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| {{About|the astronomical term|other uses|Corona (disambiguation)}}
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| [[File:Solar eclipse 1999 4 NR.jpg|thumb|right|During a total [[solar eclipse]], the solar corona can be seen by the naked eye.]] | |
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| A '''corona''' (Latin, '[[crown (headgear)|crown]]') is a type of [[Plasma (physics)|plasma]] that surrounds the [[Sun]] and other celestial bodies. The Sun's corona extends millions of kilometres into space and is most easily seen during a total [[solar eclipse]], but it is also observable with a [[coronagraph]]. The word "corona" is a [[Latin language|Latin]] word meaning '''crown''', from the [[Ancient Greek]] κορώνη (korōnē, “garland, wreath”).
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| The high temperature of the Sun's corona gives it unusual [[spectroscopy|spectral]] features, which led some in the 19th century to suggest that it contained a previously unknown element, "[[coronium]]". These spectral features have since been traced to highly ionized iron ('''Fe-XIV'''). [[Bengt Edlén]], following the work of Grotrian (1939), first identified the coronal lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green FeXIV line at 5303 Å, but also the red line FeX at 6374 Å). These high stages of [[ionisation]] indicate a plasma temperature in excess of 1,000,000 [[kelvin]].<ref name="Aschwanden">{{cite book | |
| |last=Aschwanden
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| |first=M. J.
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| |year=2004
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| |title=Physics of the Solar Corona. An Introduction
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| |publisher=Praxis Publishing
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| |isbn=3-540-22321-5}}
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| </ref>
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| 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 [[Joseph von Fraunhofer|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 (astronomy)|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
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| |last=Corfield
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| |first=Richard
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| |year=2007
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| |title=Lives of the Planets
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| |publisher=Basic Books
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| |isbn=978-0-465-01403-3}}
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| </ref> | |
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| ==Physical features==
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| 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 5800 [[kelvin]] compared to the corona's one to three million kelvin. The corona is 10<sup>−12</sup> 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 [[Magnetohydrodynamics|MHD waves]] from below. The outer edges of the Sun's corona are constantly being transported away due to open magnetic flux generating the [[solar wind]].
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| [[File: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 the following minimum. Since 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 more twisted. 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.
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| Since the corona has been photographed at high resolution in the X-rays 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 very various and complex: different zones have been immediately classified on the coronal disc
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| .<ref>{{cite journal|doi = 10.1007/BF00152731|last1 = Vaiana|first1 = G. S.|last2 = Krieger|first2 = A. S.|last3 = Timothy|first3 = A. F.|title = Identification and analysis of structures in the corona from X-ray photography | |
| | journal = Solar Physics | volume = 32| pages = 81–116| year = 1973| bibcode=1973SoPh...32...81V}}</ref><ref>{{cite journal |last = Vaiana, G.S., Tucker, W.H|title = Solar X-Ray Emission in "X-Ray Astronomy" ed. by R. Giacconi and H. Gunsky| page = 169 | year = 1974}}</ref><ref>{{cite journal|doi = 10.1146/annurev.aa.16.090178.002141|last1 = Vaiana |first1 = G S|last2 = Rosner|first2 = R | title = Recent advances in Coronae Physics
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| | journal = Ann. Rev. Astron. Astrophysics | volume = 16| pages = 393–428| year = 1978 | bibcode=1978ARA&A..16..393V}}</ref>
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| The astronomers usually distinguish several regions,<ref name="Gibson">{{cite book
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| |last=Gibson
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| |first= E. G.
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| |year=1973
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| |title= The Quiet Sun
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| |publisher=National Aeronautics and Space Administration, Washington, D.C.
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| }}</ref> as described below.
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| ===Active regions===
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| '''Active regions''' are ensembles of loop structures connecting points of opposite magnetic polarity in the photosphere, the so-called [[coronal loops]].
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| 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 10<sup>9</sup> to 10<sup>10</sup> particle per cm<sup>3</sup>.
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| [[File:Prominence (PSF).png|thumb||right|180px|[[Solar Prominence solare|Prominence]] (PSF)]]
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| Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights on the Sun's surface:<ref name="Gibson"/> [[sunspots]] and [[facula]]e, happening in the photosphere, [[spicule (solar physics)|spicules]], [[Hα]] [[Solar prominence|filaments]] and [[plage (astronomy)|plages]] in the chromosphere, [[solar prominences|prominences]] in the chromosphere and transition region, and [[Solar flare|flares]] and [[coronal mass ejection]]s happening in the corona and chromosphere, but if flares are very violent can perturb also the photosphere and generate a [[Moreton wave]], as described by Uchida. On the contrary, quiescent prominences are large, cool dense structures which are observed as dark, "snake-like" Hα ribbons (filaments) on the solar disc. Their temperature is about 5000–8000 K, and so they are usually considered as chromospheric features.
