130 years of spectroheliograms at Paris-Meudon observatories (1892-2022)

TL;DR

This paper reviews 130 years of spectroheliogram observations at Paris-Meudon observatories, highlighting technological advances, instrumental developments, and the extensive solar data collected over more than a century.

Contribution

It provides a comprehensive historical overview of the development and use of spectroheliography at Meudon, including the evolution of instruments and the long-term solar observation dataset.

Findings

Over 100,000 observations collected since 1893.

Systematic data covers 10 solar cycles in Hα and CaII K lines.

Introduction of digital technology in 2002 enhanced data quality.

Abstract

Broad-band observations of the solar photosphere began in Meudon in 1875 under the auspices of Jules Janssen. For his part, Henri Deslandres initiated imaging spectroscopy in 1892 at Paris observatory. He invented, concurrently with George Hale in Kenwood (USA) but quite independently, the spectroheliograph designed for monochromatic imagery of the solar atmosphere. Deslandres developed two kinds of spectrographs: the ''spectroh{\'e}liographe des formes'', i.e. the narrow bandpass instrument to reveal chromospheric structures (such as filaments, prominences, plages and active regions); and the ''spectroh{\'e}liographe des vitesses'', i.e. the section spectroheliograph to record line profiles of cross sections of the Sun with a 20''-30'' spatial step. This second apparatus was intended to measure the velocities (more exactly the Dopplershifts) of dynamic features. Deslandres moved to Meudon in 1898 with his instruments and tested various combinations, in order to improve the spectral and spatial resolutions, leading to the final large quadruple spectroheliograph in 1908. CaII K systematic observations started at this date and were followed in 1909 by H$\alpha$. The service was organized by Lucien d'Azambuja and continues today. Optical and mechanical parts were revisited in 1989 and the digital technology was introduced in 2002. Full line profiles are registered for all pixels of the Sun since 2018, so that the instrument produces now data-cubes. The collection is one of the longest available (more than 100 000 observations). It contains sporadic images from 1893 to 1907 (during the development phase) and systematic observations along 10 solar cycles since 1908, in H$\alpha$ and CaII K lines. This paper summarizes 130 years of observations, instrumental research and technical advances.