Radio Astronomy: Not Just for the Far-out Invisible Universe

In honor of the 2017 solar eclipse over the continental United States on Monday, August 21, I've done some digging into radio astronomy's history involving the Sun. While radio astronomy is often considered "exploring the invisible universe" (to quote the NRAO) in the context of the interstellar medium (ISM) and far-away planetary systems, the field also has an intimate past with our own solar system.

Series of stills from 2013 showing the eclipse sequence from right to left, with totality in the center. Image credit: Rick Feinberg, TravelQuest International and Wilderness Travel

The Sun has been a player in radio astronomy history since the field's earliest beginnings, first making an appearance in Karl Jansky's notebook. Karl Jansky—an American radio engineer at Bell Labs who laid the foundation for radio astronomy in the 1930s—was searching for the source of static "of unknown origin" using a large radio antenna. Jansky ruled out thunderstorms, power lines, and radio transmitters as sources of the mysterious steady hiss because of how the signal sometimes disappeared depending on the position of his antenna. Initially, the static seemed to follow the Sun, coming from the east in the morning, the south at midday, and the west in the afternoon. After about a month, however, the radio noise no longer aligned with the Sun, and six months later the source of static seemed opposite the Sun. Eventually, the radio noise was determined to be radio waves from the center of the Milky Way galaxy.

Jansky's notebook showing the position and location of his antenna in 1935. (LSST)

Over the next decade, there was little buzz over radio waves coming from outer space. During World War II, English physicist James Stanley Hey detected radio waves that were in fact coming from the Sun. Hey was employed by the Army Operational Research Group in 1940 to research anti-aircraft radar in an effort to combat the Germans who were jamming radio communications among the Allied Forces. In February 1942, radar stations along the south coast of England saw a significant increase in radio interference. While the interference was thought to be the result of Axis operatives on the French coast, the source could not be verified. Hey observed patterns in the interference, noting that the strongest radio interference came from the direction of the Sun. He was initially skeptical that the Sun could be jamming the Allies' communications, but after consulting with the Royal Observatory in Greenwich, he was convinced; the Sun was experiencing particularly strong sunspot activity at that time, resulting in the emission of strong solar radio emissions. After World War II, Hey continued to investigate radiation from the sun and how it correlated with solar activity.

Radio image of the Sun recorded by the VLA in 1981. The brightest regions are part of the corona but beyond sunspots. (NRAO/AUI, retrieved from PrimaLuceLab)

Curious about the strength of radio emissions from the Sun, British-born Australian scientist (John) Paul Wild joined the Radiophysics Division of Australia's Council for Scientific and Industrial Research in 1947. Over the course of several decades, Wild's team built several solar radio-observatories, including a radio spectrograph that pointed at the Sun 12 hours a day from Dapto and a huge radiotelescope near Narrabri at Culgoora Observatory—a radioheliograph comprising of 96 antennas in a three-kilometer ring that gave astronomers a look at the Sun's corona.

Total solar eclipse of June 1973 showing the Sun's corona (upper atmosphere), photographed at Loiyengalani, Kenya, by a research team from the High Altitude Observatory of the National Center of Atmospheric Research, Boulder, CO, USA. (CSIRO Archives)

The Australian team jump-started solar research using radio astronomy techniques, specifically to study the solar corona. Their first major discovery was reported in 1950; using the spectrograph they had discovered three types of solar burst (Type I, Type II, Type III) which is the now-accepted international standard. With the radioheliograph a decade later, they observed the mysterious solar corona—the Sun's upper atmosphere that extends millions of kilometers into space. Phenomena in the corona are invisible to optical telescopes so had not been studied by the 1960s. Keeping in mind that sunspots are regions of intense magnetic field, the team devloped the telescope such that oppositely polarized radiation (think of sunspots as opposite poles of a magnet) would be recorded separately. When the recorded data were superimposed, an image of the Sun was generated, showing unpolarized regions in white. As a result of this work in radio astronomy, the foundation for solar plasma research was set in place.

The Sun's disk, or chromosphere, photographed by the CSIRO Solar Observatory telescope. (CSIRO Archives)

The invisible Sun has long been illuminated by the field of radio astronomy. Since the field's humble of beginnings by amateur astronomers, solar radio astronomy has grown into a science that sheds light on our closest star. A solar eclipse, during which the Moon obscures not only the Sun's visible light but also some of its radio waves, provides an excellent opportunity to study the Sun and its corona. Plenty of professional astronomers and amateurs alike will certainly have their (properly protected) eyes—and dishes!—on the sky.

References

• Hewish, Antony. "James Stanley Hey, M.B.E. 3 May 1909 – 27 February 2000. Biographical Memoirs of Fellows of the Royal Society, 2002, 48, 167-178
• "John Paul Wild [1923-2008]." CSIROpedia
• "New radio waves traced to centre of the Milky Way." The New York Times, 5 May 1993
• Penley, Bill. "Dr James Stanley Hey MBE FRS." Purbeck Radar, Purbeck Radar Museum Trust. January 2011.
• "Radio Astronomy — Observing Explosions on the Sun." CSIROpedia
• Stokely, James. Atoms to Galaxies: An Introduction to Modern Astronomy. New York: Ronald Press, 1961, 37-39

Edits: A previous version of this post incorrectly listed the date of the eclipse as "October 21" instead of "August 21".