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| 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.<ref>http://www.space.com/19400-sun-corona-secrets-suborbital-telescope.html</ref>
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| ====Coronal loops====
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| {{main|Coronal loop}}
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| [[File:Traceimage.jpg|left|thumb|300px|TRACE 171Å coronal loops]]
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| [[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.<ref>{{cite journal|doi = 10.1086/427488|last1 = Katsukawa|first1 = Yukio|last2 = Tsuneta|first2 = Saku | title = Magnetic Properties at Footpoints of Hot and Cool Loops | journal = The Astrophysical Journal | volume = 621 | pages = 498–511 | year = 2005 | bibcode=2005ApJ...621..498K}}</ref> 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).
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| Solar plasma feeding these structures is heated from under 6000 K to well over 1×10<sup>6</sup> K 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,<ref>{{cite journal|doi = 10.1023/A:1005182503751|last1 = Betta |first1 = Rita|last2 = Orlando|first2 = Salvatore|last3 = Peres|first3 = Giovanni|last4 = Serio|first4 = Salvatore | title = On the Stability of Siphon Flows in Coronal Loops
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| | journal = Space Science Reviews | volume = 87 | pages = 133–136| year = 1999|bibcode = 1999SSRv...87..133B }}</ref> or asymmetric flow due to some other driver).
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| When the plasma goes upward from the footpoints towards the loop top, as it always occurs during the initial phase of a compact flare, it is defined as chromospheric [[evaporation]]. When the plasma rapidly cools falling down towards the photosphere, we have the chromospheric [[condensation]]. There may also be [[symmetric]] flow from both loop foot points, causing a buildup of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), creating dark [[Solar prominence|filaments]] in the solar disk or [[solar prominence|prominences]] off the [[limb darkening|limb]].
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| 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 ''[[wikt:quiescent|quiescent]]'' coronal loops, where there is a balance in loop energy sources and sinks ([[:File:Energyfig.png|example]]).
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| 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). The next mission of the Nasa [[Solar Probe Plus]] will approach the Sun very closely allowing more direct observations.
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| [[File:Coronal Hole Magnetic Field Lines.svg|thumb|right|180px|Coronal arches connecting regions of opposite magnetic polarity (A) and the unipolar magnetic field in the coronal hole (B)]]
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| ==== Large-scale structures ====
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| '''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.
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| They were first detected in the June 8, 1968 flare observation during a rocket flight.<ref name = Giacconi>{{cite book |last = Giacconi| first = Riccardo| title = G.S. Vaiana memorial lecture in ''Proceedinds of Physics of Solar and Stellar Coronae: G.S. Vaiana Memorial Symposium'', ed. by J. F. Linsky and S.Serio| pages = 3–19 | year = 1992 | publisher = Kluwer Academic Publishers-Printed in the Netherlands | isbn = 0-7923-2346-7}}</ref>
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| 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).
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| ==== Interconnections of active regions ====
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| The '''interconnections of active regions''' are arcs connecting zones of opposite magnetic field, in different active regions. Significant variations of these structures are often seen after a flare.
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| Some other features of this kind are [[helmet streamer]]s—large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered as sources of the slow [[solar wind]].<ref>{{cite journal| doi= 10.1029/2000GL000097| last= Ofman | first= Leon | title= Source regions of the slow solar wind in coronal streamers | journal= Geophysical Research Letters | volume = 27 | issue= 18 | pages= 2885–2888 |year=2000 | bibcode=2000GeoRL..27.2885O}}</ref>
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| ==== Filament cavities ====
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| '''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''.<ref name=Giacconi />
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| [[File:Crackling with Solar Flares.jpg|thumb|left|360px|Image taken by the [[Solar Dynamics Observatory]] on Oct 16 2010. A very long filament cavity is visible across the Sun's southern hemisphere.]]
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| 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 the 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 <math>p</math> can be calculated by the [[state equation]] of a perfect gas <math> p = n K_B T</math>, where <math>n</math> is the [[particle number density]], <math>K_B</math> the [[Boltzmann constant]] and <math>T</math> the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect makes [[sunspots]] dark in the [[photosphere]].
