Introduction

Abstract

People who have the chance to see the solar corona during a total solar eclipse are fascinated by its brilliant beauty. Appropriately, the word ‘corona’ is the Latin root of the English ‘crown’. Its optical luminosity is less than a millionth of the Sun’s total flux and is normally outshone by atmospheric stray light. In a total solar eclipse, on average about once every 18 months, a narrow strip of the Earth’s surface is shielded completely by the Moon from the brilliant disk of the Sun, and the corona appears. The phenomenon has been known from antiquity, and is described by Philostratus and Plutarch with considerable astonishment. Almost identical prehistoric paintings in Spanish caves and on rocks in Arizona, USA, seem to represent the Sun surrounded by coronal streamers and to testify to human emotions of even older times. Nevertheless, solar eclipses are rare, and the corona escaped scientific scrutiny and even interest until the middle of the 19th century, when it became clear that it is a solar phenomenon, not related to the Moon, nor an artifact produced by the Earth’s atmosphere. It entered the limelight in 1939 when W. Grotrian and B. Edlén identified coronal optical lines as originating from highly ionized atoms, suggesting a thin plasma of some million degrees. The temperature was confirmed in 1946 by the discovery of thermal radio emission at meter waves. This unexpectedly high temperature is still an enigma today: it is one of the reasons why the solar corona will be a primary target of solar research in the coming years.