The solar Zeeman effect

Spectral lines split in a magnetic field. This is known as the Zeeman effect. When Peter Zeeman presented his discovery in late 1896, he ended his paper with a speculation about the Sun. Thirty years of solar observations already contained the answer. This is a sunspot, a region on the solar surface roughly ten times the size of the Earth, where intense magnetic fields suppress convection, dropping the local temperature, and by the Stefan-Boltzmann law, making it appear darker than the surroundings. Many questions about sunspots remain open. We don't fully understand what drives their magnetic behavior, why they form where they do, why they run on 11-year cycles. Why? Why? Tell him. We don't know! In the early 1800s, astronomers didn't even have the basic mechanism for these mysterious dark patches on the Sun. One prominent idea was that sunspots were gaps in the outer layer exposing a darker, cooler interior. William Herschel, the astronomer who had discovered Uranus and mapped the structure of the Milky Way, referred to sunspots as openings of the Sun revealing its solid body, and suggested that sunspots were the consequence of volcanic explosions creating these openings. Herschel promoted a version of the Sun that was closer to a planet than a star, with a cooler interior that could possibly host solar life. A competing hypothesis was the idea that the darkness of the spots was due to the cooling effects of a descending current from the upper layers. The development of new astronomical instruments and spectroscopy as a tool for chemical identification during the 1860s led to the study of spectral lines around sunspots as an attempt to distinguish between the two popular hypotheses. The British astronomer Norman Lockyer is mostly celebrated for his discovery of helium in 1868 and as the founding editor of the journal Nature the following year. Remember the editor praising teenage Peter Zeeman, calling him professor, in 1883? But in the years before his best-known activities, Lockyer, already a skilled solar asbestoscopist, wanted to answer why sunspots were dark and made a significant discovery. In 1866 he communicated to the Royal Society the first observations of the spectrum of a sunspot. He could not settle the debate but he did find a surprise. He wrote, all the spectral lines visible in the spectrum of the photosphere were visible in the spectrum of the spot. They, moreover, appeared thicker where they crossed the spot spectrum. Although he showed no interest in the fact that spectral lines became wider when coming from the spot, today we know that Lockyer was the first person to observe the Zeeman effect in nature, 30 years before Zeeman. In his 1881 book The Sun, the American astronomer Charles Young included a brief section about the spectrum of sunspots, which he described as presenting unusual and distinctive features. In particular, he described one of his own observations from 1870 of the lines of several elements appearing thickened and especially the sodium d lines appeared widened and doubly reversed. Reversed is the name used in asbestoscopy for a line that appears split into three. He included the first published graphic illustration of what we now recognize as the Zeeman effect. Similar findings were reported by the Jesuit astronomer Eleusius Corti observing sunspots from northern England. He wrote in 1886 that the sodium d lines were not only widened and fuzzy, but were also reversed in one portion of their length. By the early 1890s, the splitting of spectral lines in sunspots into doublets and triplets was a phenomenon that solar astronomers had documented repeatedly, but without knowing what caused it. Astronomers were trying to understand properties like the structure and temperature of sunspots, and the connection between their observations of the sun and developments in fundamental physics was not obvious at the time. The astronomers found it before Zeeman. Without knowing it, they had access to powerful magnets. In the previous video, I presented the origin story of the Zeeman effect, including Faraday's failed attempt to detect any changes in the spectral lines of a flame between the poles of an electromagnet, how Zeeman's first attempt also failed, and his subsequent success with better equipment. However, almost a decade before him, the Scottish physicist Peter Tate also repeated Faraday's last experiment. He also failed to detect the effect. The first observation of the widening and splitting of the sodium d lines under a magnetic field in the lab was made by the Belgian astronomer Charles Feeve in 1885. Unfortunately, he incorrectly concluded that the magnetic field acted like a temperature effect. He reported his observations, but since his findings were not reproducible and somehow