The inhomogeneous response across the solar disc of unresolved Doppler velocity observations

TL;DR

This paper investigates how inhomogeneous instrumental effects cause uneven Doppler velocity measurements across the solar disc, affecting long-term stability and observed oscillation amplitudes.

Contribution

It models the impact of instrument-induced inhomogeneous weighting on the observed offset velocity in unresolved solar Doppler measurements.

Findings

Instrumental inhomogeneity causes measurable offset velocities.

Model explains the variation in observed offset velocities.

Implications for long-term solar observation stability.

Abstract

Unresolved Doppler velocity measurements are not homogenous across the solar disc (Brookes et al. 1978). We consider one cause of the inhomogeneity that originates from the BiSON instrumentation itself: the intensity of light observed from a 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 non-uniform weighting affects the realization of the solar noise and the amplitudes of the solar oscillations observed by a detector. An 'offset velocity', which varies with time, is observed in BiSON data and has consequences for the long-term stability of observations. We have attempted to model, in terms of the inhomogeneous weighting, the average observed offset velocity.