The Effect of Surface Layers on Solar Oscillation Frequencies

P. M. Giles
Center for Space Science and Astrophysics, Stanford University, Stanford, CA 94305-4085
415-725-9549
PGiles@solar.stanford.edu
  
T. L. Duvall, Jr.  
NASA Goddard Space Flight Center, Greenbelt, MD 20771  
415-723-6704  
TDuvall@solar.stanford.edu  
  
A. G. Kosovichev  
W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085  
415-723-7667  
AKosovichev@solar.stanford.edu  
  
P. H. Scherrer  
Center for Space Science and Astrophysics, Stanford University, Stanford, CA 94305-4085  
415-723-1504  
PScherrer@solar.stanford.edu  
  
The superadiabatic boundary layer, which occupies only a
few hundred kilometers below the surface, is the region of
greatest helioseismological uncertainty. 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. In the helioseismic
inversions, it is often assumed that perturbations of
frequencies of the oscillations can be approximated by a
smooth function of frequency. We have developed a
theoretical model of the frequency perturbations, in which
the effect of the boundary layer is modeled by variations
of the adiabatic exponent, $\gamma$.  We compare the model
results with the data obtained from the Michelson Dopper
Imager (MDI) instrument on board Solar and Heliospheric
Observatory (SOHO), and estimate the extent of the surface
perturbation.