Helioseismic studies of differential rotation in the solar envelope by thesolar oscillations investigation using the Michelson Doppler Imager

The splitting of the frequencies of the global resonant acoustic modes of t he Sun by large-scale flows and rotation permits study of the variation of angular velocity Omega with both radius and latitude within the turbulent c onvection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar Oscillations Investigat ion (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliosp heric Observatory (SOHO) spacecraft positioned at the L-1 Lagrangian point in continuous sunlight, yield oscillation power spectra with very high sign al-to-noise ratios that allow frequency splittings to be determined with ex ceptional accuracy. This paper reports on joint helioseismic analyses of so lar rotation in the convection zone and in the outer part of the radiative core. Inversions have been obtained for a medium-l mode set (involving mode s of angular degree l extending to about 250) obtained from the first 144 d ay interval of SOI-MDI observations in 1996. Drawing inferences about the s olar internal rotation from the splitting data is a subtle process. By appl ying more than one inversion technique to the data, we get some indication of what are the more robust and less robust features of our inversion solut ions. Here we have used seven different inversion methods. To test the reli ability and sensitivity of these methods, we have performed a set of contro lled experiments utilizing artificial data. This gives us some confidence i n the inferences we can draw from the real solar data. The inversions of SO I-MDI data have confirmed that the decrease of Omega with latitude seen at the surface extends with little radial variation through much of the convec tion zone, at the base of which is an adjustment layer, called the tachocli ne, leading to nearly uniform rotation deeper in the radiative interior. A prominent rotational shearing layer in which Omega increases just below the surface is discernible at low to mid latitudes. Using the new data, we hav e also been able to study the solar rotation closer to the poles than has b een achieved in previous investigations. The data have revealed that the an gular velocity is distinctly lower at high latitudes than the values previo usly extrapolated from measurements at lower latitudes based on surface Dop pler observations and helioseismology. Furthermore, we have found some evid ence near latitudes of 75 degrees of a submerged polar jet which is rotatin g more rapidly than its immediate surroundings. Superposed on the relativel y smooth latitudinal variation in Omega are alternating zonal bands of slig htly faster and slower rotation, each extending some 10 degrees to 15 degre es in latitude. These relatively weak banded flows have been followed by in version to a depth of about 5% of the solar radius and appear to coincide w ith the evolving pattern of "torsional oscillations" reported from earlier surface Doppler studies.