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Spectra of Type Ia Supernovae (SN Ia) show both continuum-like andabsorption-like line features. The pseudo equivalent width (pEW) and Dopplershift of absorption-line-like features, such as Si II 6150A, Ca II HK 3750A,and the Ca II IR-triplet 8150A quantities that are often associated with theoptical depth and the velocity of a shell of interacting material, areimportant tools for interpreting and classifying the SN Ia spectra. In thispaper, we examine the nature of spectral features in SN Ia spectra using W7model spectra and a technique that we call "knock-out" spectra. We show thatthe P-Cyg profiles of Si II 6150A and many other features are largely emissiondominated, rather than absorption dominated, and that the concepts of"absorption line" and "continuum" are therefore not adequate to describe thenature of spectral features in SNe Ia. Apparent absorption features, like Si II6150A, are frequently just coincidental troughs between two (or more)uncorrelated emission features. In this situation, the pEW measured betweenthese emission peaks is little related to the true strength of the presumedabsorption feature. Furthermore, using the same knock-out technique, wedemonstrate how spectral features overlap each other at different times afterthe explosion. This overlap distorts individual line profiles and affectsmeasured absorption-line velocities. With synthetic SN Ia ejectastratifications that are tuned to match specific observations, the methodpresented in this paper can in principle be used to quantify the effects ofspecific line opacities on measured line velocity, line strength, and lineblending, and improve the interpretation and informative value of observed SNIa spectra.

The Nature of the Si II 6150A, Ca II HK, Ca II IR-triplet, and other Spectral Features in Supernova Type Ia Spectra