Calibration of the thickness of the solar tachocline

Material circulation in the solar tachocline mixes the tachocline material with the convection zone, which causes the sound speed in and immediately b eneath the tachocline to exceed what one would expect from current standard solar models. Calibration against a helioseismic determination of the soun d speed from SOI/MDI data of a sequence of solar models having mixed layers of different thicknesses yields a value of 0.019 R. for the mean tachoclin e thickness Delta, with a formal standard error of about 5%. This value for Delta is somewhat smaller than previous estimates based on measuring shear in the seismically inferred angular velocity, and it has important implica tions concerning our understanding of the magnetohydrodynamics of the tacho cline.