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| ==== Bright points ====
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| '''Bright points''' are small active regions spread over the whole solar disk. X-ray bright points were first detected in April 8, 1969 during a rocket flight.<ref name=Giacconi />
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| 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.<ref>{{cite journal |author=Kariyappa, R.; Deluca, E. E.; Saar, S. H.; Golub, L.; Damé, L.; Pevtsov, A. A.; Varghese, B. A. |title= Temperature variability in X-ray bright points observed with Hinode/XRT | journal= Astronomy & Astrophysics| year= 2011 | volume= 526 | bibcode = 2011A&A...526A..78K | doi = 10.1051/0004-6361/201014878}}</ref>
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| === Coronal holes ===
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| {{main|Coronal hole}}
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| '''[[Coronal hole]]s''' are the polar regions which look dark in the X-rays since they do not emit much radiation.<ref>{{cite journal|doi = 10.1088/0004-637X/719/1/131|last1 = Ito |first1 = Hiroaki|last2 = Tsuneta|first2 = Saku|last3 = Shiota|first3 = Daikou|last4 = Tokumaru|first4 = Munetoshi|last5 = Fujiki|first5 = Ken'Ichi | title = Is the Polar Region Different from the Quiet Region of the Sun?| journal = The Astrophysical Journal | volume = 719 | pages = 131–142| year = 2010 | bibcode=2010ApJ...719..131I|arxiv = 1005.3667 }}</ref> 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
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| these regions.
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| 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 exactly, they are long thin streamers that project outward from the Sun's north and south poles.<ref>{{cite journal| doi=10.1051/0004-6361:20021628| last1=Del Zanna | first1=G.| last2=Bromage| first2=B. J. I.| last3=Mason| first3=H. E.| title= Spectroscopic characteristics of polar plumes| journal= Astronomy & Astrophysics| year=2003| volume=398| pages= 743–761| bibcode=2003A&A...398..743D}}</ref>
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| === The Quiet Sun ===
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| The solar regions which are not part of active regions and coronal holes are commonly identified as the '''quiet Sun'''.
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| The equatorial region has a faster velocity rotation 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 lower 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 the coronal holes.
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| == Variability of the corona ==
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| 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 in times very different among them. Studying the 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.
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| {| class="wikitable" | title="Typical length of observable coronal features"
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| |-
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| ! '''Coronal event''' !! '''Typical time-scale''' !! '''Typical length-scale (Mm)'''
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| |-
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| | Active region [[Solar flare|flare]] || 10 to 10,000 seconds || 10–100
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| |-
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| | X-ray bright point || minutes || 1–10
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| |-
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| | Transient in large-scale structures || from minutes to hours || ~100
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| |-
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| | Transient in interconnecting arcs || from minutes to hours || ~100
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| |-
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| | Quiet Sun || from hours to months || 100–1,000
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| |-
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| | [[Coronal hole]] || several rotations || 100–1,000
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| |}
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| === Flares ===
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| {{main|Solar flares}}
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| [[File:Magnificent CME Erupts on the Sun - August 31.jpg|thumb|right|On August 31, 2012 a long filament of solar material that had been hovering in the Sun's atmosphere, the Corona, erupted out into space at 4:36 p.m. EDT]]
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| Flares take place in active regions and provoke a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they interest several zones of the solar atmosphere and involve many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.
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| Flares are impulsive phenomena, of average duration of 15 minutes, even if the most energetic events can last several hours. Flares involve a high and rapid increase of the density and temperature.
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| An emission in white light is only seldom observed: usually, flares are only seen at EUV wavelengths and in the X-rays, typical of the chromospheric and coronal emission.
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| In the corona the morphology of flares, which can be grasped from the observations in the soft and hard X-rays, at the UV wavelengths and in [[Hα]], is very complex. However, two kinds of basic structures can be distinguished:
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| <ref>{{cite journal|doi = 10.1086/155452|last1 = Pallavicini |first1 = R.|last2 = Serio|first2 = S.|last3 = Vaiana|first3 = G. S.|title = A survey of soft X-ray limb flare images – The relation between their structure in the corona and other physical parameters
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| | journal = The Astrophysical Journal | volume = 216| page = 108| year = 1977 | bibcode=1977ApJ...216..108P}}</ref>
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| *'''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 10<sup>22</sup> – 10<sup>23</sup> J.
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| * '''flares of long duration''', associated to eruptions of [[Solar prominence|prominence]]s, transients in white light and ''two-ribbon flares'':<ref>{{cite journal|doi = 10.1038/344842a0|last1 = Golub |first1 = L.|last2 = Herant|first2 = M.|last3 = Kalata|first3 = K.|last4 = Lovas|first4 = I.|last5 = Nystrom|first5 = G.|last6 = Pardo|first6 = F.|last7 = Spiller|first7 = E.|last8 = Wilczynski|first8 = J. | title = Sub-arcsecond observations of the solar X-ray corona | journal = Nature | volume = 344 | pages = 842–844| year = 1990|bibcode = 1990Natur.344..842G | issue=6269}}</ref> in this case the magnetic loops change their configuration during the event. The energies emitted during these flares of such large proportions can reach 10<sup>25</sup> J.
