Supergranular power spectrum of the divergence signal at the solar equator using time-distance helioseismology. Top row: at k_y = 0. The left panel shows the spectrum for a tracking rate set to the Snodgrass (1984) rotation rate (about 20 m s⁻¹ slower than Carrington). The right panel shows the spectrum for a tracking rate that is faster by about 60 m s⁻¹. Bottom row: cut through k plane at frequency 3.2 μHz.