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Heliophysics Background By Katie Whitman Center for Computational Heliophysics in Hawaii (C2H2) http://c2h2.ifa.hawaii.edu History of Solar Discoveries Sunspots The first recorded observations of sunspots were made by Chinese astronomers. The two earliest records of sunspot observations are found in a Chinese book, the Book of Changes, dating back to about 800BC or earlier. Much more complete Chinese records began in 165BC. (Source: http://www.cora.nwra.com/~werne/eos/text/galileo.html) The first sunspot drawing that still exists (above) can be found in a book called the Chronicles of John of Worcester. The observation dates to 8 December 1128, and shows the sunspot's umbra and penumbra. The accompanying text reads, "...from morning to evening, appeared something like two black circles within the disk of the Sun, the one in the upper part being bigger, the other in the lower part smaller. As shown on the drawing." (Source: http://www.cora.nwra.com/~werne/eos/text/galileo.html) Observations of the Sun through telescopes began in 1610. Galileo noticed dark spots on the Sun one misty evening at sunset and became curious. Galileo used these spots to calculate the rotation rate of the Sun, coming up with a rotation period of about 25 days. Astronomers throughout Europe were doing the same thing with their telescopes, tracking and recording these dark blotches marring the solar surface. (Sun Kings, p. 28) But what were sunspots? Christoph Scheiner from Germany thought that they were silhouettes of undiscovered planets. Galileo showed that this could not be true because of the strange way that sunspots moved. Galileo reasoned that this was the behavior of something affixed to the surface of a spinning ball. He could also see sunspots growing and shrinking in size, behavior that could not be attributed to a planet. The issue of sunspots and what they were provoked a somewhat emotional response at the time, as the prevailing belief was that the heavens were perfect and unchanging, a reflection of God. Galileo thought they were dark clouds in the solar atmosphere, other astronomers thought that it was dark slag (the leftovers from smelting metal) on the top of a gigantic natural furnace. In the 1700’s, Newton wrote that the Sun and Stars were “great Earths vehemently hot.” Leading people to think that there was a planet inside the Sun. People then thought that the spots were smoke preceding volcanic eruptions, or mountain revealed by the ebb and flow of the Sun’s fiery oceans. It was difficult to observe the Sun through the telescope, as it was likely an astronomer could go blind if not done properly. Larger telescope focused more light, even making it harder. William Herschel made an unpolished mirror that naturally scattered light, allowing for his solar telescopes to hold larger mirrors and see higher levels of detail. He saw that Sun’s surface looked more like an orange peel and that the black spots were actually depressions in the surface. This lead him to believe that they were openings, allowing astronomers to see the dark surface of the Sun below. (The Sun Kings, p. 30) He thought that the Sun’s atmosphere consisted of a transparent layer and a bright layer. He didn’t know why the atmosphere might glow, but he pointed to the aurora seen on Earth. He also thought the Sun was richly stocked with inhabitants. He also used the fact that mountain climbers experienced a drop in temperature as they increased in elevation (close to the Sun) to assert that the Sun was not actually hot. The Sun is Hot Eventually Herschel’s mind was changed when he tried placing different colors of glass in the light path of his telescope in an effort to reduce the light. He noticed that red glass stopped most of the light, but made the eye feel intolerable hot. (p. 34) Trying different colors lead him to the realization that not all colors created equal quantities of heat, which was the prevailing belief at the time. Some scientists believed he was a rambling fool, but after a controlled experiment in which Herschel displayed the Sun’s light on the wall through a prism and measured the temperatures of the different colors (while another thermometer in the room measured the ambient temperature), did people believe him. He even showed that the hottest part of the spectrum was an invisible area next to the red color. Image Source: http://www.rudraveena.org/images/spectrum.jpg In 1814, Joseph von Fraunhofer rediscovered the dark lines in the Sun’s spectrum. Inventing the spectral grating in 1823, Fraunhofer cataloged solar spectral lines with great precision (The Sun Kings p. 93‐93). Fraunhofer died at just 37 years old, but John Herschel (son of William Herschel) and William Fox Talbot realized that elements gave