Temporal evolution of the radial gradient of the synodic rotation period (black filled circles) of the equatorial corona with 1σ estimates of the associated uncertainties obtained by fitting the data from 1.6 to 3.0 R_⊙. Blue squares and red diamonds symbols represent the radial gradients obtained by fitting the data between 1.6 and 2.2 R_⊙ (inner corona) and 2.4 and 3.0 R_⊙ (outer corona), respectively, as shown in Fig. 4. Representative error bars are also indicated in the upper right-hand corner.