contradictory, his two papers went largely unnoticed. Finally, in 1896, Zeeman, working in Cameron Owens's lab in Leiden, succeeded at detecting the effect. Using better instruments, he found that the sodium lines widen and split under a magnetic field. Lawrence, on a Sunday afternoon, developed the theoretical explanation and, after all the predictions were confirmed, the two shared the Nobel Prize in Physics in 1902. When Zeeman reported his result in late 1896, he ended his paper with a speculation. Further inquiry must also decide as to how far the strong magnetic forces, existing according to some at the surface of the Sun, might change its spectrum. An English translation of this paper appeared in the Philosophical Magazine in March 1887, and I am convinced that this last sentence was the reason for the same paper to be reprinted in the United States in the Astrophysical Journal. The journal had been established only two years before by the American astronomers James E. Keillor and George Ellery Hale. Keillor was famous for his confirmation of Maxwell's hypothesis that Saturn's rings were not solid, but composed of small particles. And Hale was the mastermind behind the planning and construction of several world-class telescopes, raising the funds himself. As an easter egg, the name George E. Hale was the alias of a popular TV character. Hale's major scientific contribution at the time was the invention of the spectroheliograph, an instrument to examine the Sun at a very specific wavelength. He had been studying sunspots for years, and was the one who would eventually connect the decades of peculiar spectra from sunspots with the Zeeman effect. Hale read Zeeman's paper, and for a solar astrophysicist, that final sentence was impossible to miss. At Princeton, a doctoral student of Charles Young, named Walter Mitchell, presented his thesis in 1906 dedicated to the spectral analysis of sunspots. Mitchell classified the many features of lines from spots and noticed that the so-called double reversals, or lines splitting into triplets, were common for spots observed near the solar limb. To properly image the structures around sunspots, astronomers needed to isolate a single spectral line, called the hydrogen alpha line, deep in the red part of the spectrum. Unfortunately, the photographic plates of the time were not sensitive in this region. During 1907, at Mount Wilson, Hale and his team developed a method to make those observations possible. Using a new chemical treatment of photographic plates, they could now photograph patterns and structures never seen before. Hale found that sunspots were surrounded by swirling structures, which he called vortices. In June 1908, Hale reported some of the shared properties of these so-called flocculi and sunspots. Out of his many conclusions, one is particularly interesting. The distribution of the hydrogen flocculi frequently resembles that of iron fillings in a magnetic field, which he extended with the following idea. If these vortices are made of ionized gas, the rotation would produce a magnetic field in the sunspot. Today we know that the vortices are not the cause of the magnetic fields, but the other way around. Despite having the wrong reasoning, Hale arrived at a correct conclusion. Sunspots have strong magnetic fields. He quickly recognized that the presence of a strong magnetic field would split spectral lines due to the Zeeman effect. I remind you that according to the Lorentz model of the Zeeman effect, the unpolarized light of a spectral line is produced by the vibrations of electrons in the gas. In the presence of a magnetic field, observing perpendicular to the field, the line splits into three linearly polarized lines. When observing parallel to the field, the line splits into two circularly polarized lines. This is why Hale wrote, When observed along the lines of force, many of the lines in the spot spectrum should be double. Hale ended his paper with an assignment. It must be determined whether the components of these double lines are circularly polarized in opposite directions. I shall attempt the necessary observations as soon as a suitable spot appears on the sun. Only four days later, Hale did the crucial observations. His key insight to test whether the Zeeman effect was responsible for the peculiar line splittings was that for sunspots near the center of the sun, the hypothetical magnetic field would be parallel to the observer, whereas for sunspots near the solar limb, the magnetic field would be perpendicular to the observer. Just like Zeeman, Hale used an optical instrument called nickel prism to analyze the polarization of light. Rotating a nickel prism varies the transmitted intensity in a way that depends on the polarization state of the light. In particular, for the