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| [[File:Solar-filament.gif|300px|thumb|right| Filament erupting during a solar flare, seen at EUV wavelengths ([[TRACE]])]]
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| As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. These times, moreover, can depend on the range of wavelengths used to observe the event even considerably:
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| *'''an initial impulsive phase''', whose duration is of the order of minutes. Strong emissions of energy are often observed even in the microwaves, at EUV wavelengths and in the hard X-rays.
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| *'''a maximum phase'''
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| *'''a decay phase''', which can last several hours.
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| Sometimes also a phase preceding the flare can be observed, usually called as "pre-flare" phase.
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| ===Transients===
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| {{main|Coronal mass ejection}}
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| 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 into [[space]] at roughly 1.5 million kilometers an hour.
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| === A solar storm ===
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| These movies have been taken by the satellite [[Solar and Heliospheric Observatory|SOHO]] during two weeks in October and November 2003. The images have been taken at the same time by the different instruments on board SOHO: the MDI, producing [[magnetogram]]s, the [[Extreme ultraviolet Imaging Telescope]] (EIT), which photographs the corona in the ultraviolets, and the [[Large Angle and Spectrometric Coronagraph]] (LASCO).
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| The first video at the top on the left (in grey) shows the magnetograms as they vary in time. At the top on the right (in yellow) the photosphere can be seen in white light as taken by the MDI.
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| Furthermore the EIT filmed the event in its four filters which are sensitive to different wavelengths, selecting plasma at different temperatures. The images in orange (on the left) refers to chromospheric plasma, while that one in green (on the right) to the corona.
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| In the last movie at the centre the Sun's images taken in the ultraviolet filter by the EIT have been combined with those taken by the coronograph LASCO blue and white in this movie.
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|
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| All the instruments registered the storm which is considered as one of the largest solar activity events observed by SOHO and maybe since the advent of space-based solar observations. The storm involved all the plasma of the solar atmosphere from the chromosphere to the corona, as can be seen from the movies, which are ordered from left to right, from top to bottom, in the outward direction of the increasing temperature on the Sun: photosphere (yellow), chromosphere-transition region (orange), low corona (green) and extended corona (blue).
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| The corona is visible to the SOHO/LASCO coronagraph instruments, which block the bright disk of the Sun so the significantly fainter corona can be seen. In this movie, the inner coronagraph (designated C2) is combined with the outer coronagraph (C3).
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| As the movie plays, we can observe a number of features of the active Sun. Long streamers radiate outward from the Sun and wave gently due to their interaction with the solar wind.
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| The bright white regions are visible due to their high density of free electrons which scatter the light from the photosphere towards the observer. Protons and other ionized atoms are there as well, but are not as visible since they do not interact with photons as strongly as electrons. Coronal Mass Ejections (CMEs) are occasionally observed launching from the Sun. Some of these launch particle events can saturate the cameras with snow-like artifacts.
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| Also visible in the coronagraphs are stars and planets. Stars are seen to drift slowly to the right, carried by the relative motion of the Sun and the Earth. The planet Mercury is visible as the bright point moving left of the Sun.
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| The horizontal "extension" in the image is called blooming and is due to a charge leakage along the readout wires in the CCD imager in the camera.
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| {{-}}<!-- If there is a better way to group these images so that they do not overrun the text, please use it. -->
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| {{-}}
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| ==Stellar coronae==
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| {{main|Coronal star}}
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| Coronal stars are ubiquitous among the [[star]]s in the cool half of the [[Hertzsprung-Russell diagram]].<ref name=Gudel>{{cite journal |author=Güdel M |title=X-ray astronomy of stellar coronae |journal=The Astronomy and Astrophysics Review|year=2004 |volume=12 |pages=71–237 |doi=10.1007/s00159-004-0023-2 |url=http://astronomy.sci.ege.edu.tr/~rpekunlu/GBDG/papers/XRayfromStellarCoronae.pdf|arxiv = astro-ph/0406661 |bibcode = 2004A&ARv..12...71G }}</ref> These coronae can be detected using [[X-ray telescope]]s. 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 [[Variable star|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 (''L''<sub>x</sub> ≥ 10<sup>32</sup> erg·s<sup>−1</sup> or 10<sup>25</sup>W) and the hottest known with dominant temperatures up to 40 MK.<ref name=Gudel/>
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| The astronomical observations planned with the [[Einstein Observatory]] by Giuseppe Vaiana and his group<ref name=Vaianaetal81>{{cite journal |doi= 10.1086/158797 |author= Vaiana, G.S., et al. |title= Results from an extensive Einstein stellar survey |journal= The Astrophysical Journal|year=1981 |volume=245 |page=163 |bibcode=1981ApJ...245..163V}}</ref> showed that F-, G-, K- and M-stars have chromospheres and often coronae much like our Sun.
