The development of the Heliometer of the Observatorio Nacional of Rio de Janeiro and application to the study of the Sun-Earth system

TL;DR

This paper details the development of a precise heliometer for monitoring solar diameter variations, its testing, and application in studying the Sun-Earth system, revealing correlations with geomagnetic activity.

Contribution

It introduces a novel heliometer with high stability and automated analysis software, enabling precise solar diameter measurements and insights into solar-terrestrial interactions.

Findings

Measured solar diameter with 0.5 arcsecond standard deviation

Identified correlation between solar diameter variations and geomagnetic field peaks

Demonstrated potential for milliarcsecond accuracy in solar measurements

Abstract

This work presents the development and construction of the Heliometer of the Observat\'orio Nacional/MCTI. This instrument is designed to monitor changes on the solar diameter with the accuracy of the next-generation solar satellites. A review of the heliometric method is made and the building and testing of 4 prototypes is described. The instrument has a mirror objective split in dihedral, formed by the hemi-sections of a parabolic mirror. The materials that form the instrument have thermal and mechanical stability to 10^(-7). The number of optical parts is minimized and their quality is greater than {\lambda}/12. An original software for the automated collection and analysis of the images was developed. With its latest version fully developed, we conducted an observational campaign of 9 days, deriving more than 70000 heliometric images of the Sun. The measured solar diameter has a standard deviation of 0.5 arcseconds, with no instrumental bias, and limited only by the provisional atmospheric modeling. Therefore, by improving the statistics, an accuracy at milliarcseconds can be achieved. A study of the correlation between the solar diameter and the geomagnetic field intensity was made. A correspondence between the peaks of the series related to solar activity, namely, the semidiameter variation, the flares index, and the sunspots counts, and the number of negative peaks on the intensity of the geomagnetic field is found. It must be cautioned though that it is a complex correspondence, requiring different modes of response and phase dependence on the stage of the cycle of solar activity. The straightforward interpretation of observational evidence indicates that the semidiameter of the Sun seems to vary significantly prior to the corresponding variations of the geomagnetic field. The effect is most evident in the downward phase of the solar cycle.