Date of Award

Spring 1-1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Mark Rast

Second Advisor

Tobin Munsat

Third Advisor

Han Uitenbroek

Fourth Advisor

Sascha Kempf

Fifth Advisor

Nils Halverson

Abstract

Spectral solar irradiance (SSI), the radiant energy flux per wavelength of the Sun received at Earth, is an important driver of chemical reactions in the Earth's atmosphere. Accurate measurements of SSI are therefore necessary as an input for global climate models. SSI models and observations, however, disagree on the sign and magnitude of the variations. In this thesis, we analyze the contributions of the currently unresolved low magnetic flux `quiet sun' to SSI variations. Using measurements of the full solar disk, we demonstrate that the sign of the low magnetic flux contribution to SSI variations depends on the defined reference pixels used to normalize a time series of observations. As these reference pixels typically represent the quiet sun, we conclude that full-disk measurements can not provide insight into the low magnetic field contributions to SSI variations.

Using high-resolution radiative magnetohydrodynamic models, we examine the contribution of the quiet sun to SSI variations. We compare the radiative output of two quiet sun simulations with differing imposed magnetic field morphologies, and find that the differences in radiative output result from the formation of magnetic structures of various sizes. The magnetic structures modify the surrounding convective energy flux based on their size, resulting in size dependent radiative output. We find that this morphology dependent difference in spectral irradiance can contribute substantially to the inferred SSI trends.

Connecting these results with upcoming high-resolution observations requires highly accurate numerical solutions and photometrically precise observations. To this end, we present an assessment and solution to numerical diffusion common to certain radiative transfer solvers used to synthesize the emergent intensity from simulations. These results retain the spatial resolution of the synthesized emergent intensity, with applications beyond solar radiative transfer. Furthermore, we assess the photometric precision of post-facto image reconstruction techniques applicable for four-meter solar telescopes. We find sufficient precision to observe radiative output variations found in the magnetohydrodynamic models. The improvement to the numerical radiative transfer method and increased certainty in the photometric precision of upcoming high-resolution observations provide the necessary tools to further enhance our understanding of solar radiative variability at the smallest spatial scales.

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