Ruby Payne-Scott and the mystery of sunspots

This post is in belated honor of International Women’s Day 2020, March 8th, and highlights an important woman physicist who I was unaware of until recently!

I think almost everybody is familiar with the phenomenon of sunspots: relatively dark patches on the surface of the sun that come and go somewhat unpredictably and can range in size from diameters of tens of miles to diameters of 100,000 miles.

Sunspots visible during solar eclipse of October 23, 2014. By user Tomruen via Wikipedia.

Sunspots are colder than the rest of the sun’s surface, 3,000-4,500 K compared to the average surface temperature of 5,780 K, which gives them their darker appearance. You may also have heard that a large amount of sunspot activity can have effects on Earth, potentially screwing up our radio communications devices.  But sunspots have also been (and remain so, to some extent) a relatively mysterious feature of the sun. A key piece of the puzzle to explaining what they are and where they come from came from experiments undertaken in the 1940s by a trio of researchers, one of whom — Ruby Payne-Scott — was one of the very first women to work in radio astronomy and an important founding member of that entire branch of astronomy. In this post, we’ll talk about Payne-Scott and her remarkable work on sunspots.

Payne-Scott as a student in the 1930s, photo provided by her son via Wikipedia.

Ruby Payne-Scott was born in South Grafton, New South Wales, in 1912. She was home-schooled until age 11, and then moved to Sydney to live with her aunt, where she eventually earned a spot in the prestigious Sydney Girls High School.  Payne-Scott graduated at age 16 with honors in mathematics and botany, and went on to earn a BSc in physics, MSc in physics, and Diploma of Education at the University of Sydney, completing her education in 1938.

She worked for a year as a grammar school teacher before accepting a job with Amalgamated Wireless, an electronics manufacturer and operator of radio communications systems in Australia.  In August of 1941, she leveraged her experience in radio systems to get a job as a physicist with the Radiophysics Laboratory of the Australian government’s Council for Scientific and Industrial Research. There, she did top secret work for the war effort, improving radar systems so that they could detect Japanese fighter planes.

When World War II ended, the Radiophysics Lab turned to purely scientific pursuits, which is where Payne-Scott’s work on sunspots began.  In 1933, radio astronomer Karl Jansky wrote a paper¹ with a title that would seem very suggestive to modern audiences, “Electrical disturbances apparently of extraterrestrial origin.”  Jansky was not talking about aliens, however, but rather the discovery of natural radio waves that appeared to be coming from somewhere beyond the Earth’s atmosphere. He concluded that the waves were coming from somewhere in the Milky Way galaxy, i.e. outside our solar system in interstellar space. Over the next decade, however, researchers found our sun to be a source as well, though different experimenters disagreed on whether the effect was small, large, or nonexistent.

Trying to resolve this uncertainty was an ideal project for the Radiophysics Lab, in particular Payne-Scott and her colleagues Joseph Lade Pawsey and Lindsay McCready. On October 3, 1945, the trio set up a radio antenna on a 400 foot hill overlooking the sea outside of Sydney. They proceeded to measure the average amount of radio-frequency energy coming from the direction of the sun at a wavelength of 1.5 meters, over the course of some twenty days.

Their key results² are illustrated below. As the upper part of the figure shows, they found very significant variation in the amount of radio emissions from the sun over the course of their month of measurements.  This could explain, for example, why some researchers saw a lot of signal from the sun and others saw little.

Even more significant, however, is the bottom half of the figure. After their experiments were concluded, they compared their radio frequency data with measurements of sunspots taken by Dr. C.W. Allen of the Mt. Stromlo Solar Physics Observatory in Canberra.  As can be seen, the data clearly shows that the total area of sunspots on the surface of the sun correlates quite closely with the amount of radio frequency energy emanating from the sun.  To reconcile their results with the previous work of Jansky, the trio of researchers suggested that the extraterrestrial signals that Jansky had observed were radio signals coming from other stars in the galaxy, not from interstellar space.

