Abstract
A formalism is developed to understand better how Doppler shifts of spectrum lines as inferred from phase shifts in the Fourier transforms of line profiles are related to the underlying velocity structures which they are intended to measure. With a standard model atmosphere and a simplified, quasi-LTE treatment of line formation, the formalism is applied to the Nii 6768 Å line which has been selected for use with a network of imaging interferometers under development by the Global Oscillations Network Group (GONG) for research in helioseismology. Fourier phase shifts are found to be a remarkably linear measure of velocity even in the presence of gradients and unresolved lateral variations in the assumed velocity field. An assumed outward increase in amplitude of a model oscillatory velocity is noticeably reflected in the center-to-limb behavior of the simulated velocity measure, and a sample model of solar granulation is found to have a strong influence on the formation of the Fourier phase.
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Jones, H.P. Formation of fourier phase shifts in the solar Nii 6768 Å line. Sol Phys 120, 211–234 (1989). https://doi.org/10.1007/BF00159876
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DOI: https://doi.org/10.1007/BF00159876