Solar cycle onset seen in SOHO Michelson Doppler Imager seismic data

We have analyzed time changes in centroid frequencies and multiplet frequency splittings of solar oscillations determined with the Michelson Doppler Imager instrument (MDI) on SOHO. The data were divided into five consecutive 72 day sets covering the period from 1996 May 1 through 1997 April 25. We have detected a significant trend in the a₄ and a₆ frequency splitting coefficients, which reflects a decrease in the P₄ distortion (described by the fourth-degree Legendre polynomial of colatitude) and an increase in the P₆ distortion. The rise of the latter distortion seems to coincide precisely with the rise in the number of new cycle sunspots. Such sharp and detailed clues to activity onset are new and do not exist in splitting data from the rising phase of the last cycle. The relative differences among the solar radii inferred from the f-mode frequencies from the five sets (at most 6 × 10^-6 or 4 km) are formally significant, reaching a minimum during the observed period.