Pawsey, McCready, and Payne-Scott had demonstrated that sunspots are a source of radio frequency energy on the sun.  Or had they? They had demonstrated that there is a correlation between sunspots and radio emissions but, as the saying goes, “correlation is not causation.” It was possible that the sunspots and radio emissions were not directly related, but both produced by some other independent phenomenon. Further testing was needed to clarify the relationship.

Pawsey, McCready, and Payne-Scott continued their measurements³ from October 1945 for another six months on a regular basis. One significant addition to their experiment was the taking of data at two different stations, Mt. Stromlo and Dover Heights, separated by some 160 miles.  Comparing the data, they found that both locations recorded the same fluctuations and bursts of radio waves at the same times; this ruled out the possibility that the radio signals were somehow produced locally in the atmosphere.  In a stroke of luck, the largest group of sunspots ever recorded appeared in February 1946, and the radio measurements went off the charts at that time, further strengthening the correlation evidence.

The great sunspot group of 1946. By Nicholson and Hickox*.

What was really needed, however, was a method to determine whether the radio waves were coming directly from the sunspots.  Here, the trio of researchers turned to a powerful tool in any sort of wave physics: inteferometry. When two or more waves overlap, they enhance each other when they are both waving “upwards” at the same time (in phase) and they cancel each other out when one is waving “upwards” and the other is waving “downwards” (out of phase). The observation of interference in light was first recognized by Thomas Young in his famous two-slit experiment in 1804; his original sketch of this experiment is shown below.

Young’s original image of the double slit experiment, showing waves emitting from the two slits combining.

The waves emerging from slits A and B are circular, like ripples spreading out from a stone thrown in a pond. Though the waves are wiggling in phase (wiggling in unison) at the slits, they travel different distances to get to the observation screen on the right, and therefore are either in phase or out of phase depending on the relative difference in their travel.  On a screen placed on the right, one will therefore see alternating bright and dark bands, representing those places where the light is in phase or out of phase. A crude simulation of what would be seen by the eye is shown below; these are called interference fringes.

In an ideal interference experiment, the dark lines have no light in them whatsoever, while the bright lines have four times the light expected from a single pinhole.  In a non-ideal case, the bright spots are dimmer, and the dark spots are brighter; when there is no interference at all, the screen would have a uniform brightness. The contrast of the interference fringes is called the visibility of the fringes.

Ruby Payne-Scott with Alec Little and “Chris” Christiansen at the Potts Hill Reservoir Division of Radiophysics field station in about 1948. Via Jessica Chapman through Wikipedia.

For natural sources of light such as a lightbulb, the sun or a star, it so turns out that the visibility of interference fringes decreases as the apparent size of the source increases. Pawsey, McCready, and Payne-Scott had determined that, with their experimental setup, the sun itself would produce no visible interference fringes, while a relatively smaller object like a sunspot would produce clear fringes. The presence or absence of fringes would therefore give evidence as to whether sunspots or the whole sun were the source of the radio waves, respectively.

Doing an interference experiment with radio waves, however, presented a different challenge for Pawsey, McCready, and Payne-Scott.  They needed a pair of sources, like the two pinholes in Young’s experiment, in order to see interference at all, and in order to see multiple bright and dark fringes, they needed their two sources to have a path length difference of multiple wavelengths. Because their operating wavelength was 1.5 meters, they therefore needed a large separation between these two sources.

Their solution was a clever one:  Since the sun was rising over the surface, and they were on the top of the cliff, their two sources of radio waves would be the waves that directly arrived at their antenna and the waves that first reflected off of the ocean surface, as shown below.

The indirect path is always longer than the direct path and, as the day progresses, the indirect path grows longer. By measuring the intensity of radio waves at the antenna during the course of the day, the researchers would have a record of the interference for a range of path length differences. If they saw fringes, it would indicate a localized source for the radio waves, such as sunspots; if they saw no fringes, it would indicate that the radio waves were emerging over much of the sun’s surface.