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| The ''O-B stars'', which do not have surface convection zones, have a strong X-ray emission. However these stars do not have a corona, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs.
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| 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.
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| == Physics of the corona ==
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| | |
| [[File:171879main LimbFlareJan12 lg.jpg|300px|thumb|left|Taken by [[Hinode]] on Jan 12 2007 this image reveals the filamentary nature of the corona.]]
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| The matter in the external part of the solar atmosphere is in the state of [[plasma (physics)|plasma]], at very high temperature (a few million Kelvins) and at very low density (of the order of 10<sup>15</sup> particle/m<sup>3</sup>).
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| According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour.
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| The composition is the same as the one in the Sun's interior, mainly hydrogen, but completely ionized, thence protons and electrons, and a small fraction of the other atoms in the same percentages as they are present in the photosphere.
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| Even 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]]. 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.
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| The corona behaves like a gas which is very hot but very light at the same time: the pressure in the photosphere is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximatively a million times higher than on the solar surface.
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| However it is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities.
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| Supposing that they have the same kinetic energy on average
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| (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.
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| Furthermore the presence of electric charges induces the generation of electric currents and high magnetic fields.
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| Magnetohydrodynamic waves (MHD waves) can also propagate in this plasma,<ref name="Jeffrey">{{cite book
| |
| |last = Jeffrey
| |
| |first = Alan
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| |year =1969
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| |title = Magneto-hydrodynamics
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| |publisher = UNIVERSITY MATHEMATICAL TEXTS
| |
| }}</ref> even if it is not still clear how they can be transmitted or generated in the corona.
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| | |
| ===Radiation===
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| {{Main|Coronal radiative losses}}
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| The corona emits radiation mainly in the X-rays, observable only from space.
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| 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.
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| 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.
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| 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.<ref>{{cite journal| doi = 10.1007/BF00873539| last = Mewe | first = R.| title = X-ray spectroscopy of stellar coronae| journal = The Astronomy and Astrophysics Review | volume = 3 | page = 127 | year = 1991 | bibcode=1991A&ARv...3..127M}}</ref>
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| | |
| === Thermal conduction ===
| |
| [[File:STEREO-A first images.jpg|thumb|right|200px|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 (195 Å—green), 60,000–80,000 °C (304 Å—red), and 2.5 million °C (286 Å—yellow).
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| ]]
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| [[File:STEREO-A first images slow anim.gif|thumb|right|200px|[[STEREO]] – First images as a slow animation]]
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| 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.
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|
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| 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.<ref name="Spitzer">{{cite book
| |
| |last=Spitzer
| |
| |first= L.
| |
| |year=1962
| |
| |title= Physics of fully ionized gas
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| |publisher= Interscience tracts of physics and astronomy
| |
| }}</ref>
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| 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.
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| 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 an 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.
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| In the direction longitudinal to the magnetic field, the [[thermal conductivity]] of the corona is<ref name="Spitzer" />
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| <math>
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| 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}~ W m^{-1}K^{-1}
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| </math>
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| | |
| where <math>k_B</math> is the [[Boltzmann constant]],
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| <math>T</math> is the temperature in Kelvin,
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| <math>m_e</math> the electron mass,
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| <math>e</math> the electric charge of the electron,
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| <math> \ln \Lambda = \ln \left(12\pi n \lambda_D^3 \right) </math>
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| the Coulomb logarithm, and
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| <math>\lambda_D = \sqrt{\frac{k_B T }{4 \pi n e^2 }}</math>
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| the [[Debye length]] of the plasma with particle density <math>n</math>.
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| The Coulomb logarithm <math> \ln \Lambda </math> is roughly 20 in the corona, with a mean temperature of 1 MK and a density of 10<sup>15</sup> particles/m<sup>3</sup>, and about 10 in the chromosphere, where the temperature is approximatively 10kK and the particle density is of the order of 10<sup>18</sup> particles/m<sup>3</sup>, and in practice it can be assumed constant.
