Analysis of a series of measurements of solar semidiameter variations during 2001 at the astrolabe of Rio de Janeiro

TL;DR

This study analyzes solar semi-diameter measurements from 2001, correcting for observational errors, comparing with other observatories, and exploring correlations with solar activity indicators.

Contribution

It provides a detailed correction and comparison of solar semi-diameter data, linking it with solar activity and cross-validating with international observations.

Findings

Corrected series show smaller deviations and consistent patterns.

East and West series become more similar after correction.

Series correlates with sunspot maxima, indicating solar activity influence.

Abstract

From 1997 the Observatorio Nacional has been observing the Sun with an astrolabe in order to study variations on the solar semi-diameter. Till now more than 20.000 observations were done. The observations taken in the year of 2001 were special because, for the first time, they can be compared with other equivalent series, and because the Sun passed by a second maximum of its cycle of activity. The observed data were at first selected, by leaving out the values relative to unsuccessful measurements. We dealt with the errors in the observed values originating from the instability of the objective prism, which increases the east values and diminishes the west ones. Then, it was possible to remove the errors by comparing the East and West series. Next, the observational conditions were investigated: the air temperature, the Fried's parameter, and the dispersion on the solar limb adjustment. Their influence on the results was accounted for. Finally we considered the errors arising from leveling defects, modeled by a dependence to the azimuth of observation. All the corrections are smaller than the standard deviation and the mean semi-diameter values stay unaffected. This means that the series have the same pattern with smaller deviations. After the correction of the prism instability the East and West series become more similar, what reinforces that they aim at the same target. The corrected series were compared with other observations of solar semi-diameter from the CERGA, Cote d'Azur Observatory in France and from Tubitak Observatoty in Antalya, Turkey. The comparison reveals alike features, which deserve further, detailed study. Also, the series was compared to the daily mean Sun spots number. It exhibited three maxima along 2001, that coincide on time with three maxima of the correct series delayed by 50 days. Again, this requires a further study.