Figure 3 : Illustrating the phase and group velocities of solar Rossby waves in this sample.

From: The detection of Rossby-like waves on the Sun

Figure 3

Hovmöller12 diagrams (longitude versus time at fixed latitude) are shown for the BP density distributions derived from 360° solar observations. a, The Hovmöller diagram for a band of latitudes 2° wide that is centred on 15° in the northern hemisphere; b, the corresponding Hovmöller diagram for 22° in the southern hemisphere. In both cases, we see that the clusters of enhanced BP density exhibit a westward (left-to-right) phase velocity over their lifetime, while there is an apparent eastward (right-to-left) group velocity. Analysis of the cluster shape yields a westward phase velocity of approximately 3 m s–1 (red dashed reference line), whereas the analysis of their longitudinal separation yields an eastward group velocity of approximately 24 m s–1 (white dashed reference line) in each hemisphere (see Methods section).