ON REDSHIFTS AND BLUESHIFTS IN THE TRANSITION REGION AND CORONA

Emission lines formed in the transition region (TR) of the Sun have long been known to show pervasive redshifts. Despite a variety of proposed explanations, these TR downflows (and the slight upflows in the low corona) remain poorly understood. We present results from comprehensive three-dimensional MHD models that span the upper convection zone up to the corona, 15 Mm above the photosphere. The TR and coronal heating in these models is caused by the stressing of the magnetic field by photospheric and convection "zone dynamics," but also in some models by the injection of emerging magnetic flux. We show that rapid, episodic heating, at low heights of the upper chromospheric plasma to coronal temperatures naturally produces downflows in TR lines, and slight upflows in low coronal lines, with similar amplitudes to those observed with EUV/UV spectrographs. We find that TR redshifts naturally arise in episodically heated models where the average volumetric heating scale height lies between that of the chromospheric pressure scale height of 200 km and the coronal scale height of 50 Mm.