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Abstract: Birmingham Solar Oscillations Network BiSON instruments use resonantscattering spectrometers to make unresolved Doppler velocity observations ofthe Sun. Unresolved measurements are not homogenous across the solar disc andso the observed data do not represent a uniform average over the entiresurface. The influence on the inhomogeneity of the solar rotation and limbdarkening has been considered previously Brookes et al. 1978a and is wellunderstood. Here we consider a further effect that originates from theinstrumentation itself. The intensity of light observed from a particularregion on the solar disc is dependent on the distance between that region onthe image of the solar disc formed in the instrument and the detector. Themajority of BiSON instruments have two detectors positioned on opposite sidesof the image of the solar disc and the observations made by each detector areweighted towards differing regions of the disc. Therefore the visibility andamplitudes of the solar oscillations and the realization of the solar noiseobserved by each detector will differ. We find that the modelled bias issensitive to many different parameters such as the width of solar absorptionlines, the strength of the magnetic field in the resonant scatteringspectrometer, the orientation of the Sun-s rotation axis, the size of the imageobserved by the instrument and the optical depth in the vapour cell. We findthat the modelled results best match the observations when the optical depth atthe centre of the vapour cell is 0.55. The inhomogeneous weighting means that a`velocity offset- is introduced into unresolved Doppler velocity observationsof the Sun, which varies with time, and so has an impact on the long-termstability of the observations.



Author: A. M. Broomhall, W. J. Chaplin, Y. Elsworth, R. New

Source: https://arxiv.org/



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