Radial variation of the equatorial synodic period (filled circles) at nine different times (specified on the right-hand side of each series of data) during solar cycle 23. The black straight lines represent weighted linear fits to the full range of data between 1.6 and 3.0 R_⊙. Blue and red straight lines represent weighted linear fits to the data between 1.6 and 2.2 R_⊙ (inner corona) and 2.4 and 3.0 R_⊙ (outer corona), respectively. The starting values of each series of data (that is, the rotation periods at 1.6 R_⊙) are specified on the left-hand side. The ticks in the y-axis are given in daily steps.