  Using full-disk observations obtained with the Michelson Doppler Imager (MDI)
on board the Solar and Heliospheric Observatory (SOHO) spacecraft, we present
variations of the solar acoustic mode frequencies caused by the solar activity
cycle. High-degree (100 < l < 900) solar acoustic modes were analyzed using
global helioseismology analysis techniques over most of solar cycle 23. We
followed the methodology described in details in Korzennik, Rabello-Soares and
Schou (2004) to infer unbiased estimates of high-degree mode parameters (see
also Rabello-Soares, Korzennik and Schou, 2006). We have removed most of the
known instrumental and observational effects that affect specifically
high-degree modes. We show that the high-degree changes are in good agreement
with the medium-degree results, except for years when the instrument was highly
defocused. We analyzed and discuss the effect of defocusing on high degree
estimation. Our results for high-degree modes confirm that the frequency shift
scaled by the relative mode inertia is a function of frequency and it is
independent of degree.