doublet, the two lines should change in relative intensity as the nickel is rotated. He failed to properly identify the polarization of lines from spots near the solar limb, but near the center, he detected the expected doublet, so he could perform his experiment. As he reported on July 3rd, the relative intensities of the two components reversed by rotating the nickel. This is the fingerprint of two circularly polarized lines in opposite directions. He confirmed this result with more than 30 lines, and he concluded that the edges of these spot lines are circularly polarized in opposite directions, similar in character to those detected by Zeeman in his first observations of radiation in a magnetic field. He performed many checks to make sure that no other phenomenon was producing the effect, but concluded that the only means of transforming a single line into a doublet, having components circularly polarized in opposite directions, is a strong magnetic field. It thus appears probable that a sunspot contains such a field, which gives rise to doublets and to widened lines in the spot spectrum. He also realized that the Zeeman effect perpendicular to the field explains the line splittings observed by Mitchell near the solar limb. This is how, in the summer of 1908, George Ellery Hale found the first magnetic field beyond Earth. Once he became convinced that sunspots have strong magnetic fields and that the Zeeman effect was responsible for all the unexpected features in their spectral lines, Hale ordered a strong electromagnet and set up his own Zeeman effect apparatus in the controlled environment of his lab to calibrate what he was seeing on the sun. I found a photograph of Hale's solar lab that shows many optical instruments, and there in the foreground is the electromagnet, identical to the one used by Zeeman. Hale submitted his results for publication on Friday, July 3rd, and the following Monday he sent a letter to Zeeman including a copy of his manuscript and a couple of photographs of the effect in the solar spectrum. Hale also asked Zeeman to send a note to Nature sharing his views on the discovery. Zeeman expressed his opinion in a brief note in Nature, published in August. He wrote, He ended his praise to Professor Hale's splendid discovery, noting, As an aside comment, in one of those rare coincidences, I noted that the paper right after Zeeman's note in Nature announced the great experimental achievement of liquid helium by Heike Kamerlingh Onnes, Zeeman's former supervisor in Leiden. This is the work that gave Kamerlingh Onnes the Nobel Prize in Physics in 1913 and helping also discover superconductivity. The strange behavior of liquid helium at very low temperatures is closely related to Bose-Einstein condensation, but that's another video. Up to this point, Hale confirmed the Zeeman effect only parallel to the magnetic field. He followed up with daily observations to test the Zeeman effect perpendicular to the field. By the end of the summer, he succeeded. On September 21, Zeeman received a telegram from Hale simply stating, With this observation, all the predictions of the Lorentz model were now confirmed in In October, Hale published an extended paper with all his observations titled On the Probable Existence of a Magnetic Field in Sunspots. He included the dedicated lab measurements used for calibrations and estimated the strength of the sunspot's magnetic field ranging from 2,000 to roughly 5,000 gauss, which is thousands of times stronger than Earth's magnetic field. Hale is considered one of the pioneers of solar physics. His discovery of magnetic fields in sunspots was just the beginning of a long list of discoveries, including that sunspots always come in pairs of opposite polarities, that polarities are reversed in opposite hemispheres, and that sunspots follow the now called Hale cycle, a period of 22 years, twice the sunspot cycle, for the magnetic polarity to return to its original state. The Zeeman effect is a robust mechanism for directly detecting and measuring stellar magnetic fields. The first detection beyond the sun came in 1947. During the 1970s, astronomers used the Zeeman effect to detect fields of up to millions of gauss in white dwarfs. X-ray and radio bands revealed that neutron stars possess fields reaching trillions of gauss. Zeeman speculated in 1896 that solar magnetic forces might affect the sun's spectrum. Hale confirmed it in 1908. Hale never stopped building. His last telescope, the 200-inch at Mount Palomar, now bears his name. Zeeman had been fascinated by the aurora since he was a teenager, when he sketched it and sent the drawings to nature. And in 2025, NASA launched a set of satellites to map exactly those atmospheric disturbances named after the effect. The Electrojet Zeeman Imaging Explorer.