| Abstract : | Between the chromosphere and corona, there lies a transition region of about a couple of hundred kilometers. The physical parameters such as density and temperatures show abrupt change. Although there have been numerous studies on transition region dynamics using both imaging and spectroscopy, including theoretical modeling, the transition region heating and dynamics are far from complete comprehension. One of the ideas proposed to explain the properties of the transition region is that field-aligned downflows from the corona (enthalpy) may explain the radiation and the Doppler shifts observed in the transition region. While this may be true for active region moss (Tripathi et al. 2010), it does not explain the Doppler shifts that we measure in the lower transition region using Si IV lines using observations taken from the Interface Region Imaging Spectrometer (IRIS). Moreover, for field-aligned flows to be the cause, we should observe centre to limb variation (CLV) of measured Doppler shifts. In this work, we study centre to limb variation of Doppler shifts in Si IV line using IRIS observations. Our results show that although there are hints of CLV in the data, there are sizeable Doppler shifts near the limb. These results do not support the idea of the heating of the transition region with field-aligned downflows from the corona. To explain these results, we propose that the Doppler shifts observed using the Si IV line are primarily due to Type-II spicules. We introduce the idea of a chromospheric wall (associated with classical Type-I spicules) that diminishes the CLV of the Doppler shifts. |
