Supernova (SN) explosions, through the metals they release, play a pivotal role in the chemical evolution of the Universe and the origin of life. Nebular phase spectroscopy constrains such metal yields, for example through forbidden line emission associated with OI, CaII, FeII, or FeIII. Fluid instabilities during the explosion produce a complex 3D ejecta structure, with considerable macroscopic, but no microscopic, mixing of elements. This structure sets a formidable challenge for detailed nonlocal thermodynamic equilibrium radiative transfer modeling, which is generally limited to 1D in grid-based codes. Here, we present a novel and simple method that allows for macroscopic mixing without any microscopic mixing, thereby capturing the essence of mixing in SN explosions. With this new technique, the macroscopically mixed ejecta is built by shuffling in mass space, or equivalently in velocity space, the shells from the unmixed coasting ejecta. The method requires no change to the radiative transfer, but necessitates high spatial resolution to resolve the rapid variation in composition with depth inherent to this shuffled-shell structure. We show results for a few radiative-transfer simulations for a Type II SN explosion from a 15Msun progenitor star. Our simulations capture the strong variations in temperature or ionization between the various shells that are rich in H, He, O, or Si. Because of nonlocal energy deposition, gamma rays permeate through an extended region of the ejecta, making the details of the shell arrangement unimportant. The greater physical consistency of the method delivers spectral properties at nebular times that are more reliable, in particular in terms of individual emission line strengths, which may serve to constrain the SN yields and, for core collapse SNe, the progenitor mass. The method works for all SN types.
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