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| Thence, if we indicate with <math>q</math> the heat for a volume unit, expressed in J m<sup>−3</sup>, the Fourier equation of heat transfer, to be computed only along the direction <math>x</math> of the field line, becomes
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| <math> \frac{\partial q}{\partial t}= 0.9~10^{-11}~ \frac{\partial^2 T^{7/2}}{\partial x ^2 }</math>.
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| Numerical calculations have shown that the thermal conductivity of the corona is comparable to that of copper.
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| ===Coronal seismology===
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| {{main|Coronal seismology}}
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| '''Coronal seismology''' is a new way of studying the [[plasma (physics)|plasma]] of the solar corona with the use of [[magnetohydrodynamics|magnetohydrodynamic]] (MHD) waves. Magnetohydrodynamics studies the [[dynamics (mechanics)|dynamics]] of [[electrical conduction|electrically conducting]] [[fluid]]s—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 kind 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.
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| == Coronal heating problem ==
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| {{unsolved|physics|Why is the Sun's Corona so much hotter than the Sun's surface?}}
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| [[File:Van Gogh Sun.ogv|thumb|350px|A new visualisation technique can provide clues to the coronal heating problem.]]
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| | |
| The ''coronal heating problem'' in [[solar physics]] relates to the question of why the temperature of the Sun's 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 K, to the much hotter corona at about 1 to 3 [[SI prefix|MK]] (parts of the corona can even reach 10 MK).
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| The 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.
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| 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, or 1/40000 of the amount of light energy that escapes the Sun.
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| Many coronal heating theories have been proposed,<ref>{{cite book |last = Ulmshneider |first= Peter |title = Heating of Chromospheres and Coronae in ''Space Solar Physics'', Proceedings, Orsay, France, edited by J.C. Vial, K. Bocchialini and P. Boumier| publisher = Springer | pages = 77–106| year = 1997| isbn= 3-540-64307-9}}</ref> but two theories have remained as the ''most likely'' candidates, ''wave heating'' and ''[[magnetic reconnection]]'' (or ''[[nanoflares]]'').<ref>{{cite book |last = Malara, F., Velli, M. |title = Observations and Models of Coronal Heating in ''Recent Insights into the Physics of the Sun and Heliosphere: Highlights from SOHO and Other Space Missions'', Proceedings of IAU Symposium 203, edited by Pål Brekke, Bernhard Fleck, and Joseph B. Gurman| publisher = Astronomical Society of the Pacific | pages = 456–466| year = 2001| isbn= 1-58381-069-2}}</ref> Through most of the past 50 years, neither theory has been able to account for the extreme coronal temperatures.
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| 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.
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| In 2012, high resolution (<0.2″) [[soft X-ray]] imaging with the [[High Resolution Coronal Imager]] on board of a [[sounding rocket]] revealed tightly wound braids in the corona. The authors hypothesized that the reconnection and unraveling of braids can 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) in this case is supposed to be MHD waves.<ref name=Cirtain2013>{{cite doi|10.1038/nature11772}}</ref>
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| | |
| {| class="wikitable" style="margin: 1em auto 1em auto"
| |
| |+'''Competing heating mechanisms'''
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| |-
| |
| ! colspan="3" |Heating Models
| |
| |-
| |
| ! Hydrodynamic
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| ! colspan="2" |Magnetic
| |
| |-
| |
| | rowspan="2" |
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| * No magnetic field
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| * Slow rotating stars
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| ! [[Direct current|DC]] (''reconnection'')
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| ! [[Alternating current|AC]] (''waves'')
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| |-
| |
| |
| |
| * B-field stresses
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| * Reconnection events
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| * [[Solar flare|Flares]]-[[nanoflares]]
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| * ''Uniform heating rates''
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| |
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| * Photospheric foot point ''shuffling''
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| * MHD wave propagation
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| * High Alfvén wave flux
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| * ''Non-uniform heating rates''
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| |-
| |
| ! Not our Sun!
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| ! colspan="2" |Competing theories
| |
| |}
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| | |
| ===Wave heating theory===
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| 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
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| | last = Alfvén
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| | first = Hannes
| |
| | title = Magneto hydrodynamic waves, and the heating of the solar corona
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| | journal = MNRAS
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| | volume = 107
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| | pages = 211–219
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| | year = 1947
| |
| |bibcode = 1947MNRAS.107..211A }}</ref> 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|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 [[Granule (solar physics)|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.
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| 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 [[reflection (physics)|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.
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| | |
| 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 [[extreme ultraviolet]] (EUV) 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.
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| | |
| 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 [[Solar and Heliospheric Observatory|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.
