Editing Stellar corona (section)

==Coronal heating problem==
{{unsolved|physics|Why is the Sun's corona so much hotter than the Sun's surface?}}
[[File:Van Gogh Sun.ogv|thumb|A new visualisation technique can provide clues to the coronal heating problem.]]
The coronal heating problem in [[solar physics]] relates to the question of why the temperature of the Sun's corona is millions of kelvins greater than the thousands of kelvins of the surface. Several theories have been proposed to explain this phenomenon, but it is still challenging to determine which is correct.<ref name=":0">{{Cite web|url=http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=2004ESASP.575....2K&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES|title=2004ESASP.575....2K Page 2|website=adsbit.harvard.edu|access-date=2019-02-28}}</ref> The problem first emerged after the identification of unknown spectral lines in the solar spectrum with highly ionized iron and calcium atoms.<ref name=":1">{{Cite book|title=Physics of the Solar Corona: An Introduction with Problems and Solutions|url=https://link.springer.com/chapter/10.1007/3-540-30766-4_9|url-access=limited|last=Aschwanden|first=Markus|publisher=Springer Science & Business Media|year=2006|isbn=978-3-540-30765-5|location=Berlin|page=355|doi=10.1007/3-540-30766-4_9 }}</ref><ref name=":0" /> The comparison of the coronal and the photospheric temperatures of {{gaps|6|000|K}}, leads to the question of how the 200-times-hotter coronal temperature can be maintained.<ref name=":1" /> The problem is primarily concerned with how the energy is transported up into the corona and then converted into heat within a few solar radii.<ref>{{Cite book|title=Turbulence and Magnetic Fields in Astrophysics|url=https://archive.org/details/turbulencemagnet00falg_199|url-access=limited|last1=Falgarone|first1=Edith|last2=Passot|first2=Thierry|publisher=Springer Science & Business Media|year=2003|isbn=978-3-540-00274-1|location=Berlin|pages=[https://archive.org/details/turbulencemagnet00falg_199/page/n33 28]}}</ref>

The high temperatures require energy to be carried from the solar interior to the corona by non-thermal processes, because the [[second law of thermodynamics]] prevents heat from flowing directly from the solar photosphere (surface), which is at about {{gaps|5|800|K}}, to the much hotter corona at about 1 to 3 [[SI prefix|MK]] (parts of the corona can even reach {{gaps|10|MK}}).

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

The amount of power required to heat the solar corona can easily be calculated as the difference between coronal radiative losses and heating by thermal conduction toward the chromosphere through the transition region. It is about 1 kilowatt for every square meter of surface area on the Sun's chromosphere, or 1/{{gaps|40|000}} of the amount of light energy that escapes the Sun.

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 |editor = J.C. Vial |editor2 = K. Bocchialini |editor3 = P. Boumier| publisher = Springer | pages = 77–106| year = 1997| isbn= 978-3-540-64307-4}}</ref> but two theories have remained as the most likely candidates: wave heating and [[magnetic reconnection]] (or [[nanoflares]]).<ref>{{cite book |last1=Malara |first1= F. |last2=Velli |first2= 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 |editor = Pål Brekke |editor2 = Bernhard Fleck |editor3 = Joseph B. Gurman| publisher = Astronomical Society of the Pacific | pages = 456–466| year = 2001| isbn= 978-1-58381-069-9}}</ref> Through most of the past 50 years, neither theory has been able to account for the extreme coronal temperatures.

