The Solar Chromosphere-Corona Transition Region

In the solar atmosphere there is a thin region which separates the hot, diffuse corona from the cool, dense chromosphere known as the transition region (TR). This region is defined to extend in temperature from 10('4) < T(, )< 10('6)(DEGREES)K. Observationally, we find it convenient to separate the transition region into an upper (T(, )> 10('5)(DEGREES)K) and lower (T(, )< 10('5)(DEGREES)K) region based upon the existence of a minimum in the emission measure (EM) near 10('5)(DEGREES)K. It is found that although theory can predict fairly well the observed form of the EM above 10('5)(DEGREES)K, the theoretical prediction of the emission in the lower TR can be several orders of magnitude below what is observed. Solving the one-dimensional, time-independent equations of momentum and energy conservation we find the existence of a class of solutions in which shallow temperature gradients exist with signifi- cant emission at low temperatures. These solutions are found for a damping length form of the heating function when the momentum deposition associated with the mechanical heating is neglected. The slope of the EM curve produced by such a solution is much too steep compared with observations. We have also investigated the effect of wave pressure when the mechanical heating is assumed to be the result of an upward traveling wave flux. We find that when wave pres- sure effects are incorporated there are no physically valid solutions for the exponential form of the heating function. To discover the extent to which this result is dependent on the assumed form of the heating function, we subsequently use the observed EM to specify the temperature gradients and thereby solve for the form of the wave energy flux. We find that if the wave mode travels at the sound speed, then there is no physically valid form of the wave flux. However, if the wave mode travels at the Alfven speed, then in magnetic regions of moderate strength ((DBLTURN)10 gauss) it is possible to find a functional form of the mechanical heating which is consistent with the observations if a filling factor argument is invoked to reduce the results down to observationally acceptable levels.