Connecting Light Emissions from Aurora and Other Astronomical Phenomena to Chemistry Courses

The year 2024 has continued to be outstanding for even casual observers of astronomical phenomena. There are opportunities to connect these phenomena to chemical concepts. Chemistry connections to the April, 2024, total solar eclipse were already discussed in a previous post.^1,2 As noted in that post, many observers saw eruptions of gas on the surface of the sun, known as solar prominences, during totality of the eclipse.³ These appeared as small reddish dots at the edge of the moon shadow. Their reddish glow was largely due to the excited hydrogen that they contained. The hydrogen-alpha electron energy transition (n=3 to n=2) used extensively in astronomy produces red light with a wavelength of about 656 nm.⁴ I have described seeing the prominences during totality and their reddish glow due to hydrogen while showing gas discharge tubes in class⁵ and STEM outreach events. However, not very many people saw the prominences during the eclipse, and total eclipses are rather rare phenomena at any given location. Additional chemistry connections can be made using other astronomical phenomena. These phenomena include the aurora, also known as the northern and southern lights.

An excellent resource for both learning about and also tracking the aurora is the Aurora Dashboard (Experimental) and associated webpages hosted by the National Oceanic and Atmospheric Administration Space Weather Prediction Center.⁶ Aurora are formed when particles produced by the Sun, e.g., from outbursts such as coronal mass ejections, reach the Earth’s magnetosphere. Electrons can be knocked out of the Earth’s magnetosphere and channeled towards the poles. These electrons strike nitrogen and oxygen-based species in the upper atmosphere, causing them to produce visible light. The air becomes denser as altitude decreases, and the collisions producing the glow in the atmosphere do not reach the surface of the earth. A very common color associated with the aurora is green. Another, somewhat less common color, is red. Both colors are produced by excited oxygen atoms, which are much more common in the upper atmosphere than near the Earth’s surface where molecular oxygen dominates. The electronic states associated with this light production are described in Figure 1. This figure shows an image of a PowerPoint slide that I showed to my General Chemistry 1 course and my Environmental Chemistry courses. The left side describes the orbital diagrams for oxygen atom electrons in the ground and two excited states. The right side shows a picture of aurora recently viewed from near Peoria, IL. An electron striking an oxygen atom can excite its electrons from the ground state (a triplet state with the term symbol ³P) to an excited state such as a singlet state. Both the ground and excited states shown in Figure 1 all have the electron configuration 1s²2s²2p⁴, however, the excited states have unusual electron arrangements. In the ¹S singlet electron arrangement, one of the unpaired spins is “spin down”, as opposed to the typical “spin up” for a ground state unpaired spin.⁷ In the ¹D singlet electron arrangement, the four electrons in the 2p orbitals are all in two pairs, rather than having two paired and two unpaired electrons like the ³P ground state or the ¹S excited state.⁷

When an atom with the excited ¹S singlet state converts to the excited ¹D singlet state, green light is produced, often at altitudes of 120 to 400 km. This is a forbidden transition, but it happens somewhat quickly, with an excited state lifetime of about one second.^6,8 As a result, the green color is most commonly observed with aurora. When an atom with the excited ¹D singlet state converts to the ground ³P triplet state, red light is produced. But why does the red tend to appear higher in the sky than the green? That is not an illusion. It takes the ¹D state a relatively long time (often longer than 150 seconds) to undergo the forbidden transition to emit red light. That is such a long time that the excited state often loses energy via collisions with other atoms before it can emit light. Way up in the atmosphere (above 300 km) there are fewer atoms for collisions, so the ¹D state can last long enough to emit red light. However, at that altitude the number of available oxygen atoms is very small, so observations of the red color tend to require the higher electron flux of more intense aurora.^6,8 Other colors can be sometimes be observed from aurora, e.g., blue and red from N₂⁺.⁸

Figure 1. Slide describing origin of aurora colors from atomic oxygen and aurora visible from Central Illinois. Note the red color is higher in the sky than the green color.

