Rotational radial shear in the low solar photosphere

Abstract

Context. Radial differential rotation is an important physical ingredient in stellar dynamo theory. In the case of the Sun, heliosismology techniques have revealed the existence of a near-surface shear layer covering 15–20% of the upper convection zone. It was recently shown that the rotation velocity gradient is not uniform in this layer and that it displays a steep increase in a shallow layer near the surface. Aims. We report the detection of a rotation velocity depth-gradient in the low photosphere that is not accessible to heliosismology techniques. Methods. We applied differential interferometric methods to spectroscopic data obtained with the solar telescope THEMIS, which is equipped with an efficient adaptative optics system. The detection was based on the measurement of a systematic east-west shift between images of the solar granulation at different depths in the FeI 630.15 nm at the center of the solar disk. The same technique was applied to obtain the depth-difference between the images from their perspective shift when they are observed away from the disk center. Both THEMIS and HINODE/SOT data were used for the height-difference measurement, giving similar results. Results. At the center of the solar disk, we measured a systematic retrograde shift of the photospheric granular structures on the east-west axis and with no shift in the north-south direction. The retrograde shift increases linearly with height. We interpret these findings as a signature of a steep decrease in the angular velocity in the low photosphere. Conclusions. The rotational radial shear in the low solar photosphere is likely related to the dynamics of the subsurface shear layer. Its measurement yields a valuable constraint on the numerical simulations of the solar upper convection zone.