat. The Solar and Heliospheric Observatory (SOHO) is a spacecraft that was launched on an Atlas IIAS launch vehicle on 2 December 1995 to study the Sun, and began normal operations in May 1996. ... For other uses, see Ultraviolet (disambiguation). ... Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical objects electromagnetic radiation. ... This article is about the SI unit of frequency. ...

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 100 MHz (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 200 Hz, 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 1–10 Hz range. Look up Trace in Wiktionary, the free dictionary. ... ... Williams College is a private, liberal arts college located in Williamstown, Massachusetts. ...

Magnetic reconnection theory

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 a 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 the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing the overall topology of a magnetic field. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery Standard atomic weight 200. ... Annual mean sea surface salinity for the World Ocean. ... Magnetic field lines shown by iron filings Magnetostatics Electrodynamics Electrical Network Tensors in Relativity This box:      In physics, the magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. ... A Möbius strip, an object with only one surface and one edge; such shapes are an object of study in topology. ... Magnetic lines of force of a bar magnet shown by iron filings on paper A magnet is an object that has a magnetic field. ...

Magnetic reconnection is hypothesized to be the mechanism behind solar flares, the largest explosions in our solar system. Furthermore, the surface of Sun covered with million of small magnetized regions 50–1,000 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. Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains are spliced to one another, changing the overall topology of a magnetic field. ...

The idea that micro flares might heat the corona was put forward 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. Eugene N. Parker (1927 - ) is a solar astrophysicist. ... For other uses, see Ultraviolet (disambiguation). ...

References

1. ^ Aschwanden, M. J. (2004). Physics of the Solar Corona. An Introduction. Praxis Publishing Ltd.. ISBN 3-540-22321-5. 
2. ^ Alfvén, Hannes (1947). "Magneto hydrodynamic waves, and the heating of the solar corona". MNRAS 107: 211–219. 

External links

• 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
• Sun|trek website An educational resource for teachers and students about the Sun and its effect on the Earth

v • d • e The Sun
Structure	Solar core - Radiation zone - Convection zone
Atmosphere	Photosphere - Chromosphere - Transition region - Corona
Extended structure	Termination Shock - Heliosphere - Heliopause - Heliosheath - Bow Shock
Solar phenomena	Sunspots - Faculae - Granules - Supergranulation - Solar Wind - Spicules - Coronal loops - Solar Flares - Solar Prominences - Coronal Mass Ejections - Moreton Waves - Coronal Holes
Other	Solar System - Solar Variation - Solar Dynamo - Heliospheric Current Sheet - Solar Radiation - Solar Eclipse - Spectral Class: G2