In 2012, high resolution (<0.2″) [[soft X-ray]] imaging with the [[High Resolution Coronal Imager]] aboard a [[sounding rocket]] revealed tightly wound braids in the corona. It is hypothesized that the reconnection and unravelling of braids can act as primary sources of heating of the active solar corona to temperatures of up to 4 million kelvin. The main heat source in the quiescent corona (about 1.5 million kelvin) is assumed to originate from MHD waves.<ref name=Cirtain2013>{{Cite journal | last1 = Cirtain | first1 = J. W. | last2 = Golub | first2 = L. | last3 = Winebarger | first3 = A. R. | last4 = De Pontieu | first4 = B. | last5 = Kobayashi | first5 = K. | last6 = Moore | first6 = R. L. | last7 = Walsh | first7 = R. W. | last8 = Korreck | first8 = K. E. | last9 = Weber | first9 = M. | doi = 10.1038/nature11772 | last10 = McCauley | first10 = P. | last11 = Title | first11 = A. | last12 = Kuzin | first12 = S. | last13 = Deforest | first13 = C. E. | title = Energy release in the solar corona from spatially resolved magnetic braids | journal = Nature | volume = 493 | issue = 7433 | pages = 501–503 | year = 2013 | pmid =  23344359|bibcode = 2013Natur.493..501C | s2cid = 205232074 }}</ref>

[[NASA]]'s [[Parker Solar Probe]] is intended to approach the Sun to a distance of approximately 9.5 solar radii to investigate coronal heating and the origin of the solar wind. It was successfully launched on August 12, 2018<ref>{{cite web |url=http://parkersolarprobe.jhuapl.edu/The-Mission/index.php#Journey-to-the-Sun |title= Parker Solar Probe: The Mission|website=parkersolarprobe.jhuapl.edu |archive-url=https://web.archive.org/web/20170822222954/http://parkersolarprobe.jhuapl.edu/The-Mission/index.php |archive-date=2017-08-22}}</ref> and by late 2022 had completed the first 13 of more than 20 planned close approaches to the Sun.<ref>{{Cite web|url=https://blogs.nasa.gov/parkersolarprobe/2019/09/03/parker-solar-probe-completes-third-close-approach-of-the-sun/|title=Parker Solar Probe Completes Third Close Approach of the Sun|website=blogs.nasa.gov|date=3 September 2019 |access-date=2019-12-06}}</ref>

{| class="wikitable" style="margin: 1em auto 1em auto"
|+Competing theories of 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
* Flares-nanoflares
* ''Uniform heating rates''
|
* Photospheric foot point ''shuffling''
* MHD wave propagation
* High Alfvén wave flux
* ''Non-uniform heating rates''
|}

===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 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
  | issue = 2
  | pages = 211–219
  | year = 1947
  |bibcode = 1947MNRAS.107..211A
  | doi=10.1093/mnras/107.2.211| doi-access = free
  }}</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]] [[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.

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 through the chromosphere and transition region and finally into the corona where it dissipates it as heat.

Another problem with wave heating has been the complete absence, until the late 1990s, of any direct evidence of waves propagating through the solar corona. The first direct observation of waves propagating into and through the solar corona was made in 1997 with the [[Solar and Heliospheric Observatory]] space-borne solar observatory, the first platform capable of observing the Sun in the [[extreme ultraviolet]] (EUV) for long periods of time with stable [[Photometry (astronomy)|photometry]]. Those were magneto-acoustic waves with a frequency of about 1 [[hertz|millihertz]] (mHz, corresponding to a {{gaps|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 {{gaps|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 {{gaps|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–{{gaps|10|Hz}} range.

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 |url-status=live |archive-url=https://web.archive.org/web/20110723053337/http://www.science20.com/news_releases/alfven_waves_our_sun_doing_magnetic_twist |archive-date=2011-07-23 }}</ref><ref>{{cite journal | doi = 10.1126/science.1168680 | last1 = Jess | first1 = D. B. | last2 = Mathioudakis | first2 = M. | last3 = Erdélyi | first3 = R. | last4 = Crockett | first4 = P. J. | last5 = Keenan | first5 = F. P. | last6 = Christian | first6 = D. J. | 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 | hdl = 10211.3/172550 | s2cid = 14522616 }}</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|last1=McIntosh |first1= S. W. |last2=de Pontieu |first2= B. |last3=Carlsson |first3= M. |last4=Hansteen |first4= V. H. |author5=The Sdo |author6=Aia Mission Team | title = Ubiquitous Alfvenic Motions in Quiet Sun, Coronal Hole and Active Region Corona
  | journal = American Geophysical Union
  | volume = abstract #SH14A-01
  | date = Fall 2010
  |pages=SH14A–01 |bibcode=2010AGUFMSH14A..01M }}</ref>
These Alfvénic oscillations have significant power, and seem to be connected to the chromospheric Alfvénic oscillations previously reported with the [[Hinode (satellite)|Hinode]] spacecraft.<ref>{{cite web
 |url         = http://www.space.com/scienceastronomy/080122-st-sunshine-hinode.html
 |title       = Sun's Magnetic Secret Revealed
 |website = [[Space.com]]
 |date = 22 January 2008
 |access-date = January 6, 2011<!--assume this is what's meant by "read on..."-->
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20101224061437/http://www.space.com/scienceastronomy/080122-st-sunshine-hinode.html
 |archive-date = 2010-12-24
}}</ref>

