Folklore for the Aurora and Beautiful Medieval Art

Madison Rae Smith ASTR 101 Honors

The Aurora

The aurora has awed, inspired, and terrified humans throughout our history on this planet. On both North and South poles we have an aurora; in the northern hemisphere we call it the Aurora Borealis or the Northern Lights, in the southern hemisphere it is known as the Aurora Australis or the Southern Lights. People who lived in the northern or southern most regions of the world were the most frequent witnesses to the auroras and they invented stories to explain their origins or causes. Such myths attributed the strange motion and ethereal light to the dead, mythical beasts, or gods.

Folklore for the aurora and beautiful medieval art: http://www.webexhibits.org/causesofcolor/4C.html

Earliest physical record of the Aurora Borealis is from Babylonian tablet dated from 567 B.C.: http://astrogeo.oxfordjournals.org/content/45/6/6.15.full

Many scientists made educated guesses as to the cause of the strange lights; even Benjamin Franklin had a theory concerning electricity (Scientist). An interesting, but incorrect, early scientific theory as to the origin of the natural phenomenon was that the aurora was caused by florescence in icebergs reflected onto ice crystals in the air. Our uncertainty persisted into the mid-twentieth century before we could make observations in Earth’s upper atmosphere. The first scientist to develop an accurate theory for the auroral phenomena was Norwegian scientist Kristian Birkeland. By leading the Norwegian Polar Expedition from 1899-1900, Birkeland was able to determine the patterns of global electric currents in Polar Regions from ground magnetic field measurements. In the laboratory Birkeland experimented with magnets, cathode rays, and terrellas. He discovered that rings of light form around the magnetic poles of a magnetized terrella when a stream of charge particles is directed at it. His theory was that charged particles from the sun must collide with the Earth’s magnetosphere in much the same manner, causing the aurora. Birkeland’s theory was not very well received and he was not validated until long after his death in 1917. Eventually when a satellite could be sent above the ionosphere with a magnetometer, the results, received in 1967, showed magnetic disturbances in high latitude regions related to Earth’s magnetic poles; proving Birkeland’s theory to have been largely correct. The currents that flow along geomagnetic field lines are now known as Birkeland currents in honor of the Norwegian scientist (Egeland).

Birkeland, along with his terrella and a stylized representation of the Birkeland currents, is portrayed on the 200 kroner:

Early statistical data showed a correlation between the Sun’s magnetic activity (sun spots in particular) and the varying activity of the aurora. It is now known the auroral displays are evidence of the Earth’s magnetosphere interacting with solar wind. The Sun undergoes cycles of activity and we can verify this by monitoring the aurora’s activity. However, even a quiet Sun radiates not only electromagnetic waves, but also particles (Brekke 23). Electromagnetic storms, aside from the impressive aurora they produce, can have real and terrible repercussions on Earth, particularly in our technological age. The Solar Storm of 1859, or the Carrington Event, was caused by a massive coronal mass ejection that disrupted telegraph communications, in some instances shocking operators and causing fires. Auroras were seen as far south as Hawaii, Italy, and Cuba. Another severe storm could potentially affect the health of fauna around the world, along with bringing modern civilization to a halt by destroying transformers and disrupting satellites; severing communications and navigation (Davis 93).

Daily updates for Solar activity are available online: http://www.spaceweatherlive.com/en/news

The basic processes that cause the aurora are these: The Sun is continuously ejecting charged particles (of which are protons and electrons) which stream toward Earth in a flow called the solar wind. As the solar wind approaches Earth, most of the charged particles are routed around Earth by our magnetic field (this region of redirection is known as the Van Allen Belt). The magnetosphere is not a perfect obstacle so some particles manage to enter Earth’s atmosphere and come under the control of the Earth’s magnetic field (and any electric fields there). “The electric fields speed up the charged particles, and the magnetic field guides them into the Earth’s polar atmosphere” (10, Davis). In each hemisphere the auroras occur most often in an elongated strip known as the auroral zone, which encompasses the polar region. The outer edge of the auroral zone, which by its nature is purely a statistical concept, is known as the auroral oval. The brightest aurora is associated with the Birkeland currents, which create the Auroral Oval. The auroral oval is, curiously, is a seemingly fixed to the geomagnetic pole, but is skewed toward the night side of Earth. During times of low solar activity, the oval is narrow or contracted and it expands down toward the equator with greater activity. The Auroral Oval as seen from space:

