what we see in the aurora by joseph a. shaborealis (northern lights) and aurora australis (southern...

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What We See in the Aurora By Joseph A. Shaw

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  • What We See in the AuroraBy Joseph A. Shaw

  • O p t i c s & P h o t o n i c s N e w s / N o v e m b e r 1 9 9 9 21

    Of all nature’s optical splendors, the auroramay be the most spectacular. The auroracaptures audiences like few other naturalphenomena because of its relative rarity inpopulous regions, since it occurs primarilyat high latitudes, near midnight. The besttime to see an aurora is during fall, winter

    and spring, when the sun is scarce. For this reason, eventhe large number of visitors to places like Alaska rarelysee auroras because they tend to plan their trips in thesummer, when the sun lights the night sky. The next fewwinters should offer greater opportunities for viewingthe aurora because the eleven-year sunspot cycle, towhich auroral activity is closely correlated, is reachingits maximum.

    The aurora is a particularly spectacular optical phe-nomenon because of its great variety: the aurora’s color,shape, and motion change, sometimes slowly, some-times rapidly, before the viewer’s eye. On some long,quiet Arctic nights, the aurora takes the form of a dimgreen glow on the horizon; on other nights, the colorsof the aurora swirl and dance overhead like a grand bal-let. Even people who have lived long under the aurora’ssplendor pause on such nights to stare skyward.

    What causes the auroraThe aurora1–5 occurs at high latitudes in both theNorthern and Southern Hemispheres. The AuroraBorealis (Northern Lights) and Aurora Australis(Southern Lights) occur simultaneously, and as mir-ror images. Charged protons and electrons in a plas-ma called the solar wind stream out from the sun, andthroughout the solar system, at a speed of about 500-1,000 km/sec. The electrically conductive solar wind isdeflected by the earth’s magnetic field so that it flowsin a long, cylindrical region around the earth; thisregion is called the magnetosphere (Figure 1). Some-times, when the solar wind blows it causes the sun’smagnetic field lines to stretch out and interconnectwith the earth’s magnetic field. Charged particles ofthe solar wind blowing through these interconnectedmagnetic fields generate huge quantities of electricity,up to a million megawatts (the largest man-madegenerators produce about a thousand times less). Theinteraction of the solar wind with the geomagneticfield causes electrical currents to flow in a great cir-cuit, incorporating a vast region of space around theearth. These currents intersect the ionosphere in alarge, nearly oval region centered approximately onthe Geomagnetic Poles.

    At this point the charged particles begin collidingwith atmospheric gas molecules, creating bursts oflight; the colors of these bursts of light are deter-mined by the energy emitted in the collision. Thesecollisions occur as part of a process similar to thatwhich takes place inside a neon light. In a neon signor other low-pressure gas discharge tube, high voltageis applied to the electrodes at each end of a glass tube.The tube contains a small amount of gas at a low levelof pressure, similar to that which characterizes theupper atmosphere where auroras occur. Inside the

    tube, electrons collide with gas molecules as the elec-trical discharge current flows from one electrode tothe other. Light is emitted as a result of these colli-sions; the color of the light is determined by the ener-gy of the process, and therefore by the type of gasinvolved in the collision. The colors of the aurora aredetermined by a similar process, except that in thiscase the electrons strike atoms or molecules of oxygenand nitrogen in the upper atmosphere, and the col-ored patterns wander freely with the geomagneticfield in complex patterns.

    The most common auroral color is pale green(optical wavelength of 557.7 nm), due to emission byatomic oxygen approximately 100 km above the earth’ssurface. Bright red seems to be everyone’s favoriteauroral color, perhaps because it is relatively rare. Themost common red, created by emission from nitrogenmolecules, is a pinkish-red fringe at the bottom of agreen auroral arc. During particularly intense auroralactivity, one might see large regions of bright-redaurora situated above the green emission. This red isemission from oxygen atoms higher in the atmospherethan the green-emitting oxygen. Because of its heightand the intensity of the magnetic storms causing it,this relatively rare bright red color can be seen in mostauroras visible from mid-latitudes.

