MDI and GONG Inferences of the Changing Sun

Intensity and velocity filtergrams, analyzed with ring diagram, lead to different mode frequencies. The frequency differences increase with increasing distance from disk center (shown for four longitudes east of the central meridian from MDI data from 1997, April 19 to 28) and occur mainly at higher p-modes (n 4). Intensity ring diagrams lead to higher frequencies that increase toward the limb by about 20 Hz and the difference between intensity and velocity is about 50–60 Hz. The daily (1664 images) power spectra were averaged to enhance the signal-to-noise ratio and fit with symmetric profiles.