An Explanation of the Solar Limb Shift

Absorption lines in the solar spectrum are shifted as a result of collisions of the absorbing atoms with neutral hydrogen atoms. The shift, which would be to the red if only van der Waals forces between atoms were considered, is generally to the blue when the more realistic Lennard-Jones potential is used. The shift is proportional to the density of perturbing atoms, and is therefore smaller at the limb than it is at the center of the disk, where we see to greater depths in the atmosphere. This differential shift is suggested as the explanation of the solar limb effect. Computations using the Lennard-Jones potential and standard photospheric models correctly predict the magnitude and direction of the limb shift, as well as the functional forms of the observed variations of line shift and damping constant with distance from the center of the disk. Subject headings: line formation - solar atmospheric motions - spectra, solar