2. The data
We used a sequence of solar oscillation measurements obtained with the Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO). The sequence lasted as long as MDI operated in the full-disk mode (see Scherrer et al. 1995) and consists of about 17 hours of MDI Dopplergrams taken every minute on June 10, 1996. Within these data there were only very few gaps; we reconstructed the missing data from values adjacent to those gaps. For the center-to-limb variation we selected areas on the Sun which at the beginning of the observing period were located on the solar meridian. The Sun was very quiet on the meridian on that day, but there was some strong activity in the western hemisphere (active region NOAA 7968, 2o N, 42o W).
The data arrays are 1024 1024 integer values of velocities, corrected for instrumental effects and calibrated to m/s. In these images the solar radius equals to 483 pixels. For the center of the solar disk the square pixel size corresponds to km2 on the Sun.
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999