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On Doppler shift and its Center-to-Limb Variation in Active Regions in the Transition Region

A comprehensive understanding of the structure of Doppler motions in transition region including the center-to-limb variation and its relationship with the magnetic field structure is vital for the understanding of mass and energy transfer in the solar atmosphere. In this paper, we have performed such a study in an active region using the Si IV 1394~Å emission line recorded by the Interface Region Imaging Spectrograph (IRIS) and the line-of-sight photospheric magnetic field obtained by the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). The active region has two opposite polarity strong field regions separated by a weak field corridor, which widened as the active region evolved. On average the strong field regions (corridor) show(s) redshifts of 5{--}10 (3{--}9)~km~s$^{-1}$ (depending on the date of observation). There is, however, a narrow lane in the middle of the corridor with near-zero Doppler shifts at all disk positions, suggesting that any flows there are very slow. The Doppler velocity distributions in the corridor seem to have two components---a low velocity component centered near 0 km/s and a high velocity component centered near 10~km~s$^{-1}$. The high velocity component is similar to the velocity distributions in the strong field regions, which have just one component. Both exhibit a small center-to limb variation and seem to come from the same population of flows. To explain these results, we suggest that the emission from the lower transition region comes primarily from warm type II spicules, and we introduce the idea of a `chromospheric wall'---associated with classical cold spicules---to account for a diminished center-to-limb variation.