Observations of global-scale photospheric Fraunhofer line shifts

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Fraunhofer Lines, Frequency Shift, Photosphere, Solar Atmosphere, Solar Rotation, Solar Spectra, Convection, Line Of Sight, Power Spectra, Spectral Signatures, Velocity Measurement

Scientific paper

Transform techniques have been applied to a 1 yr sequence of solar photospheric Fraunhofer line shift data. In agreement with earlier studies, no evidence is found of large-scale structure in mean spatial power spectra near the level of 12 m/s. 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, statistical evidence (at the 3 sigma level) is found of a large-scale, long-lived, photospheric line shift field. A simple model suggests spatial scales of 100,000-1,000,000 km, with lifetimes of at least 3 days and line shifts corresponding to velocity amplitudes near 2 m/s. While the interpretation of the residual line shift is not unambiguous, it is suggested that this is the Doppler velocity signature of large-scale convective cells.

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