Doppler Velocity Variation near the Solar Limb in the Solar Photosphere Observed with Hinode

Abstract

The Doppler velocity and bisector of photospheric lines show significant center-to-limb variation. This variation has been considered to be caused by the projection angle of the granular convection which has a corrugated surface. This study aimed to show the spectral line shifts up to limb and investigate how these shifts are caused by granular convection. We analyzed the Fe I 630.15 nm absorption line by spatially resolving the granulation very close to the limb using datasets of polar observations obtained with the Spectro-polarimeter (SP) of the Solar Optical Telescope (SOT) onboard Hinode. We also analyzed the Fe I 630.15 nm emission line, which was observed within 1" above the limb. This study revealed the following: 1) the spatially averaged spectral lines showed an increase in redshift toward the limb, then became constant, and finally became nearly zero, 2) the Doppler velocities of granules, which were classified as bright and non-magnetic regions, showed as increase in redshifts toward the limb, 3) the bisector of granules changed more than that of the intergranules, and 4) the emission lines showed small blueshifts. These results indicate that the increase in redshift toward the limb is caused by the increase in the redshift of granules. Because the bisector reflects the atmospheric velocity along the line-of-sight, our observations can be interpreted as suggesting that the line-forming region in granules is geometrically thicker than that in intergranules.