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'{{Otheruses4|the astronomical term|other uses|Corona (disambiguation)}} A '''corona''' is a type of [[Plasma (physics)|plasma]] "[[celestial body's atmosphere|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]]. The [[Latin]] root of the word corona means '''crown'''. [[File:Solar eclips 1999 4 NR.jpg|thumb|right|During a total [[solar eclipse]], the solar [[corona]] can be seen with the naked eye.]] The high temperature of the corona gives it unusual [[spectroscopy|spectral]] features, which led some to suggest, in the 19th century, that it contained a previously unknown element, "[[coronium]]". These spectral features have since been traced to highly ionized Iron ('''Fe-XIV''') which indicates a plasma temperature in excess of 10⁶ [[kelvin]].<ref name="Aschwanden">{{cite book |last=Aschwanden |first=M. J. |year=2004 |title=Physics of the Solar Corona. An Introduction |publisher=Praxis Publishing Ltd. |isbn=3-540-22321-5}} </ref> Light from the corona comes from three primary sources, which are called by different names although all of them share the same volume of space. The K-corona (K for ''kontinuierlich'', "continuous" in German) is created by sunlight scattering off free [[electron]]s; [[Doppler broadening]] of the reflected photospheric absorption lines completely obscures 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 line|forbidden]] or hot [[spectral line|spectral emission lines]] and is the main source of information about the corona's composition.<ref name="Corfield">{{cite book |last=Corfield |first=Richard |year=2007 |title=Lives of the Planets |publisher=Basic Books |isbn=978-0-465-01403-3}} </ref> ==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|kelvin]] compared to the corona's one to three million kelvin. The corona is 10^&minus;12 times 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 [[sound|sonic]] pressure waves from below (the latter being less probable now that coronae are known to be present in early-type, highly magnetic [[star]]s). The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux generating the [[solar wind]]. [[Image:Twistedflux.png|right|thumb|300px|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 [[equator]]ial regions, with [[coronal hole]]s covering the [[Geographical pole|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 loop]]s, 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. ===Coronal Loops=== {{main|Coronal loop}} [[Image:Traceimage.jpg|left|thumb|300px|TRACE 171Å coronal loops]] [[Coronal loop]]s 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 precursor to [[solar flares]] and [[coronal mass ejection]]s (CMEs). Solar plasma feeding these structures is 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 chromospheric [[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 [[Solar prominence|filaments]] in the solar disk or [[solar prominence|prominences]] off the [[limb darkening|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 ([[:Image:Energyfig.png|example]]). 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 [[Line-of-sight propagation|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). ===Transients=== Accompanying [[solar flare]]s or large [[solar prominence]]s, '''"coronal transients"''' (also called [[coronal mass ejection]]s) 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 accompanies them. Some larger ejections can propel hundreds of millions of tons of material in to [[space]] at roughly at 1.5 million kilometers an hour. ==Other stars== Stars other than the Sun have coronae, which can be detected using [[X-ray]] [[telescope]]s. Some stellar coronae, particularly in young stars, are much more luminous than the Sun's. == 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 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 [[SI prefix|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. {{unsolved|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. 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 physics|solar physicists]] now believe that some combination of the two theories can probably explain coronal heating, although the details are not yet complete. The [[NASA]] mission [[NASA Solar probe|Solar Probe +]] is intended to approach the sun to a distance of approximately 9.5 solar radii in order to investigate coronal heating and the origin of the solar wind. {| class="wikitable" style="margin: 1em auto 1em auto" |+'''Competing heating mechanisms''' |- ! colspan="3" |Heating Models |- ! Hydrodynamic ! colspan="2" |Magnetic |- | rowspan="2" | * No magnetic field * Slow rotating stars ! [[Direct current|DC]] (''reconnection'') ! [[Alternating current|AC]] (''waves'') |- | * B-field stresses * Reconnection events * [[Solar flare|Flares]] * ''Uniform heating rates'' | * Photospheric foot point ''shuffling'' * MHD wave propagation * High Alfvén wave flux * ''Non-uniform heating rates'' |- ! Not our Sun! ! colspan="2" |Competing theories |} ===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 physics|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 wave]]s and [[Alfvén wave]]s.<ref>{{cite journal | last = Alfvén | first = Hannes | title = Magneto hydrodynamic waves, and the heating of the solar corona | journal = MNRAS | volume = 107 | pages = 211–219 | year = 1947 }}</ref> 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. 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. 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|SOHO]] space-borne solar observatory, the first platform capable of observing the Sun in the [[EUV|extreme ultraviolet]] for long periods of time with stable [[Photometry (astronomy)|photometry]]. Those were magneto-acoustic waves with a frequency of about 1 [[hertz|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. 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&nbsp;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&ndash;10&nbsp;Hz range. ===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 (element)|mercury]] or [[seawater]]). In a plasma, [[Magnetic field#Magnetic_field_lines|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 our solar system. Furthermore, the surface of Sun is covered with millions of small magnetized regions 50&ndash;1,000&nbsp;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 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. ==References== {{Reflist}} ==Further reading== Thorsten Dambeck: ''[http://www.mpg.de/english/illustrationsDocumentation/multimedia/mpResearch/2008/heft02/011/pdf13.pdf Seething Cauldron in the Suns's Furnace]'', MaxPlanckResearch, 2/2008, p. 28 - 33 == External links == * [http://www.innovations-report.com/html/reports/physics_astronomy/report-33153.html Coronal heating problem at Innovation Reports] * [http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/corona.html NASA/GSFC description of the coronal heating problem] * [http://solar-center.stanford.edu/FAQ/Qcorona.html FAQ about coronal heating] * [http://sohowww.nascom.nasa.gov Solar and Heliospheric Observatory, including near-real-time images of the solar corona] * [http://xrt.cfa.harvard.edu/ Coronal x-ray images from the Hinode XRT] * [http://antwrp.gsfc.nasa.gov/apod/ap090726.html 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 of 2006 *[http://alienworlds.glam.ac.uk/sunStructure.html#/corona Animated explanation of the core of the Sun] (University of Glamorgan) *[http://alienworlds.glam.ac.uk/sunStructure.html#/coronatemp Animated explanation of the temperature of the Corona] (University of Glamorgan) {{The Sun}} {{Star}} [[Category:Sun]] [[Category:Space plasmas]] [[Category:Plasma physics]] [[be-x-old:Сонечная карона]] [[ca:Corona solar]] [[cs:Koróna]] [[da:Korona (solen)]] [[de:Korona (Sonne)]] [[es:Corona solar]] [[eo:Korono]] [[eu:Eguzki koroa]] [[fa:تاج خورشیدی]] [[fr:Couronne solaire]] [[ko:코로나]] [[id:Korona]] [[it:Corona solare]] [[he:עטרה (שמש)]] [[lb:Sonnekorona]] [[lt:Saulės vainikas]] [[mk:Корона]] [[nl:Corona (astronomie)]] [[ja:コロナ]] [[nn:Korona]] [[pl:Korona słoneczna]] [[pt:Coroa solar]] [[ru:Солнечная корона]] [[sk:Koróna]] [[sl:Korona]] [[fi:Korona (aurinko)]] [[sv:Korona]] [[tr:Güneş tacı]] [[vi:Vành nhật hoa]] [[zh:日冕]]'