Helioseismic Data Inclusion in Solar Dynamo Models

The Astrophysical Journal (Impact Factor: 6.28). 06/2009; 698:461-478. DOI: 10.1088/0004-637X/698/1/461
Source: arXiv

ABSTRACT An essential ingredient in kinematic dynamo models of the solar cycle is the internal velocity field within the simulation domain—the solar convection zone (SCZ). In the last decade or so, the field of helioseismology has revolutionized our understanding of this velocity field. In particular, the internal differential rotation of the Sun is now fairly well constrained by helioseismic observations almost throughout the SCZ. Helioseismology also gives us some information about the depth dependence of the meridional circulation in the near-surface layers of the Sun. The typical velocity inputs used in solar dynamo models, however, continue to be an analytic fit to the observed differential rotation profile and a theoretically constructed meridional circulation profile that is made to match the flow speed only at the solar surface. Here, we take the first steps toward the use of more accurate velocity fields in solar dynamo models by presenting methodologies for constructing differential rotation and meridional circulation profiles that more closely conform to the best observational constraints currently available. We also present kinematic dynamo simulations driven by direct helioseismic measurements for the rotation and four plausible profiles for the internal meridional circulation—all of which are made to match the helioseismically inferred near-surface depth dependence, but whose magnitudes are made to vary. We discuss how the results from these dynamo simulations compare with those that are driven by purely analytic fits to the velocity field. Our results and analysis indicate that the latitudinal shear in the rotation in the bulk of the SCZ plays a more important role, than either the tachocline or surface radial shear, in the induction of the toroidal field. We also find that it is the speed of the equatorward counterflow in the meridional circulation right at the base of the SCZ, and not how far into the radiative interior it penetrates, that primarily determines the dynamo cycle period. Improved helioseismic constraints are expected to be available from future space missions such as the Solar Dynamics Observatory and through analysis of more long-term continuous data sets from ground-based instruments such as the Global Oscillation Network Group. Our analysis lays the basis for the assimilation of these helioseismic data within dynamo models to make future solar cycle simulations more realistic.

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