New aspects of Doppler imaging in Sun-as-a-star observations

TL;DR

This paper investigates how instrument-related inhomogeneities affect unresolved Doppler velocity measurements of the Sun, revealing biases that influence long-term observational stability and depend on various instrument and solar parameters.

Contribution

It introduces a model accounting for instrument-induced biases in Doppler observations, highlighting their impact on data interpretation and stability.

Findings

Model matches observations best with optical depth of 0.55

Instrument biases cause time-varying velocity offsets

Inhomogeneous weighting affects long-term data stability

Abstract

Birmingham Solar Oscillations Network (BiSON) instruments use resonant scattering spectrometers to make unresolved Doppler velocity observations of the Sun. Unresolved measurements are not homogenous across the solar disc and so the observed data do not represent a uniform average over the entire surface. The influence on the inhomogeneity of the solar rotation and limb darkening has been considered previously (Brookes et al. 1978a) and is well understood. Here we consider a further effect that originates from the instrumentation itself. The intensity of light observed from a particular region on the solar disc is dependent on the distance between that region on the image of the solar disc formed in the instrument and the detector. The majority of BiSON instruments have two detectors positioned on opposite sides of the image of the solar disc and the observations made by each detector are weighted towards differing regions of the disc. Therefore the visibility and amplitudes of the solar oscillations and the realization of the solar noise observed by each detector will differ. We find that the modelled bias is sensitive to many different parameters such as the width of solar absorption lines, the strength of the magnetic field in the resonant scattering spectrometer, the orientation of the Sun's rotation axis, the size of the image observed by the instrument and the optical depth in the vapour cell. We find that the modelled results best match the observations when the optical depth at the centre of the vapour cell is 0.55. The inhomogeneous weighting means that a `velocity offset' is introduced into unresolved Doppler velocity observations of the Sun, which varies with time, and so has an impact on the long-term stability of the observations.