Each tile is 16 degrees square, and the tile centers are separated by 7.5 degrees in latitude and longitude. For each observational day and for each tile, we have computed the frequency shift measured relative to the temporal and spatial average of the entire set of frequencies sampled over a full rotation. The motion of active regions as they rotate across the solar disk is vividly traced by changes in the frequencies.
Active regions appear as locations of large positive frequency shifts.
Depending on the radial order and wavenumber of the observed acoustic modes the frequency shifts can be as much as 10 to 30 microHz. The magnitude and frequency dependence of the large frequency shifts are consistent with the far smaller changes measured in global oscillation frequencies over the solar cycle, provided the local frequency shifts are averaged over the solar disk and are scaled to the appropriate wavenumber regimes. The frequency dependence of the shifts indicates that the physical phenomena inducing the shifts is largely confined to the surface layers of the sun, although there is some indication that there may be a deeper structural component as well.