The light produced by the aurora is an electrical discharge from excited, or ionized atoms in Earth’s upper atmosphere, nitrogen and oxygen in particular. Oxygen emissions are green or reddish-orange depending on the amount of energy absorbed. Nitrogen emissions are blue or red. If a nitrogen atom is ionized and regains an electron then it will emit blue light, red if it is returning from an excited state (Omholt 31). Because of the distribution of certain atoms and molecules in our atmosphere, the aurora takes on particular shades at different altitudes. Green is the most common color seen in auroral zones. Red is more common at the top and bottom edges of an auroral display. All red auroras are usually only visible during large displays when the aurora reaches lower atmospheres. Blue or purple auroras occur when tall auroras are exposed to sunlight or bright moonlight (Davis 10) Interestingly, as humans we did not evolve with eyes sensitive to seeing color at night, thusly the weakest auroras appear colorless. The most accurate way to determine the color of an aurora is to photograph it with a high- resolution camera and a long exposure. Because the emission of the light requires very low-density gas conditions, auroras can never touch the ground. The closer the atmosphere is to Earth’s surface, the denser it becomes. Too many collisions of particles would eliminate the specific electronic transition needed to produce the specific auroral emission lines. The density of the atmosphere near the lower part of the auroral limit is near 70 km, and is nearly the same as what is found inside a neon bulb. At the upper range of the auroral display, 1000 km, the atmosphere is even more rarified (Omholt 87). Over the years there have been many different classification for auroral types. While there are many names applied to auroral structure “-arc, band, drapery, corona, veil, diffuse surface, flaming aurora, pulsating surface, etc.-“ (Davis 43), there are two basic types. There is the discrete aurora, which is sharply defined and easily seen. A second, and important, category is the diffuse aurora; which varies in brightness, but is always dim, and sometimes pulsates. The discrete aurora’s form is very distinguishable; they are always aligned along the direction of local magnetic fields. The lower edge is roughly 80-120 km above Earth’s surface and can extend for hundreds of kilometers. “Discrete auroras are horizontally elongated along magnetic east west and may stretch thousands of kilometers.” (Omholt 101) Despite its massive size, discrete aurora are often very thin, no wider than a hundred meters. Diffuse auroras are sometimes seen from Earth’s surface but are most often viewed by orbiting satellites. When energetic particles are found in the equatorial part of the auroral oval (where the magnetic field is almost dipolar) they precipitate and drift around Earth, electrons to the east and protons to the west, creating the weak and sometimes pulsating glow of the diffused aurora. “The strongest diffuse auroras are found in the post-midnight and produced by electrons” (Space Physics Textbook). While it is obvious that the aurora emits electromagnetic waves at least in the visible, it is also known that x-rays and radio waves are also produced. Auroral backscatter, as the radio transmissions are known, can be tuned into tuned into with a very low frequency radio receiver or listened to online as several websites have streaming feeds.

http://www.aurora-service.eu/radio/

There are also numerous reports of physical noise corresponding to the aurora; enough reports that they have not been dismissed even though we have no current understanding of this phenomenon. Those who have experienced the sounds of the aurora say that it sounds much like radio static or rustling of leaves. One possible explanation of aurorally associated sounds is that low frequencies radio waves propagate to the ground are converted into sound waves by nearby conductors. (Alaska) Earth is not the only planet in our solar system have an aurora, we are one of seven planets that have a magnetic field. Auroras have been witnessed on Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter has a massive magnetic field and extensive radiation belts, these feature produce an impressive auroral display. Additionally, several of Jupiter’s moons (Io, Europa, and Ganymede) have their own aurora. We have learned from the varying planetary forms in our solar system that there are many factors that create an aurora. For example; Jupiter has a very complex magnetic field that is fed mostly by its most active moon, Io. Venusian aurora are not the result of a magnetic field but rather the impact of electrons carried by the solar wind, and the aurora takes on a very diffuse and patchy form in comparison to our well defined auroral oval (Case). Just this year, 2015, the first extra-solar aurora was discovered on brown dwarf LSR J1835-3259, the cause of this extraordinarily bright aurora (more than a million times brighter than the northern lights) is unknown but is perhaps caused by a yet to be discovered moon or planet, similar to Io’s relationship with Jupiter. (Jet Propulsion Lab) Artist’s impression of Brown Dwarf with aurora:

Our appreciation of the aurora’s beauty and the powerful information contained within it’s comprehension is compelling science forward with several cutting edge missions, as well as ongoing research into many of the poorly understood processes that form the aurora. The CAPER rocket is currently in action studying the relationship between the “planetary magnetosphere’s and their ionospheres” (NASA), whose relationship is most easily studied in high-magnetic latitudes. Meanwhile, at UCLA scientists are working to understand what triggers sub storms in space (those same storms that knock out satellites) in order to better defend us from a potential global catastrophe. The aurora is our gateway to comprehension of so many fundamental forces that shape our planet and our solar system. While the fundamental aspects of auroral formation are understood, much is yet to be learned about plasma physics, the acceleration of charged particles in the magnetosphere, or the varying forces that my play into the atmospheric production of auroral light. By exploring the regions of our own planet we can hope to learn more about the rest of our solar system, at the very least! Our understanding of the inner workings and inner connectivity of solar and planetary bodies will light the way to a more complete understanding of the entire universe. Works Cited

Alaska Dispatch News. “Confirmed: Aurora Borealis makes sound”. 10 July 2012. Retrieved 6 December 2015. < http://www.adn.com/article/confirmed-aurora-borealis-makes- sounds>

Brekke, Asgeir. “Physics of the Upper Polar Atmosphere”. Springer Atmospheric Sciences 2013.

Case, Nathan. PHYS.ORG. “What is it like to see aurora’s on other planets?” 10 November 2015. Retrieved 6 December 2015. < http://phys.org/news/2015-11-auroras- planets.html>

Davis, Neil. “The Aurora Watcher’s Handbook”. University of Fairbanks Alaska 1992.

Egeland, Alv. “Kristian Birkeland: the first space scientist”. Journal of Atmospheric and Solar-Terrestrial Physics. September 2009. Retrieved 28 November 2015. http://www.dnva.no/binfil/download.php?tid=44836

Jet Propulsion Lab, CalTec. “Powerful Auroras Found at Brown Dwarf”. 31 July 2015. Retrieved 6 December 2015. < http://www.jpl.nasa.gov/news/news.php? feature=4676>

NASA Sounding Rockets Program Office. “Cusp Alfven and Plasma Electrodynamics Rocket (CAPER)”. Retrieved 28 November 2015.

Omholt, A. “The Optical Aurora”. Springer-Verlag Berlin Heidelberg New York 1971.

"Scientist and Inventor: Benjamin Franklin: In His Own Words... (AmericanTreasures of the Library of Congress)". Loc.gov. 16 August 2010. Archived from the original on 28 June 2011. Retrieved 26 July 2011.

Space Physics Textbook. “Aurora”. 23 November 1998. Retrieved 6 December 2015. < http://magbase.rssi.ru/REFMAN/SPPHTEXT/Welcome.html>