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Session 2 - Everything Else.
Display session, Friday, June 27
Ballroom C, Chair: Richard Canfield

[2.04] Convective Flow Patterns in Time-Distance Measurements and "Magnetic Corrections" in Vertically Stratified Atmosphere.

M. Ryutova, P. Scherrer (Stanford University)

Time-distance measurements for the reconstruction of subsurface flows and horizontal magnetic fields proved to be very efficient. However, if one can expect a reasonable accuracy of reconstructed maps for the annuli of the radius small compared to the characteristic scale of the convection, the situation changes when annular distances become comparable with the scale of granular, mezogranular, or supergranular convective motions: in each of these cases the uncertainty in the measurements of travel time perturbations increases dramatically. We present here a quantitative analysis of the problem for a particular model of convective motions and compute the travel time perturbations as a function of annular distances and the supergranule radius. It is shown that at annular sizes close to the size of convective cell there occurs: the apparent reduction of the local velocity, and (2) appearance of additional terms in the corrections to perturbation travel time which cause a large error in reconstruction of the velocity field. We discuss the importance of "directionally sensitive" measurements and show that Fourier \sin n\theta, \cos n\theta transforms of travel times measured as a function of direction, "kills" the largest source of errors. We discuss the role of vertical motions. We also present the expressions for the "magnetic corrections" in a vertically stratified atmosphere.

This research is supported by NASA contract NAG5-3077 at Stanford University and the MDI contract PR 9162 at Lockheed.

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