Editing X-ray astronomy (section)

====Coronal heating problem====
In the area of solar X-ray astronomy, there is the [[Solar corona#Coronal heating problem|coronal heating problem]]. The [[photosphere]] of the Sun has an effective temperature of 5,570 K<ref name=Massey>{{Cite journal|author=Massey P|author2= Silva DR|author3=Levesque EM|author4=Plez B|author5=Olsen KAG|author6=Clayton GC|author7=Meynet G|author8=Maeder A |title=Red Supergiants in the Andromeda Galaxy (M31) |journal=Astrophys J |volume=703|date=2009|issue=1 |page=420|doi=10.1088/0004-637X/703/1/420 |bibcode=2009ApJ...703..420M|arxiv = 0907.3767 |s2cid= 119293010}}</ref> yet its corona has an average temperature of 1–2 × 10<sup>6</sup> K.<ref name=Erdelyi>{{Cite journal|author=Erdèlyi R|author2=Ballai, I |title=Heating of the solar and stellar coronae: a review |date=2007|journal=Astron Nachr|volume=328|issue=8|page=726|doi=10.1002/asna.200710803|bibcode=2007AN....328..726E|doi-access=free}}</ref> However, the hottest regions are 8–20 × 10<sup>6</sup> K.<ref name=Erdelyi/> The high temperature of the corona shows that it is heated by something other than direct [[heat conduction]] from the photosphere.<ref name=Russell2001>{{Cite book|author=Russell CT |title=Space Weather (Geophysical Monograph) |date=2001 |publisher=[[American Geophysical Union]] |chapter=Solar wind and interplanetary magnetic filed: A tutorial |editor=Song, Paul |editor2=Singer, Howard J. |editor3-link=George Siscoe |editor3=Siscoe, George L. |isbn=978-0-87590-984-4|pages=73–88|url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-url=https://web.archive.org/web/20030828214153/http://www-ssc.igpp.ucla.edu/personnel/russell/papers/SolWindTutorial.pdf |archive-date=2003-08-28 |url-status=live}}</ref>

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=Erdelyi/> The first is [[wave]] heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.<ref name=Erdelyi/> These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat.<ref name=Alfven>{{Cite journal|author=Alfvén H |title=Magneto-hydrodynamic waves, and the heating of the solar corona |bibcode=1947MNRAS.107..211A|journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=107|issue=2 |page=211 |date=1947 |doi=10.1093/mnras/107.2.211|doi-access=free }}</ref> The other is [[magnetic field|magnetic]] heating, in which magnetic energy is continuously built up by photospheric motion and released through [[magnetic reconnection]] in the form of large [[solar flare]]s and myriad similar but smaller events—[[nanoflares]].<ref name=Parker2>{{Cite journal|author=Parker EN |title=Nanoflares and the solar X-ray corona|journal=Astrophys J|volume=330 |page=474 |date=1988 |doi=10.1086/166485 |bibcode=1988ApJ...330..474P}}</ref>

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except [[Alfvén wave]]s have been found to dissipate or refract before reaching the corona.<ref name=Sturrock>{{Cite journal|author=Sturrock PA|author2=Uchida Y |title=Coronal heating by stochastic magnetic pumping|journal=Astrophys J|volume=246|page=331 |date=1981 |doi=10.1086/158926 |bibcode=1981ApJ...246..331S|hdl=2060/19800019786|hdl-access=free}}</ref> In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.<ref name=Erdelyi/>