As noted above, the Aurora Dashboard (Experimental) and associated webpages hosted by the National Oceanic and Atmospheric Administration Space Weather Prediction Center is a good resource for tracking the aurora. A key parameter to track is the Kp value. NOAA describes the Kp as “…a common index used to indicate the severity of the global magnetic disturbances in near-Earth space. Kp is an index based on the average of weighted K indices at 13 ground magnetic field observatories. It is based on the range of the magnetic field variation within 3 hour intervals that is caused by phenomena other than the diurnal variation and the long-term components of the storm time variations. The values of the Kp range from 0 (very quiet) to 9 (very disturbed)…”.⁶ The greater the Kp value, the more likely they will be visible at lower latitudes. Here in Central Illinois, it takes Kp values of at least 5 or 6 to pique my interest as being possibly visible. The Aurora Dashboard displays recent Kp values as a Planetary K Index bar graph that updates every three hours (note the times are Universal Time). The challenge is that the data are looking backward to what has happened and not necessarily what will happen. The Dashboard forecasts upcoming Kp values in a number of ways, with varying degrees of reliability. Satellites can detect solar wind parameters to give good estimates of aurora intensity with only 15-45 minutes of lead time. The Dashboard also has the 3-Day Forecast, projecting Kp values at three-hour intervals (in Universal Time) for about three days ahead. In my opinion, this forecast has some value in noting how high or low the Kp values will be, but the timing of when that will happen can be challenging. I have seen peaks in solar activity arrive one or two days later than originally forecasted by this Dashboard product. The delayed arrival of coronal mass ejections in Earth’s space weather is reminiscent of the delayed arrival of warm and cold fronts locally in earthly weather. Another forecast product on the Dashboard, the Viewline Forecasts for Tonight and Tomorrow Night, faces the same challenges as the 3-Day Forecast – a somewhat useful general guide, but be ready for possible delays. My general strategy is to use the Viewline and 3-Day Forecasts to advise me when something might happen, but then I monitor the Planetary K Index graph to track when the Kp values really begin to rise. Solar storm chasing this way is imperfect at best, and I welcome advice on how to do it better.

If the forecast seems right, get to a place with the darkest, clearest northern view. Maps showing possible dark sky locations are available online.⁹ Once there, settle in to watch the fun. Even if nothing seems obviously visible, try using a cell phone camera to hunt for aurora. Some cell phone cameras do an excellent job sensing low light intensity. On my phone (an iPhone 13 mini), I have pushed the camera sensitivity even further by boosting image saturation as far possible to detect weak green aurora signals. Be advised that aurora intensity can vary quite a bit over shorter timescales than three-hour intervals. For example, before I shot the picture shown in Figure 1, I had viewed a half-hour of less-intense aurora from the darkest location that I could find. When those appeared to taper off, I headed home. When I was almost home, I turned northward briefly and was surprised to see aurora that were so bright that I could see them with my car headlights on.

Meteors are astronomical phenomena that can also be connected to light emission of chemical species. I have only ever observed white and green meteors and fireballs, but other colors can be observed. As the rocks from space heat up from friction with the atmosphere, vaporized metals like Na, Mg, Ca, and Fe produce colors of light in the yellow to violet range.¹⁰ Excited atmospheric gases like nitrogen and oxygen can produce red and green colors.¹⁰

Comets are astronomical phenomena with multiple chemistry connections. Comet Tsuchinshan-ATLAS was a comet recently visible to the naked eyes of many observers. At times the comet’s tail was estimated to be 18 million miles long (29 million km).¹¹ This is nearly one-fifth of the 93 million mile (150 million km) distance from the Earth to the Sun.¹² When placed in the perspective of the Community Solar System model centered in Peoria, IL, where the scaling factor is 99,000,000 to one,¹³ the 29 million mile tail becomes 0.29 km, or 290 m long. The planet Earth is approximately 13 cm in diameter in this model.^13,14 Figure 2 shows two views of Comet Tsuchinshan-ATLAS. Possible chemistry connections can be made with the composition of the comet, which can by analyzed spectroscopically. A typical emission spectrum of a comet can be found in the literature.¹⁵ The spectrum has a curved baseline produced by reflected sunlight from dust in the comet. The spectrum also has sharp peaks corresponding to emissions from specific chemical species, e.g. hydroxyl radicals. I brought these spectral features to the attention of my Environmental Chemistry class, since we cover the backbody emission curve from the sun and we also discuss hydroxyl radicals in atmospheric chemistry.¹⁶ Similar to hunting the aurora described above, using my cell phone camera and image saturation really helped me to search for the comet even when I could not see it with my eyes.

Figure 2. Views of Comet Tsuchinshan-ATLAS 

There has been much information made available to the public about these and other astronomical phenomena – certainly more than can be fit into this blog format. I hope you might have found something here to help connect these phenomena to your chemistry classes and outreach events.

Safety Exercise caution when working in the dark outside and make sure that you are visible to other people (especially car drivers) nearby.

Supporting Information

I have attached the Figure 1 PowerPoint slide of the aurora that I have shown in class in the Supporting Information below.

Acknowledgements I thank Karen, Kristine, and Katie Campbell for accompanying me on many trips in the dark to watch the sky with me. This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium. The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA. This work was supported through a NASA grant awarded to the Illinois/NASA Space Grant Consortium.

References

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