Solar-cycle variation of the rotational shear near the solar surface

Context. Helioseismology has revealed that the angular velocity of the Sun increases with depth in the outermost 35 Mm of the Sun. Recently, we have shown that the logarithmic radial gradient (dln Omega / dln r) in the upper 10 Mm is close to 1 from the equator to 60 degrees latitude. Aims. We aim to measure the temporal variation of the rotational shear over solar cycle 23 and the rising phase of cycle 24 (1996 2015). Methods. We used f mode frequency splitting data spanning 1996 to 2011 from the Michelson Doppler Imager (MDI) and 2010 to 2015 from the Helioseismic Magnetic Imager (HMI). In a first for such studies, the f mode frequency splitting data were obtained from 360-day time series. We used the same method as in our previous work for measuring dln Omega / dln r from the equator to 80 degrees latitude in the outer 13 Mm of the Sun. Then, we calculated the variation of the gradient at annual cadence relative to the average over 1996 to 2015. Results. We found the rotational shear at low latitudes (0 degrees to 30 degrees) to vary in-phase with the solar activity, varying by similar to +/- 10% over the period 1996 to 2015. At high latitudes (60 degrees to 80 degrees), we found rotational shear to vary in anti-phase with the solar activity. By comparing the radial gradient obtained from the splittings of the 360-day and the corresponding 72-day time series of HMI and MDI data, we suggest that the splittings obtained from the 72-day HMI time series suffer from systematic errors. Conclusions. We provide a quantitative measurement of the temporal variation of the outer part of the near surface shear layer which may provide useful constraints on dynamo models and differential rotation theory.