The mechanism that heats the corona to such high temperatures is unclear, however, so this question is the focus of much of modern solar research. The coronal gas close to the Sun is visible with the naked eye during eclipses because it scatters photospheric light from electrons in the plasma. The hot coronal plasma also emits its own ultraviolet light and X-ray light when rapidly moving electrons collide with ions of the heavier elements. For instance, the lines of 9-times-ionized magnesium and 11-times-ionized silicon are prominent in the ultraviolet spectrum. The heating of the corona is not a matter of simple heat flow from the cooler photosphere, because such a heat flow would violate the second law of thermodynamics. Most likely, acoustic or other forms of waves generated by gas motions at the photosphere may carry energy into the coronal medium and dissipate it into heat, balancing the corona's losses. Another alternative is dissipation of electric currents in the highly conducting coronal plasma, in much the same way that joule heating raises the temperature of a common resistor. Some scientists suggest that numerous small flares - the term "small" is relative; the flares are in the size range of the Earth - feed magnetic energy into the corona, heating it in this way. Another explanation derives from images obtained by NASA's Trace (Transition Region and Coronal Explorer) satellite in 2000, suggesting that the heating comes from pillars of fiery gas at the bottoms of looping arcs of magnetic fields that stretch hundreds of kilometers above the solar surface.
The Structure of the Sun: The Corona
During a total solar eclipse, or with a coronagraph, the Sun's atmosphere can be seen extending to several solar radii beyond the photospheric limb as a faint glow, about one million times less bright than the disk. The reason that the corona extends to such heights is that it exists at temperatures of several million K. Because a hot gas is compressed relatively less by the weight of overlying layers than a cool one, the corona remains visible much farther above the limb than might be expected.