Population diagram for the SO₂ lines without signatures of nonthermal emission in the innermost and intermediate regions defined as annular areas between 30 and 60 mas (~1 and 2 R_cont) and between 60 and 100 mas (~2 and 3.3 R_cont), respectively. The red and blue dots represent the measurements in the innermost and intermediate regions, respectively. The solid red and blue lines represent the fits in the respective regions. The excitation temperatures (T_ex) and column densities (N_SO2) derived in two regions are also shown.