  The spectral properties of type Ia supernovae in the ultraviolet (UV) are
investigated using the early-time spectra of SN 2001ep and SN 2001eh obtained
using the Hubble Space Telescope (HST). A series of spectral models is computed
with a Monte Carlo spectral synthesis code, and the dependence of the UV flux
on the elemental abundances and the density gradient in the outer layers of the
ejecta is tested. A large fraction of the UV flux is formed by reverse
fluorescence scattering of photons from red to blue wavelengths. This process,
combined with ionization shifts due to enhanced line blocking, can lead to a
stronger UV flux as the iron-group abundance in the outer layers is increased,
contrary to previous claims.
