MDI 30-Minute Averaged Dopplergram
This image is the average of 30 minutes of a series of Dopplergrams obtained on 13 January 1996. The full series extended for 7.4 hours. This frame is the result of averaging 30 full disk velocity maps and subtracting the contribution from the Sun's rotation. The colour scale is such that dark is motion towards the observer and bright is motion away from the observer. The signature of the waves is nearly cancelled in this image since the wave periods are mostly about 5 minutes.
The resulting image clearly shows the supergranulation pattern. The "smooth" area in the center is where the supergranules do not contribute to the signal since what observers see are horizontal motion and MDI measures only the component of motion directed towards or away from SOHO. (Note that supergranules are convective cells, so their motion is convective, but they are on a much larger scale than granules, and observers usually see mostly the horizontal motion.) Close inspection shows that the supergranules flow outwards from their centers so that the edges towards the center are dark (motion toward SOHO) and the edges towards the Sun's limb are bright (motion away from SOHO). These flows are about 400 metres per second. SOHO will allow a detailed study of such large scale convective motions.