Rotational radial shear in the low solar photosphere

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Context. Radial differential rotation is an important factor in stellar dynamo theory. In the Sun, helioseismology has revealed a near-surface shear layer in the upper 5–10 percent of the convection zone. At low to midlatitudes, the rotation velocity gradient decreases sharply near the surface. A depth gradient in rotational velocity was recently detected in the low photosphere using a differential interferometric method on spectroscopic data. Granular structures at different depths in the Fe I 630.15 nm line showed a systematic retrograde shift compared to continuum structures, which suggests a height-related decrease in angular velocity. This estimate depends on the assumed granulation coherence time. Aims. We use a more direct approach to measure the differential rotational velocity at different photospheric heights. Methods. We performed spectroscopic scans of the same granular region in Fe I 630.15 nm and Ca I 616.2 nm lines, and measured displacements of images at different line chords between consecutive scans. These observations require excellent seeing, stable adaptive optics correction, and scanning times shorter than the granulation lifetime. Adaptive optics stabilizes continuum images but not higher-altitude rotation differences. We used both THEMIS and HINODE Solar Optical Telescope Fe I 630.15 nm data to measure formation height differences via perspective shifts observed away from the disk center with the slit radially oriented. Results. Measurements at disk center and ±25° latitude along the central meridian show a parabolic decrease in rotational velocity with height that reaches about 16% slower rotation at 80 km above the continuum. No significant difference is found between equator and ±25° latitudes. Conclusions. The low photosphere is a transition zone between the convective and radiative layers. Our measurements provide new constraints on its dynamical behavior and valuable boundary conditions for numerical simulations of the Sun’s upper convection zone.

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