Editing Stellar corona (section)

===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>