Temporal variations in μHz of the frequency shifts, ⟨ Δν_{n,l = 0} ⟩, ⟨ Δν_{n,l = 1} ⟩, ⟨ Δν_{n,l = 2} ⟩, at each individual angular degree l (from top to bottom), and calculated for four different frequency ranges with the QBO signature removed (black dots). From left to right, the frequency ranges are the following: a) 1800 μHz ≤ ν< 3790μHz; b) 1800 μHz ≤ ν< 2450μHz; c) 2450 μHz ≤ ν< 3110μHz; and d) 3110 μHz ≤ ν< 3790μHz. The 10.7 cm radio flux, F_10.7, smoothed in the same way and scaled to match the rising phase and maximum of Cycle 23, is shown as a proxy of the solar surface activity (solid line).