off unique spectral patterns. Kirchhoff and Bunsen created pure samples for flame tests. In 1859, Kirchhoff figured out that a hot solid dense object will give off a continuous spectrum of light (image below); a hot tenuous gas will produce an emission line spectrum; and a hot object with a cooler tenuous gas around it will create a spectrum with dark absorption lines. Because they identified the elements on the Sun’s surface and it was well known that these metals could only be in a gas form at very high temperatures, the Sun had to be incredibly hot (The Sun Kings, p. 95 – 97). Image Source: https://www.e‐ education.psu.edu/astro801/book/export/html/1549 Solar Variability Going through his records in 1801, Herschel noticed that the number of sunspots seemed to come and go. He looked through scientific journals and identified possible periods over which he saw this variability. (The Sun Kings, p. 36) The Sun and Climate or How Sunspots Affect the Sun’s Temperature Herschel became interested in the Sun’s affect on climate. He didn’t have long‐term solar records, but he had a genius stroke to look at the price of wheat as a proxy for temperatures. Around 1802, he thought that years with many sunspots (when the Sun was supposedly cooler) would have higher wheat prices, but he actually found the opposite. (The Sun Kings, p. 37) Herschel then flipped his original idea about sunspot, now proclaiming that transparent clouds of hot gas were welling up in the Sun and the sunspots were the result of this emission. Those in the scientific community either did not listen or chose to criticize and even ridicule him. The Edinborough Review stated that Herschel’s speculations were “all eclipsed by the grand absurdity which he has there committed; in his hasty and erroneous theory concerning the influence of the solar spots on the price of grain. Since the publication of Gulliver’s voyage to Laputa, nothing so ridiculous has ever been offered to the world.” The Beginning of the Sun­Earth Magnetic Connection In 1802, Alexander von Humboldt was traveling in Peru, concerned about the effects that land clearing and agriculture had on the climate. In Venezuela, he arrived at a community that was wondering why the water levels in their lake were dropping. Humboldt investigated and found that the forests they had been clearing helped trap moisture and increase rainfall. Humboldt was exploring, but he also had the position of the Earth’s magnetic equator. He measured the orientation of the Earth’s magnetic equator at the geographic equator. Back in Berlin in 1806, Humboldt began monitoring the daily movement of magnetic needles. On the 21st of December, Humboldt saw his magnetic needles varying wildly and noted that aurora were in the sky. In the 1740’s, Hoirtier and Celsius had observed this same phenomenon. In 1827, Humboldt organized a series of magnetic observatories around the globe (p. 52). He convinced Carl Friedrich Gauss to participate in the project. Humboldt also worked with Russia and the British Empire, which held land across the world. Colonel Edward Sabine was also greatly interested and joined the effort. He tirelessly fought for expeditions to the southern hemisphere. In 1840, magnetic stations were founded at Greenwich, Dublin, Toronto, St. Helena in the South Atlantic, the Cape of Good Hope, Van Dieman’s Land, Madras, Simla, Bombay, and Singapore. (The Sun Kings p. 57) Heinrich Schwabe began observing sunspots every day since 1825. He noticed a pattern and, by 1843, had enough to see a repeating pattern (The Sun Kings, p. 69). Humboldt recognized the importance of being able to predict sunspots and published it in his book Kosmos. Sabine’s wife was translating this book and George Sabine noticed the patter immediately. He saw that magnetic storms and sunspot numbers moved in lockstep. (The Sun Kings, p. 69) In 1852, Edward Sabine showed that global magnetic fluctuations are synchronized with the Sun’s 11 year cycle. (The Sun, Earth and Sky) Solar Differential Rotation Richard Carrington of England set up his own observatory at Redhill, Surrey. He was an extremely careful observer whose northern star catalog was published by the British admiralty with public funds because of its importance for navigation. Starting in 1853, Carrington applied his detail‐oriented technique to observing sunspots, trying to determine the Sun’s rotation period once and for all, since astronomers could not seem to agree on a value, finding somewhere between 25 – 28 days (The Sun Kings, p. 77). In 1858, Carrington had enough information to show that spots at higher latitudes moved more slowly than those at the equator. This differential rotation was proof that the Sun was a gaseous body and not a solid one.

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