Their first measurements seemed to indicate the latter case; however, they later learned that there were sunspots broadly distributed over the surface of the sun at that time, making the “size” of the sunspots comparable to that of the sun itself. At the end of January 1946, though, a “compact sunspot group dominated the sun,” and measurements at that time showed clear interference fringes, as shown below.

The appearance of fringes indicated that the radio waves were coming from a localized part of the sun, and only seemed to appear when sunspots were very localized on the sun’s surface; this was good evidence that sunspots were the origin of the radio signals.

But Pawsey, McCready, and Payne-Scott weren’t done: by making a careful measurement of exactly when the bright and dark fringes appeared in their instrument, they could use a technique called Fourier analysis to estimate where in a horizontal strip across the sun the radio waves appeared to be emanating. As shown in one of their figures below, the horizontal strip (bounded by dashed lines) always included the biggest sunspot groups.

So Pawsey, McCready, and Payne-Scott had provided conclusive evidence that radio waves emanate from the sun and, specifically, emanate from sunspots. This was a major milestone in the development of radio astronomy, setting the stage — and techniques — for future research.

So what does cause radio emissions from sunspots?  Sunspots are regions of the sun where there is an intense concentration of magnetic fields. When charged particles — like the solar plasma — encounter a magnetic field, they are forced to move in a circular or helical path. But particles moving along such a trajectory are accelerating, and accelerating particles produce electromagnetic waves: the radio waves detected by Pawsey, McCready and Payne-Scott. The magnetic fields in sunspots also inhibit heat convection, which results in their reduced temperature and their darker color.

With such incredible discoveries to her name, one might expect that Ruby Payne-Scott had a bright future in physics. Unfortunately, politics and oppressive gender norms of the era ended her career prematurely. In 1944, Payne-Scott, a feminist, atheist, environmental conservationist and communist, married William Hall, who shared her views. But the Australian government had declared that married women were not allowed to work in public service, so the pair kept their marriage secret. Her colleagues helped keep this secret, but in 1950 a restructuring and audit of her department uncovered the marriage, and demanded her resignation. She fought the dismissal fiercely, but in the end was forced to step down from her post.

Pawsey hired her back soon after on “temporary” status, with a raise, but the arrangement didn’t last, no doubt in part due to the bitterness of the original fight. She resigned in 1951, a few months before her first son was born; there was also no maternity leave available at the time.

Ruby Payne-Scott’s story is now a familiar one, as are the obstacles she faced in trying to have a career in science.  Given the incredible work she managed to accomplish before her dismissal, it is certain that science suffered a great loss in her departure.

Her colleagues apparently felt the same way. In August 1952, one year after she left her job, Ruby Payne-Scott participated in the 10th International Union of Radio Science General Assembly at the University of Sydney. It was a chance to visit once more with colleagues that she should have had a lifetime of collaborations with.  Here’s hoping that they let her know how important her research was and how much of an impact it would have on radio astronomy.

Payne-Scott died in 1981. In 2018, The New York Times finally wrote an obituary giving credit to this remarkable woman and her scientific work.

The 1952 International Union of Radio Science conference. Via Wikipedia.

********************************************

¹ K. Jansky, “Electrical disturbances apparently of extraterrestrial origin,” Proc. IRE 21 (1933), 1387.

² J.L. Pawsey, R. Payne-Scott, L.L. McCready, “Radio-frequency energy from the sun,” Nature 157 (1946), 158.

³ L.L. McCready, J.L. Pawsey, R. Payne-Scott, “Solar radiation at radio frequencies and
its relation to sunspots,” Proc. Roy. Soc. A 190 (1947), 357.

* S.B Nicholson and J.O. Hickox, “The great sunspot group of February 1946,” Publications of the Astronomical Society of the Pacific 58 (1946), 86.

This entry was posted in History of science, Physics, Women in science. Bookmark the permalink.

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