SOLAR INTERNAL-ROTATION FROM LOWL DATA - A 2D REGULARIZED LEAST-SQUARES INVERSION USING B-SPLINES

Observations of surface oscillations of the Sun can be analyzed to pro be the solar interior. We use data obtained by the LOWL instrument (LO WL is an abbreviation for low degree with degree denoted by L) install ed on Mauna Loa, Hawaii, since 1994 to investigate solar internal rota tion. A 2 Dimensional Regularized Least-Squares (2D RLS) inverse metho d based on an expansion of the solution on B-splines of arbitrary orde r is presented and applied to a 2 year dataset. This method insures th e regularity of the solution in the center and introduces surface cons traints. The choice of trade-off parameters in the regularization term is discussed using an L-curves analysis and we discuss the influence of the choice of the order of derivatives in the regularization terms for the description of the deep interior. We study the latitudinal res olution of the inversion of a-coefficients compared to that of the inv ersion of individual splittings built from these coefficients. Compare d to the previous inversion of the first three months of LOWL data mad e by Tomczyk et al. (1995b), our solution is extended up to the surfac e by adding high degree modes and constraining the rotation to fit the spectrographic observations (Snodgrass 1984). In the radiative zone w e obtain more rigid rotation and our solution is compatible with a rot ation of the solar core of the order or smaller than the surface rotat ion at mid latitude.