What Produces the Transition Region of the Quiet Sun?

The existence of the transition region, a mysterious sharp change, by a factor of 20 times, in the temperature and density from the chromosphere to the corona, may be among the most difficult outstanding problems in solar physics. The conventional understanding of the formation of the transition region by many is a runaway temperature increase when heating can raise the critical temperature that produces the maximum radiation function, so that further temperature rise can be easily achieved by a much smaller radiative loss. However, this critical temperature of maximum radiation function occurs around 2´105 K which is near the upper end of the transition region and cannot explain why the transition region starts in the first place, well below 104 K. An alternative view is that it is formed by the heat flux from the corona flowing downward. When the available heat flux is used up, the temperature below remains low. The topside temperature profile produced by the heat conduction indeed looks like the observed one. However, the heating due to the conduction below the transition region is too small to account for heating the chromosphere. More problematic is that the heat conduction process tends to smooth the temperature profile, not to steepen it. Conduction cannot cause or sustain the sharp profile. In recent years, studies have been focused less on the physical understanding of the processes that produce and maintain the structure of the transition region. In these studies, the transition region is considered a discontinuity that is difficult to resolve numerically because the numerical codes seem to resist delivering a stable solution at the discontinuity. The arguments often go like that since the governing equations of a code include all possible physical processes, the structure can be resolved if the code can be stabilized. Therefore, the research has focused more on numerical techniques or with large artificial dissipations. We investigate the formation of the transition region during quiet time based on two-fluid treatment of partially ionized plasma with ionization, recombination, Alfvenic collisional heating, and radiation. From the derived physical understanding, we explain the location, thickness, and profiles of the transition region.