Solar wind observations with the [[Wind (spacecraft)|''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.|title=Hot Solar-Wind Helium: Direct Evidence for Local Heating by Alfven-Cyclotron Dissipation|journal=Physical Review Letters |date=December 2008|volume=101|pmid=19113766|issue=26|page=261103|doi=10.1103/PhysRevLett.101.261103|bibcode=2008PhRvL.101z1103K|display-authors=etal}}</ref>

===Magnetic reconnection theory===
{{main|Magnetic reconnection}}
[[File:Arcing Active Region.jpg|thumb|Arcing active region by [[Solar Dynamics Observatory]]]]

The magnetic reconnection theory relies on the solar magnetic field to induce electric currents in the solar corona.<ref>{{cite book
  | last = Priest
  | first = Eric
  | title = Solar Magneto-hydrodynamics
  | publisher = D.Reidel |location=Dordrecht, Holland
  | year = 1982
  | isbn = 978-90-277-1833-4
}}</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 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.

Magnetic reconnection is hypothesized to be the mechanism behind solar flares, the largest explosions in the Solar System. Furthermore, the surface of the Sun is covered with millions of small magnetized regions 50–{{gaps|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.

The idea that nanoflares might heat the corona was proposed by Eugene Parker in the 1980s but is still controversial. In particular, [[ultraviolet]] telescopes such as [[TRACE]] and [[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
  | issue = 3
  | pages = 1073–1077
  | year = 2002
| bibcode=2002A&A...385.1073P
| doi-access = free
  }}</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 does not appear well in the TRACE data. Variations on the micro-flare hypothesis use other mechanisms to stress the magnetic field or to release the energy, and are a subject of active research in 2005.

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

As per studies performed in 2010 at the ''National Center for Atmospheric Research'' in [[Colorado]], in collaboration with the ''Lockheed Martin's Solar and Astrophysics Laboratory'' (LMSAL) and the ''Institute of Theoretical Astrophysics'' of the [[University of Oslo]], a new class of spicules (TYPE II) discovered in 2007, which travel faster (up to 100&nbsp;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' |work=Rediff |date=2011-01-07 |access-date=2012-05-21 |url-status=live |archive-url=https://web.archive.org/web/20120415111049/http://www.rediff.com/news/slide-show/slide-show-1-mystery-of-suns-hot-outer-atmosphere-solved/20110107.htm |archive-date=2012-04-15 }}</ref> These jets insert heated plasma into the Sun's outer atmosphere.

The Atmospheric Imaging Assembly on NASA's Solar Dynamics Observatory and NASA's Focal Plane Package for the Solar Optical Telescope on the Japanese Hinode satellite were used to test this hypothesis. The high spatial and temporal resolutions of the newer instruments reveal this coronal mass supply.

According to analysis in 2011 by de Pontieu and colleagues, 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 = S. W. | last3 = Carlsson | first3 = M. | last4 = Hansteen | first4 = V. H. | last5 = Tarbell | first5 = T.D. | last6 = Boerner | first6 = P. | last7 = Martinez-Sykora | first7 = J. | last8 = Schrijver | first8 = C. J. | last9 = Title | first9 = A. M. | 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 | s2cid = 42068767 }}</ref>