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| | |
| Recently, Alfvénic motions have been found in the lower solar atmosphere <ref>{{cite web|url=http://www.science20.com/news_releases/alfven_waves_our_sun_doing_magnetic_twist |title = Alfven Waves – Our Sun Is Doing The Magnetic Twist|publisher=read on Jan 6 2011}}</ref>
| |
| <ref>{{cite journal | doi = 10.1126/science.1168680 | last1 = Jess | first1 = DB | last2 = Mathioudakis | first2 = M | last3 = Erdélyi | first3 = R | last4 = Crockett | first4 = PJ | last5 = Keenan | first5 = FP | last6 = Christian | first6 = DJ | title = Alfvén Waves in the Lower Solar Atmosphere | journal = Science | volume = 323| issue = 5921 | pages = 1582–1585 | year = 2009 | pmid = 19299614|bibcode = 2009Sci...323.1582J |arxiv = 0903.3546 }}</ref> and also in the quiet Sun, in coronal holes and in active regions using observations with AIA on board the [[Solar Dynamics Observatory]].<ref>{{cite journal| author = McIntosh, S. W.; de Pontieu, B.; Carlsson, M.; Hansteen, V. H.; The Sdo/Aia Mission Team
| |
| | title = Ubiquitous Alfvenic Motions in Quiet Sun, Coronal Hole and Active Region Corona
| |
| | journal = American Geophysical Union, Fall Meeting 2010
| |
| | volume = abstract #SH14A-01
| |
| | year = 2010
| |
| }}</ref>
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| These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the [[Hinode]] spacecraft
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| .<ref>{{cite web|url=http://www.space.com/scienceastronomy/080122-st-sunshine-hinode.html
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| |title= Sun's Magnetic Secret Revealed|publisher=read on Jan 6 2011}}</ref>
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| | |
| Solar wind observations with the [[WIND (spacecraft)]] have recently shown evidence to support theories of Alfvén-cyclotron dissipation, leading to local ion heating.<ref>{{cite journal|last=Kasper|first=J.C.|coauthors=et al.|title=Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfven-Cyclotron Dissipation|journal=Phys. Rev. Lett.|year=2008|month=December|volume=101|pmid=19113766|issue=26|page=261103|doi=10.1103/PhysRevLett.101.261103|bibcode=2008PhRvL.101z1103K}}</ref>
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| | |
| ===Magnetic reconnection theory===
| |
| {{main|magnetic reconnection}}
| |
| | |
| [[File:Arcing Active Region.jpg|thumb|right|360px|Arcing active region by [[Solar Dynamics Observatory]]]]
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| The [[magnetic reconnection]] theory relies on the solar magnetic field to induce electric currents in the solar corona.<ref>{{cite book
| |
| | last = Priest
| |
| | first = Eric
| |
| | title = Solar Magneto-hydrodynamics
| |
| | publisher = D.Reidel Publishing Company, Dordrecht, Holland
| |
| | year = 1982
| |
| | isbn = 90-277-1833-4
| |
| }}</ref> 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 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 [[Poles of astronomical bodies#Magnetic poles|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.
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| | |
| [[Magnetic reconnection]] is hypothesized to be the mechanism behind solar flares, the largest explosions in our solar system. Furthermore, the surface of the Sun is covered with millions 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.
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| | |
| The idea that [[nanoflares]] 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 [[Solar and Heliospheric Observatory|SOHO]]/EIT can observe individual micro-flares as small brightenings in extreme ultraviolet light,<ref>{{cite journal
| |
| | doi = 10.1051/0004-6361:20020151
| |
| | last1 = Patsourakos
| |
| | first1 = S.
| |
| | last2 = Vial
| |
| | first2 = J.-C.
| |
| | title = Intermittent behavior in the transition region and the low corona of the quiet Sun
| |
| | journal = Astronomy and Astrophysics
| |
| | volume = 385
| |
| | pages = 1073–1077
| |
| | year = 2002
| |
| | bibcode=2002A&A...385.1073P
| |
| }}</ref> 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.
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| | |
| ===Spicules (type II)===
| |
| For decades, researchers believed [[spicule (solar physics)|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 Centre 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.<ref>{{cite web|url=http://www.rediff.com/news/slide-show/slide-show-1-mystery-of-suns-hot-outer-atmosphere-solved/20110107.htm |title=Mystery of Sun's hot outer atmosphere 'solved' – Rediff.com News |publisher=Rediff.com |date=2011-01-07 |accessdate=2012-05-21}}</ref> These jets insert heated plasma into the Sun's outer atmosphere.
