42 Years of Continuous Observations of the Solar Diameter from 1974 to 2015

TL;DR

This paper reviews 42 years of ground-based solar diameter observations from multiple global stations, demonstrating the consistency and importance of long-term measurements for understanding solar radius variations over the last century.

Contribution

It compiles and analyzes decades of solar diameter data from various observatories, addressing challenges of data shifts and emphasizing the value of ground-based measurements alongside space data.

Findings

Overall increase in solar radius from 1891 to 2015.

Ground-based measurements are crucial for continuous solar monitoring.

Data consistency across different instruments and observatories.

Abstract

Several group in the World followed the solar diameter with dedicated instruments, namely solar astrolabes, since 1974. Their data have been gathered in several observing stations connected in the R2S3 (Reseau de Suivi au Sol du Rayon Solaire) network and through reciprocal visits and exchanges: Nice/Calern Observatory, Rio de Janeiro Observatorio Nacional/Brazil, IAG/Universidade de Sao Paulo/Brazil, Antalya Observatory/Turkey, San Fernando/Spain. The tradition of these observational efforts is here briefly sketched with the aim to evidence the possibility to analyze against the solar activity all these 42 years data at once by overcoming the problem of the shift between the different series. Each instrument has its own density filter with a prismatic effect responsible of that shift. The overall change of the solar radius during the last century is evident by comparing the Auwers' radius of 959.63" (1891, present IAU standard) with 959.94" (2015, from eclipses and Venus transit data, the latter either observed from space) and the role of ground-based daily measurements has been and it is crucial nowadays with Picard-sol and the Reflecting Heliometer of Rio de Janeiro, to ensure an homogeneus and continuous monitor of the solar diameter without the lifetime limitation of the satellites.