Rotation and zonal flows in the solar envelope from the SOHO/MDI observations

Abstract

We report on the latest inferences concerning solar differential rotation that have been drawn from the helioseismic data that are now available from the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). That spacecraft is positioned in a halo orbit near the Sun-Earth Lagrangian point L_1, in order to obtain continuous Doppler-imaged observations of the sun with high spatial fidelity. Doppler velocity, intensity and magnetic field images are recorded, based on modulations of the 676.8 nm Ni I solar absorption line. The high spatial resolution of MDI thereby permits the study of many millions of global resonant modes of solar oscillation. Determination and subsequent inversion of the frequencies of these modes, including the degeneracy-splitting by the rotation of the sun, enables us to infer how the sun's angular velocity varies throughout much of the interior. The current MDI data are providing substantial refinements to the helioseismic deductions that can be made about differential rotation both within the convection zone and in its transition to the radiative interior. The shearing layer evident in the angular velocity Omega just below the solar surface is becoming better defined, as is the adjustment layer or tachocline near the base of the convection zone. The MDI data are also revealing a prominent decrease in Omega at high latitudes from the rotation rate expressed by a simple three-term expansion in latitude that was originally deduced from surface Doppler measurements. Further, there are indications that a submerged polar vortex involving somewhat faster Omega than its surroundings exists at about 75(deg) in latitudes.