Some comments on the limb shift of solar lines

Abstract

It has been suggested that pressure shifts are the sole cause for the systematic blue shift of solar fraunhofer lines (Hart, 1974). To check this we evaluate the significance of pressure shifts for Fe I lines. The observed wavelength shifts of a large number of lines are compared with the shifts calculated for the Lennard-Jones potential following Hindmarsh et al. (1967). The Lennard-Jones potential for the interatomic forces yield pressure shifts caused by neutral hydrogen, which explain only a small fraction of the observed blue shift. It is also shown that the quadratic Stark effect contributes insignificantly to the position of Fe I lines. Table I summarizes the average line shifts for iron lines at the center of the solar disk after correction for pressure shifts.

Examples of Non-Thermal Motions as Seen on the Sun

“…The origin of the limb effect as a convective line shift receives further support from the properties of the limb effect: (i) the decrease of the line shift with increasing line strength (e.g. Beckers and DeVegvar, 1978) as the consequence of the decrease of the convective overshoot with height, (ii) the absence of the line shift in sunspots (Beckers, 1977) as the consequence of the suppression of convection in sunspots, and (iii) the explanation of the center-to-limb variation as the consequence of the effect of both vertical and horizontal motions in convective elements (Beckers and Nelson, 1978).…”

“…Because of its peculiar center to limb variation this shift has been called the "limb effect." Of the three most likely causes for the limb effect, pressure shifts, wave shifts and convective shifts, only the latter remains as a possible one after Beckers and DeVegvar (1978) and Cram et_ aj_. (1979) showed that the former two are much too small and perhaps of the wrong sign.…”

The Limited Influence of Pressure Gradients on Late‐Type Stellar Line Asymmetries

“…When applied to stellar atmospheres, since the -4line formation takes place along a density stratified region, asymmetries and a net shift in the central wavelength of the emergent profile will appear. These line shifts were suggested to be the origin for the solar limb effect (Spitzer 1950;Hart 1974), although later the convective blue-shift, resulting from the correlation between intensity and velocity fields in the solar photosphere, became a better candidate (Beckers & de Vegvar 1978;Beckers & Nelson 1978). In these previous studies, making use of the Lennard-Jones potential (Hindmarsh, Petford, & Smith 1967) and the classical impact theory, pressure shifts in the line absorption profile were estimated at a representative atmospheric height only.…”