  We test the adequacy of ultraviolet (UV) spectra for characterizing the outer
structure of Type Ia supernova (SN) ejecta. For this purpose, we perform
spectroscopic analysis for ASASSN-14lp, a normal SN Ia showing low continuum in
the mid-UV regime. To explain the strong UV suppression, two possible origins
have been investigated by mapping the chemical profiles over a significant part
of their ejecta. We fit the spectral time series with mid-UV coverage obtained
before and around maximum light by HST, supplemented with ground-based optical
observations for the earliest epochs. The synthetic spectra are calculated with
the one dimensional MC radiative-transfer code TARDIS from self-consistent
ejecta models. Among several physical parameters, we constrain the abundance
profiles of nine chemical elements. We find that a distribution of $^{56}$Ni
(and other iron-group elements) that extends toward the highest velocities
reproduces the observed UV flux well. The presence of radioactive material in
the outer layers of the ejecta, if confirmed, implies strong constraints on the
possible explosion scenarios. We investigate the impact of the inferred
$^{56}$Ni distribution on the early light curves with the radiative transfer
code TURTLS, and confront the results with the observed light curves of
ASASSN-14lp. The inferred abundances are not in conflict with the observed
photometry. We also test whether the UV suppression can be reproduced if the
radiation at the photosphere is significantly lower in the UV regime than the
pure Planck function. In this case, solar metallicity might be sufficient
enough at the highest velocities to reproduce the UV suppression.