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| 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 were used to test this hypothesis. The high spatial and temporal resolution of the newer instruments reveal this coronal mass supply.
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| 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.<ref>{{cite journal | doi = 10.1126/science.1197738 | last1 = De Pontieu | first1 = B | last2 = McIntosh | first2 = SW | last3 = Carlsson | first3 = M | last4 = Hansteen | first4 = VH | last5 = Tarbell | first5 = TD | last6 = Boerner | first6 = P | last7 = Martinez-Sykora | first7 = J | last8 = Schrijver | first8 = CJ | last9 = Title | first9 = AM | title = The Origins of Hot Plasma in the Solar Corona | journal = Science | volume = 331| issue = 6013 | pages = 55–58| year = 2011 | pmid = 21212351|bibcode = 2011Sci...331...55D }}</ref>
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| ==See also==
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| {{Portal|Solar System|Astronomy|Space|Star}}
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| {{Div col}}
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| * [[Advanced Composition Explorer]]
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| * [[Alfvén waves]]
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| * [[Chromosphere]]
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| * [[Coronal hole]]
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| * [[Coronal loop]]
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| * [[Coronal mass ejection]]
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| * [[Coronal radiative losses]]
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| * [[Coronal seismology]]
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| * [[Geocorona]]
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| * [[Heliosphere]]
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| * [[Helmet streamer]]
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| * [[Magnetic reconnection]]
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| * [[Magnetohydrodynamics#MHD waves|Magnetohydrodynamic waves]]
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| * [[Magnetohydrodynamics]]
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| * [[Nanoflares]]
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| * [[Photosphere]]
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| * [[Solar and Heliospheric Observatory]] (SOHO)
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| * [[Solar cycle]]
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| * [[Solar flare|Flares]]
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| * [[Solar prominence]]
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| * [[Solar wind]]
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| * [[STEREO]]
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| * [[Sun]]
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| * [[Transition region]]
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| * [[WIND (spacecraft)]]
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| * [[X-ray astronomy]]
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| {{Div col end}}
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| ==References==
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| {{reflist|colwidth=30em}}
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| ==Further reading==
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| * 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
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| * B. N. Dwivedi and A. K. Srivastava [http://www.ias.ac.in/currsci/10feb2010/295.pdf Coronal heating by Alfvén waves] CURRENT 296 SCIENCE, VOL. 98, NO. 3, 10 FEBRUARY 2010, pp. 295–296
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| == External links ==
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| {{commons category|Solar corona}}
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| * [http://solarscience.msfc.nasa.gov/corona.shtml NASA description of the solar corona]
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| * [http://www.innovations-report.com/html/reports/physics_astronomy/report-33153.html Coronal heating problem at Innovation Reports]
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| * [http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/corona.html NASA/GSFC description of the coronal heating problem]
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| * [http://solar-center.stanford.edu/FAQ/Qcorona.html FAQ about coronal heating]
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| * [http://sohowww.nascom.nasa.gov Solar and Heliospheric Observatory, including near-real-time images of the solar corona]
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| * [http://xrt.cfa.harvard.edu/ Coronal x-ray images from the Hinode XRT]
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| * [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 2006
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| *[http://alienworlds.southwales.ac.uk/sunStructure.html#/corona Animated explanation of the core of the Sun] (University of South Wales)
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| *[http://alienworlds.southwales.ac.uk/sunStructure.html#/coronatemp Animated explanation of the temperature of the Corona] (University of South Wales)
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| * [https://sites.google.com/site/ecanrio/Home/referencias/ETMV-Libro_new.pdf?attredirects=0yd=1 Space,time,matter and vacuum: The Solar Corona. A sign of Quantum Gravity?(Spanish)]
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| * [http://blogs.physicstoday.org/update/2009/03/alfven-waves-may-heat-the-suns.html Alfvén waves may heat the Sun's corona]
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| * [http://news.discovery.com/space/sun-heat-mystery-110106.html New Clue May Solve Solar Mystery]
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| * [http://www.sciencemag.org/content/331/6013/55.abstract?sid=18d15db8-169c-4ebf-a391-dc1ad55f0fa9 The Origins of Hot Plasma in the Solar Corona]
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| * [http://www.youtube.com/watch?v=o-v9kLLiK4s Solar Interface Region – Bart de Pontieu (SETI Talks) Video]
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| {{The Sun}}
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| {{Star}}
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| [[Category:Light sources]]
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| [[Category:Plasma physics]]
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| [[Category:Solar phenomena]]
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| [[Category:Space plasmas]]
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| [[Category:Sun]]
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| [[Category:Unsolved problems in astronomy]]
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