Si IV formation diagram in the pixel marked by a white circle in Fig. 4. The panels show (a) Line profiles of Si IV 1394 Å (solid black) and Si IV 1403 Å (dashed black). (b) Intensity ratio of Si IV 1394 Å and Si IV 1403 Å lines. (c) Emissivity j of the 1394 Å line (Eq. (4), j^0.6) and vertical velocity v_z (blue). The vertical dashed line indicates where Δv = 0 km s⁻¹. The orange line marks the height at which τ = 1 is reached. (d) Gas temperature T (red) and electron density n_e (black) of Si IV. The dashed vertical lines represents the LTE Si IV formation temperature interval and the dotted vertical lines represent the coronal equilibrium formation temperature interval. Positive Δν indicates an upwards velocity.