Editing Sun (section)

=== Coronal heating ===
{{Main|Stellar corona}}
{{unsolved|astronomy|Why is the Sun's corona so much hotter than the Sun's surface?}}

The temperature of the photosphere is approximately 6,000&nbsp;K, whereas the temperature of the corona reaches {{val|1000000|-|2000000|u=K|fmt=commas}}.<ref name="Erdelyi2007" /> The high temperature of the corona shows that it is heated by something other than direct [[heat conduction]] from the photosphere.<ref name="Russell2001" />

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating.<ref name="Erdelyi2007" /> The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.<ref name="Erdelyi2007" /> These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat.<ref name="Alfven">{{Cite journal |last=Alfvén |first=H. |date=1947 |title=Magneto-hydrodynamic waves, and the heating of the solar corona |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=107 |issue=2 |pages=211–219 |bibcode=1947MNRAS.107..211A |doi=10.1093/mnras/107.2.211 |doi-access=free}}</ref> The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through [[magnetic reconnection]] in the form of large solar flares and myriad similar but smaller events—[[nanoflares]].<ref name="Parker2">{{Cite journal |last=Parker |first=E. N. |date=1988 |title=Nanoflares and the solar X-ray corona |journal=[[The Astrophysical Journal]] |volume=330 |issue=1 |page=474 |bibcode=1988ApJ...330..474P |doi=10.1086/166485}}</ref>

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona.<ref name="Sturrock">{{Cite journal |last1=Sturrock |first1=P. A. |last2=Uchida |first2=Y. |year=1981 |title=Coronal heating by stochastic magnetic pumping |journal=[[The Astrophysical Journal]] |volume=246 |issue=1 |page=331 |bibcode=1981ApJ...246..331S |doi=10.1086/158926 |hdl-access=free |hdl=2060/19800019786}}</ref> In addition, Alfvén waves do not easily dissipate in the corona. The current research focus has therefore shifted toward flare heating mechanisms.<ref name="Erdelyi2007" />