  Here we present a detailed analysis of solar acoustic mode frequencies and
their rotational splittings for modes with degree up to 900. They were obtained
by applying spherical harmonic decomposition to full-disk solar images observed
by the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory
spacecraft. Global helioseismology analysis of high-degree modes is complicated
by the fact that the individual modes cannot be isolated, which has limited so
far the use of high-degree data for structure inversion of the near-surface
layers (r > 0.97 R). In this work, we took great care to recover the actual
mode characteristics using a physically motivated model which included a
complete leakage matrix. We included in our analysis the following instrumental
characteristics: the correct instantaneous image scale, the radial and
non-radial image distortions, the effective position angle of the solar
rotation axis and a correction to the Carrington elements. We also present
variations of the mode frequencies caused by the solar activity cycle. We have
analyzed seven observational periods from 1999 to 2005 and correlated their
frequency shift with four different solar indices. The frequency shift scaled
by the relative mode inertia is a function of frequency alone and follows a
simple power law, where the exponent obtained for the p modes is twice the
value obtained for the f modes. The different solar indices present the same
result.
