A Brief, Basic Guide to Terms and Concepts of Solar Radio Astronomy

Abstract

Humanity has observed the sun for many centuries. Among the millions of stars observable through optical telescopes, only the sun is close enough to be studied in all of its activity, in exquisite detail. But because it is so bright, it was difficult for anyone to see features on the sun’s surface until more modern times. Chinese astronomers likely observed sunspots as early as 364 BC with naked eye observations; the observations would have been at sunrise or sunset when the solar radiation is attenuated by the earth’s atmosphere or even through dense terrestrial clouds. In the early seventeenth century Galileo and others began detailed studies of the sun with some of the first optical telescopes; their detection of sunspots was a major discovery that impacted the understanding of the universe. Detailed telescopic solar studies began in the nineteenth century, including spectroscopic identification of many known and even some unknown elements in the solar spectrum. The sun emits a continuum of electromagnetic radiation from X-ray, to ultraviolet, optical, infrared and radio wavelengths. In the optical wavelengths (similar to the receptivity of the human eye at 400–800 nm), the solar spectrum shows absorption lines that enable astronomers to determine the chemical composition of the sun. The solar spectrum and associated discoveries were made by the German astronomer Fraunhofer in 1817 using the newly invented spectroscope.