  Following our previous study of Artificial Intelligence Assisted Inversion
(AIAI) of supernova analyses (Chen et al. 2020), we train a set of deep neural
networks based on the one-dimensional radiative transfer code TARDIS
(Kerzendorf & Sim 2014) to simulate the optical spectra of Type Ia supernovae
(SNe Ia) between 10 and 40 days after the explosion. The neural networks are
applied to derive the mass of 56Ni in velocity ranges well above the
photosphere for a sample of 153 well-observed SNe Ia. Many SNe have multi-epoch
observations for which the decay of the radioactive 56Ni can be tested
quantitatively. The 56Ni mass derived from AIAI using the observed spectra as
input for the sample is found to agree with the theoretical 56Ni decay rate.
The AIAI reveals a spectral signature near 3890 \AA which can be identified as
being produced by multiple Ni II lines between 3950 and 4100 \AA. The mass
deduced from AIAI is correlated to the light-curve shapes of SNe Ia, with the
SNe Ia with broader light curves showing larger 56Ni mass in the envelope. AIAI
enables spectral data of SNe to be quantitatively analyzed under theoretical
frameworks based on well-defined physical assumptions.
