Solar Interior and Variability (SIV) Section

The scientific purview of the Solar Interior and Variability (SIV) section extends upward from the center of the Sun, through the radiative and convective zones, to the photospheric layers at the base of the solar atmosphere. The diverse research activities of the section scientists are directed toward understanding the structure and dynamics of the solar interior, the nature and internal origins of the Sun's magnetism and magnetic activity cycle, and the mechanisms contributing to the variable solar radiative output. In pursuit of these objectives, SIV researchers investigate convection and related flows, hydromagnetic dynamo processes, and magnetic flux transport in the solar interior. These efforts are further enhanced by viewing the Sun-Earth system within a wider, astrophysical context; section scientists also study analogous physical processes and phenomena in solar-type stars and extra-solar planets.

Solar Convection and Internal Rotation

Turbulent convection in the solar envelope transports energy to the surface where it is radiated into space and also redistributes momentum and entropy, producing differential rotation and global meridional circulations. Of particular importance is a layer of rotational shear near the base of the convection zone known as the solar tachocline where much of the observed magnetic activity of the Sun is thought to originate. HAO scientists investigate the multi-faceted dynamics of the solar interior using a suite of theoretical and numerical models ranging from three-dimensional magnetohydrodynamic (MHD) simulations to mean-field dynamo models to tachocline models based on a hydromagnetic generalization of the shallow-water equations commonly used in geophysics. Such models provide valuable interpretive insight and guidance in support of ongoing investigations of solar internal structure and dynamics based on helioseismology and on observations of magnetic activity patterns in the solar atmosphere.

The Solar Dynamo

The interaction of the solar dynamo with differential rotation and meridional flow is investigated through a coupled meanfield model including the Lorentz-force feedback on differential rotation and meridional flow. This feedback gives rise to periodic changes of the rotation rate, known as torsional oscillations. This dynamo model allows to incorporate the additional constraints given by observations of the variable internal rotation of the sun. It is also possible to address the energy budget of the dynamo.

Magnetic Flux Emergence

The current prevailing picture is that magnetic active regions on the solar surface originate from strong, predominantly toroidal magnetic fields generated by the solar dynamo mechanism at the thin tachocline layer at the base of the solar convection zone. Thus the magnetic fields need to traverse the entire convection zone (the outer 30% of the solar interior) before they reach the photosphere to form the observed sunspots and solar active regions, which are centers of solar eruptions. Understanding the process of magnetic flux emergence through the solar convection zone is therefore crucial for understanding the link between the observed magnetic activities at the surface and the dynamo-generated magnetic fields in the interior. Using MHD numerical simulations, HAO scientists have been modeling the formation and rise of buoyant of magnetic flux tubes in the solar convection zone and their emergence through the photosphere into the solar atmosphere.

Sunspots and Photospheric Dynamics

In collaboration with the Max-Planck Institute for Solar System Research (MPS) in Germany and the University of Utrecht in the Netherlands 2D and 3D MHD simulations with radiative transfer are used to investigate the subsurface structure of sunspots. The simulations are based on the MURaM code developed by the MPS and the University of Chicago.

The Solar-Stellar Connection

Just as helioseismology revolutionized our understanding of the interior structure of the Sun, asteroseismology is now placing this knowledge into a broader context, by providing structural information for other solar-type stars. Scientists at HAO are developing a stellar model-fitting pipeline, using a parallel genetic algorithm, to prepare for the asteroseismic data soon expected from several satellite missions. Meanwhile, the Solar Oscillations Network Group (SONG) is a concept for a global network of small ground-based telescopes dedicated to asteroseismology and extrasolar planet searches, currently being organized though the Danish AsteroSeismology Center (DASC) at the University of Aarhus. The High Altitude Observatory is participating in the design and development phase of the SONG effort, with the intent to build and operate one of the SONG telescopes at HAO's Mauna Loa Observatory in Hawaii.

Effects associated with rotation can modify stellar properties, altering the luminosities, surface temperatures, sizes, and shapes of stars in ways that are unaccounted for in nonrotating models. HAO scientists have developed methods for constructing self-consistent models of differentially rotating, chemically homogeneous stars, whereby the equations of stellar structure and Poisson's equation for the gravitational potential are iteratively solved for an assumed conservative internal rotation law. Such models provide the means of interpreting observations of stars that are known to be rapid rotators.

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