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 that of the surface. Several theories have been proposed to explain this phenomenon but it is still challenging to determine which of these is correct. The problem first emerged when Bengt Edlen and Walter Grotrian identified Fe IX and Ca XIV lines in the solar spectrum. This led to the discovery that the emission lines seen during solar eclipses are not caused by an unknown element called "coronium" but known elements at very high stages of ionization. The comparison of the coronal and the photospheric temperatures of , leads to the question of how the 200 times hotter coronal temperature can be maintained. 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.