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Exploring the Dynamics of Near-Surface Solar Convection with Helioseismology

TitleExploring the Dynamics of Near-Surface Solar Convection with Helioseismology
Publication TypeThesis
Year of Publication2015
AuthorsGreer, BJ
Academic DepartmentAstrophysical and Planetary Sciences
Number of Pages202
Date Published12-2015
UniversityUniversity of Colorado
CityBoulder, CO

I present a new implementation of local helioseismology along with observations of near-surface solar convection made with this method. The upper 5% of the solar radius (35 Mm) is known as the Near-Surface Shear Layer (NSSL) and is characterized by strong rotational shear. While the physical origin of this layer remains unknown, current theories point to convective motions playing an important role. In this thesis I investigate the properties of convection in the NSSL using a newly-developed high-resolution ring-diagram analysis. I present measurements of the speeds and spatial scales of near-surface flows and from these infer that the degree of rotational constraint on convective flows varies significantly across this layer. In depth analysis of the convective patterns reveals the pervasive influence of coherent downflow plumes generated at the photosphere. These structures link the convective pattern of supergranulation seen in surface observations with the deeper motions found within the NSSL and further hint at the importance of rotation in this layer.

These observations of transient, small-scale convective motions are enabled by the use of improved local helioseismic techniques. Local helioseismology relies on observations of the solar wavefield to produce measurements of plasma flows beneath the surface. In general, this has the capability to map out the subsurface convective flows in three-dimensions, but is often limited in accuracy, resolution, and depth range by the specifics of the analysis procedure. Here, I focus on a particular implementation of local helioseismology called ring-diagram analysis that involves analyzing small patches of the solar surface to build up three-dimensional maps. I will present a new analysis scheme for ring-diagram helioseismology that produces maps of the subsurface flow fields with higher fidelity and vastly higher resolution than previously possible. This is achieved through a combination of novel tools including a robust nonlinear fitting procedure and a highly efficient linear inversion technique. I present these new methods and demonstrate how they enable a new class of high-resolution helioseismic observations. The scientific results made possible with these methods display the power of the new techniques and aid our understanding of near-surface solar dynamics.

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