in model B extend over the whole range of the observed radial velocities from $-15\,000$ kms$^{-1}$ to $15\,000$ kms$^{-1}$ (Fig. \[fig:hmd117\]c). Unfortunately, the calculated absorption component runs above the observed one in a radial velocity range between $-15\,000$ kms$^{-1}$ and $-12\,500$ kms$^{-1}$. On day 52.14 the emission component of the H$\alpha$ line computed with the time-dependent approach with model B fits the observed one fairly well, while the absorption component is still stronger than the observed one (Fig. \[fig:hmd117\]d). Thus, we may state that the above hydrodynamic and atmosphere models are in a good agreement with the photometric and spectroscopic observations of SN 1999em.

Now let us pay attention to the time-dependent effects in hydrogen lines of SN 1999em, a normal SN IIP. The time-dependent approach with model B reproduces the strength of the H$\alpha$ line at day 26.24 and day 52.14 fairly well (Figs. \[fig:hmd117\]c and \[fig:hmd117\]d). In contrast, a steady-state model B demonstrates an extremely weak H$\alpha$ line on days 26.24 and 52.14. This reflects the fact that the steady-state ionization is significantly lower than in the time-dependent model, while the electron temperature is too low for the collisional excitation of hydrogen. The radioactive $^{56}$Ni is mixed too 