Observations of global-scale photospheric Fraunhofer line shifts

Abstract

Transform techniques have been applied to a 1 yr sequence of solar photospheric Fraunhofer line shift data. In agreement with earlier studies, we find no evidence of large-scale structure in mean spatial power spectra near the level of 12 m s/sup -1/. It is argued that such spectra are easily dominated by random supergranule noise, and that these data may even be used to estimate supergranule characteristics. By considering a rotation signal in the temporal transformed data, we find statistical evidence (at the 3 sigma level) of a large-scale, long-lived, photospheric line shift field. A simple model suggests spatial scales of 10/sup 5/--10/sup 6/ km, with lifetimes of at least 3 days and line shifts corresponding to velocity amplitudes near 2 m s/sup -1/. While the interpretation of the residual line shift is not unambiguous, we suggest that this is the Doppler velocity signature of large-scale convective cells.