Time modulation of the frequency shifts of the retrograde (m =  − 1) and prograde (m =  + 1) components of the triplet centered at 2224 μHz. The frequency shift f(m = 0) – f(m =  − 1) for the retrograde component ( × ) and f(m =  + 1) – f(m = 0) for the prograde component (o) are plotted as function of time, relatively to their average value marked by the continuous horizontal line.