A NEW INTERPRETATION OF THE REDSHIFT OBSERVED IN OPTICALLY THIN TRANSITION REGION LINES

We propose that the pervasive redshift observed in transition region spectral lines is caused by downward propagating acoustic waves. The waves are assumed to be generated in the corona as a result of nanoflares or some other form of episodic heating. The dynamic response of a coronal loop to energy released as heat near the loop apex is studied by solving the hydrodynamic equations numerically, consistently including the effects of nonequilibrium ionization on the radiative losses and on the internal energy. We find that the radiative loss curve may change by a factor of 2 during the loop evolution as a result of flows and waves. Spectral line intensities and line profiles are computed in accordance with the numerical model for the resonance lines of C IV, 0 IV, 0 VI, and Ne VIII. The three lower temperature lines display an average redshift on the order of 1 km s-1 while the high-temperature line (of Ne VIII) displays little average shift. The largest line shifts occur as the loop undergoes transient heating. A simple analytical analysis is performed to isolate the physical effects relevant to the line formation process. We find that the amplitude of the line shift depends on the characteristic time scale for ionization of the radiating ion as well as the global loop parameters; the time scales for loop cooling, the maximum temperature, and the base pressure. The importance of nonequilibrium ionization in the line-forming process will also have impact on the interpretation of the emission measure derived from nonsteady plasmas.