Seeing-Induced Errors in Solar Doppler Velocity Measurements

TL;DR

This paper investigates how variable atmospheric seeing conditions introduce errors in solar Doppler velocity measurements obtained from narrow-band imaging, highlighting the significance of adaptive optics in reducing these errors.

Contribution

It provides a quantitative analysis of seeing-induced errors in solar Doppler measurements and demonstrates the effectiveness of adaptive optics in mitigating these errors.

Findings

Seeing conditions can cause spurious velocities up to 1 km/s.

Errors are more significant in umbral regions of sunspots.

Adaptive optics reduces the impact of seeing-induced errors.

Abstract

Imaging systems based on a narrow-band tunable filter are used to obtain Doppler velocity maps of solar features. These velocity maps are created by taking the difference between the blue- and red-wing intensity images of a chosen spectral line. This method has the inherent assumption that these two images are obtained under identical conditions. With the dynamical nature of the solar features as well as the Earth's atmosphere, systematic errors can be introduced in such measurements. In this paper, a quantitative estimate of the errors introduced due to variable seeing conditions for ground-based observations is simulated and compared with real observational data for identifying their reliability. It is shown, under such conditions, that there is a strong cross-talk from the total intensity to the velocity estimates. These spurious velocities are larger in magnitude for the umbral regions compared to the penumbra or quiet-sun regions surrounding the sunspots. The variable seeing can induce spurious velocities up to about 1 km/s It is also shown that adaptive optics, in general, helps in minimising this effect.