DETERMINATION OF THE SUN SEISMIC RADIUS FROM THE SOHO MICHELSON DOPPLER IMAGER

Dopplergrams from the Michelson Doppler Imager (MDI) instrument on boa rd the SOHO spacecraft have been used to accurately measure frequencie s of the Sun's fundamental(f) mode in the medium angular degree range, l = 88-250. The comparison of these frequencies with the correspondin g frequencies of the standard solar models suggests that the apparent photospheric solar radius (695.99 Mm) used to calibrate the models sho uld be reduced by approximately 0.3 Mm. The precise value of the seism ologically determined solar radius depends on the description of the s ubsurface layer of superadiabatic convection. The discrepancy between the ''seismic'' and apparent photospheric radii is not explained by th e known systematic errors in the helioseismic and photospheric measure ments. If confirmed, this discrepancy represents an interesting new ch allenge to theories of solar convection and solar modelling.