History of Solar Magnetic Fields since George Ellery Hale

TL;DR

This paper provides a historical overview of solar magnetic field research over the past century, highlighting technological, theoretical, and observational advancements and key personalities involved.

Contribution

It offers a comprehensive historical account of the development of solar magnetic field studies, including measurement techniques, theoretical foundations, and recent high-resolution diagnostics.

Findings

Development of measurement techniques from photographic to imaging spectro-polarimetry

Introduction of the Hanle effect and the Second Solar Spectrum in astrophysics

Recognition of the small-scale nature of solar magnetic fields

Abstract

As my own work on the Sun's magnetic field started exactly 50 years ago at Crimea in the USSR, I have been a participant in the field during nearly half the time span since Hale's discovery in 1908 of magnetic fields in sunspots. The present historical account is accompanied by photos from my personal slide collection, which show a number of the leading personalities who advanced the field in different areas: measurement techniques, from photographic to photoelectric and imaging methods in spectro-polarimetry; theoretical foundations of MHD and the origin of cosmic magnetic fields (birth of dynamo theory); the quest for increased angular resolution from national projects to international consortia (for instruments both on ground and in space); introduction of the Hanle effect in astrophysics and the Second Solar Spectrum as its playground; small-scale nature of the field, the fundamental resolution limit, and transcending it by resolution-independent diagnostics.