B18 rewrite (from C1)
The Mystery Behind the Northern Lights
The composition of aurora borealis became a subject of scientific interest with the introduction of spectroscopy, but for centuries the auroras refused to reveal its secrets. However, after nearly 50 years of research, the understanding of how these auroras come about came in 1912 by a group of spectroscopy researchers (1). Norwegian scientist Lars Vegard suggested a new picture of the atmosphere, which explained the green lights in terms of excitation states of the nitrogen atom (1). The determined wavelength of this green line was 5577 angstroms. However, Canadian physicist John McLellan challenged Vegard’s theory and claimed that the green auroral lines were caused by the transition between two metastable oxygen atoms by experimenting with helium-oxygen mixtures (1). The purpose of this paper is to give an introductive perspective of aurora borealis, the reason why and how they occur, and most importantly, how studies on these lights expanded the field of spectroscopy and helped scientists understand the quantum theory of atoms better. Aurora Borealis is a natural light phenomenon, which occurs mainly in the high latitude areas such as the Artic and Antarctica. There are two types of auroras: diffuse and discrete. Diffuse auroras are those that cannot be seen by the naked eye where discrete auroras are. Auroras are generally seen at night or dusk when the sky is dark. The first sighting of these lights was documented on December 17, 1719 in New England (2). Many people believed the lights were the appearance of the “Son of Man” (2). Regardless of religious stance, these lights were awe-inspiring, and a spellbinding mystery. The beautiful array of colors are caused by charged particles emitted from the sun when solar flares permeate the earth’s magnetic shield and crash into atoms and molecules in the atmosphere (3). These millions of collisions produce little bursts of light called photons that make up the auroras. This phenomenon can be seen in the figure below which captures the entire process (3).
Figure 1: diagram explaining how auroras are produced. The theory behind these lights was a mystery for centuries because the recorded research was in the fields of astronomy, geology, and meteorology, but it did not belong to any of these disciplines. Moreover, geologists were inclined to ignore auroral research because auroras are not geological in nature, while astronomers ignored it due to them not being astronomical in nature. The actual spectra of the auroras were also difficult to obtain because the lights are weak and flicker. It was in 1869 where physicist Anders Angstrom determined the wavelength to be 5567 angstroms (1). Furthermore, auroral research was difficult because the field of spectroscopy was not as expansive and understood as it is today. In the mid 1800s, early spectroscopists had to guess the elements that induced the light phenomenon by comparing the color of lights emitted from known laboratory experiments. However, the lights emitted were different because they are in the upper atmosphere where conditions like temperature and pressure are different. Instead, auroras are understood under the fields of physics and chemistry. Aforementioned, the leading scientist in auroral research was Norwegian scientist Lars Vegard. Although he failed to explain how auroras were produced, Vegard transformed auroral research into a laboratory science. His low temperature experiments mimicked the conditions of the upper atmosphere, and he was able to come up with several theories on the green line wavelength. Vegard mimicked the Earth’s atmosphere by creating a magnetized sphere and then bouncing cathode rays off of it to produce the auroras (1). His results showed that the emitted light from the cathode rays were largely due to nitrogen. Figure 2 below shows some of Vegard’s spectrograms that show the N at 5577 Angstroms.
Figure 2: Vegard’s spectrogram from 1924 which shows N at the 5577 Angstrom band. Concluded that auroras were composed of nitrogen atoms.
The other scientist was the Canadian Physicist John McLellan. Although McLellan did not have previous experience with Northern Lights and no interest as well, unlike Lars Vegard, McLellan succeeded where he failed. McLellan’s experiments determined that the characteristic line of the auroras originated in oxygen, and not nitrogen (1). McLellan was a Canadian of Scottish descent, who studied at the Cavendish Laboratory in 1900 and was the first person awarded a PhD in Physics at the University of Toronto (1). In early 1923, a cryogenic lab with equipment to liquefy helium and hydrogen was built at the university. Here is where the first liquefied helium was produced (1). Armed with a new cryogenic lab that could produce extreme cold temperatures, McLellan and his PhD student, Gordon Schrum, determined that the nitrogen lines were at 5556 angstroms and not 5577. This proved that Vegard’s experiment was wrong. However, they could not explain which elements were at the 5577 band. Schrum finally determined the green line one day in February, but was not able to reproduce it and show it to McLellan. Interestingly, Schrum figured out that it was because the helium he tested was not pure enough. After this finding, he was able to show McLellan the green line and they both determined that the auroras were caused by oxygen and helium in special conditions. To further prove that nitrogen was not a factor, they produced the same conditions in the cryogenic lab with nitrogen and helium, and the green line was not produced (1). Schrum and McLellan were also able to deduce that the upper atmospheric conditions, where the auroras were located, was composed of 5 percent oxygen and 95 percent helium in the temperature range of 300 K from their experiments (1). Their results can be seen below in figure 3 (1).
Figure 3: spectrogram of McLellan and Schrum’s experiment, which shows the oxygen-helium mixture at 5577 angstroms.
The Northern lights are one of mother nature’s wonders, but the research and work that went into understanding them has proven to be far more awe-inspiring. The experiments by Vegard and McLellan/Schrum placed spectroscopy “on the map” in the world of science. Not only did they determine which elements caused the production of these lights, but also they showed how valuable spectroscopy could be, as an analytical tool. References
1. Kragh, Helge. "The Spectrum of the Aurora Borealis: From Enigma to Laboratory Science." Historical Studies in the Natural Sciences 39.4 (2009): 377- 417. Print. 2. Lovering, Joseph. "On the Secular Periodicity of the Aurora Borealis." Memoirs of the American Academy of Arts and Sciences New Series.Vol. 9, No. 1 (1867): 101-20. JSTOR. Web. 24 Feb. 2014.