Lead Investigator: A.G. Kosovichev
Other Team Members: T.L. Duvall, P. Giles, K. Murawski, B. Roberts
Abstract/Technical Summary
Measure mean properties (such as sound speed, turbulent pressure, parameter of
convective stability) of the convective boundary layer by analysing
high-degree p and f modes and 3D tomographic maps from MDI High-resolution
data; study interaction between oscillations and random flows.
Investigation Plan
The upper convective boundary layer, which is only a few hundred km thick, is a subphotospheric region of superadiabatic temperature gradient, where convection takes the form of extremely unstable turbulent eddies (granules) and is characterized by large-amplitude fluctuations of the thermodynamic state and nearly sonic velocities. It is the place in the Sun where Reynolds stresses and nonadiabatic effects have their greatest influence on the frequencies and stability of five-minute oscillations. A reliable theory of the interaction of the oscillations with the turbulent layer has yet to be developed. Helioseismology provides a valuable tool for studying the interaction and also for uncovering errors made in the standard solar model due to poor understanding of the structure and dynamics of the layer. Such studies are of great importance for improving the accuracy of diagnostics of the Sun's deep interior.
The turbulent pressure (Reynolds stresses), often neglected in the standard solar model, significantly affects the equilibrium stratification of the convective boundary layer and is responsible for most of the difference between the observed and theoretical frequencies of high-degree p modes. Therefore, the turbulent pressure effects must be taken into account in future realizations of the standard solar model, though a consistent approach to these effects has yet to be developed. The remaining frequency residuals are probably due to the interaction between the oscillations and random fluctuations of velocity, density and magnetic field in the granulation layer. These effects are most apparent in the residuals of frequencies of the f mode, which is not sensitive to variations in the mean hydrostatic stratification. Therefore, accurate measurements of f-mode frequencies (and their temporal variation) over a broad range of horizontal wavenumbers is important for studying effects of the convective inhomogeneities and the magnetic field.
We propose to use accurate measurements of high-degree f- and p-mode frequencies from the MDI data to determine the average properties of the superadiabatic layer: the radial gradient of the specific entropy and the turbulent pressure. These properties inferred directly from oscillation frequencies can be used for testing and calibrating convection theories and 3D numerical simulations. The helioseismic tomography will provide complementary information about 3D structure and flows in the layer. Interpretation of the results will be supported by theoretical studies of the interaction between waves and random convective flows and by numerical simulations of subsurface convection.
The current investigation plan is