DETECTION OF A STEEP HEIGHT GRADIENT OF THE ROTATIONAL VELOCITY IN THE LOW PHOTOSPHERE OF THE SUN

We present a new method to measure the rotational height gradient in the solar photosphere. The method is inspired from differential interferometric techniques, it is applied to spectroscopic observations in the FeI 630.15 nm obtained at the solar telescope THEMIS equipped with an efficient adaptative optics system. At the center of the solar disk, we measured systematic retrograde shifts between images of the granulation pattern obtained at different line cords that are formed at different heights. The shift varies linearly with height. We interprete this as the signature of a steep decrease of the rotational velocity in the low photosphere.