Implications of the correlation between Doppler shifts and line widths in Si IV spectral lines from the active region transition region

Abstract:

Emission lines from the 100,000 K solar transition region are generally red-shifted and broadened beyond what can be attributed to their temperature. The reason for either remains unclear at this time. We use observations of the Si IV1402.77A line in quiescent active regions, made by IRIS, to show that the Doppler shift and non-thermal broadening are strongly correlated. The correlation is approximately linear for red-shifts between 5 and 20 km/s. Subtracting this linear trend from the measured non-thermal broadening leaves a quantity we call the Doppler-compensated broadening, which is uncorrelated with the Doppler velocity. It turns out to be very nearly statistically independent of it, suggesting that different, physically independent mechanisms are responsible for the persistent red-shift and for a part of the ubiquitous non-thermal broadening in the transition region. The fact that the red-shift contributes proportionately to non-thermal broadening offers an important clue to the nature of the mechanism responsible for it. The other mechanism independently produces broadening which is smaller than the full broadening (16 vs. 21 km/s) and far less variable within one region or between different regions (varying by roughly 25%). Their different properties suggest that Doppler-compensated broadening is generated by a mechanism operating below the transition region, perhaps in the photosphere, while Doppler shifts are generated above in the corona. Of several candidates for the latter mechanism we find two to be most consistent with a broadening linearly proportional to red-shift. These are downward propagating acoustic waves or downflows in a coronal loop quasi-statically cooling following impulsive heating.