An Assessment of and Solution to the Intensity Diffusion Error Intrinsic to Short-characteristic Radiative Transfer Methods

Radiative transfer coupled with highly realistic simulations of the solar atmosphere is routinely used to infer the physical properties underlying solar observations. Due to its computational efficiency, the method of short-characteristics is often employed, despite it introducing numerical diffusion as an interpolation artifact. In this paper, we quantify the effect of the numerical diffusion on the spatial resolution of synthesized emergent intensity images, and derive a closed form analytical model of the diffusive error made as a function of viewing angle when using linear interpolation. We demonstrate that the consequent image degradation adversely affects the comparison between simulated data and observations away from disk center, unless the simulations are computed at much higher intrinsic resolutions than the observations. We also show that the diffusive error is readily avoided by interpolating the simulation solution on a viewing angle aligned grid prior to computing the radiative transfer. Doing this will be critical for comparisons with observations using the upcoming large aperture telescopes—the Daniel K. Inouye Solar Telescope and the European Solar Telescope.