    Where and when to see the auroraThe best place to see the aurora is from beneath the“auroral oval,” a doughnut-shaped area floating abovethe earth’s polar regions like a loose, rotating crown. It isalways tilted away from the Pole, toward the night side ofthe planet. In the Northern Hemisphere, the oval oftenlies over central Alaska, Canada, and northern Europe(Figure 2). The probability of seeing the aurora decreasesas one travels either south or north of these regions (Fig-ure 3). During intense solar activity, the auroral oval

    Figure 1. The solar wind consists of charged particles (electrons and pro-tons) streaming away from the sun. This stream is deflected by the earth’smagnetic field, creating a long, oval-shaped region called the magnetos-phere, which extends for many earth radii beyond the night side of earthand about 10 earth radii in front of the day side. The interaction of the solarwind and the geomagnetic field comprises a magneto-hydrodynamic powergenerator that is the aurora’s energy source. The inset indicates the electri-cal circuit between the earth and the magnetosphere, intersecting theatmosphere on the auroral oval.

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  • expands, bringing the aurora to locations further souththan usual. Also, the oval drops farther south on the eastside of the United States than it does on the west, so peo-ple living in the northeastern states have a higher proba-bility of seeing auroras than do those in the northwesternstates, cloud cover not withstanding.

    The aurora is most frequently visible at mid-latitudelocations during the maximum of the sunspot cycle,which will take place next around 2000–2001. The strongmagnetic storms required to bring the aurora to themid-latitudes occur as a result of intense solar flares neara large group of sunspots. At high latitudes, the auroratends to occur more frequently in the declining phase ofthe sunspot cycle, in the years just after the sunspot peak.Besides clouds, other factors influencing auroral visibili-ty include interfering light from cities, the sun or themoon. Unfortunately for human observers, the clearArctic nights that are best for viewing auroras are alsooften extremely cold! Because of this, it is nice to have awarm house or a warm vehicle close by—with the lightsturned off, of course. To avoid the intense mid-wintercold, the best aurora viewing seasons are late fall andearly spring, which offer frequent auroras and sufficient-ly dark skies, but usually warmer temperatures.

    Photographing the auroraIf you wish to capture the aurora on film, you will needat least a tripod and a camera with a “bulb” setting thatallows you to lock the shutter open for a long time. It isbest to use a camera not entirely dependent on batterypower; otherwise you will have to find a way to keep thebattery warm. Most modern, electronic 35 mm cameraslast minutes, at best, outside on cold winter nightsbefore the battery freezes. Because of this, I prefer to useolder manual cameras with a shutter-release cable; otheroptions are heating pads or external battery packs that

    can be kept inside your coat.Reasonably fast film and lenses help you capture the

    aurora with minimal blurring, especially during rapid-ly moving displays. The photographs in Figures 4through 6 illustrate the variety of auroral colors andforms, and also give an idea of the photographic tech-nique employed. Figure 4 shows a calm, sweeping,green aurora photographed with a Nikon EL2 cameraand a 28 mm f/3.5 lens in October 1998 at Fairbanks,Alaska (65˚ N); the exposure was 40 seconds onEktachrome ISO 1600 film. Figure 5 is a more dramat-ic aurora, showing vertical ray structure and high-alti-tude red emission: it was photographed with a CanonAE1 camera and a 24 mm f/2.8 lens, on EktachromeISO 1600 film, with a 15 second exposure, in February1990 in Snow Lake, Manitoba, Canada (~55˚ N).

    Figure 6 should provide motivation for mid-lati-tude photographers to watch carefully for auroras dur-ing the next few years. This photograph was taken atNederland, Colorado (~40˚ N), in the Rocky Moun-tains just west of Boulder, on June 4, 1991. The expo-sure (~30 s, f/1.8 on ISO 1600 film with a Canon 50-mm lens) may enhance the color intensity somewhatbeyond what can be seen with the naked eye, but this isan impressive example of the best aurora I have seenphotographed at such southern latitudes. Not only didthis display occur at a modest latitude, it also occurredduring the summer. At the Alaskan and Canadian loca-tions that were the sites of the photographs depicted inFigures 4 and 5, this aurora would not have been visi-ble because of the bright, sunlit nocturnal summer sky.This photograph was taken during the declining phaseof the solar cycle, immediately after the last sunspotmaximum in about 1989–90. Despite its intensity, thisaurora would not have been readily apparent in prox-imity of bright city lights.

    22 O p t i c s & P h o t o n i c s N e w s / N o v e m b e r 1 9 9 9

    Figure 3. Average percentage of nights with aurora over the Northern Hemisphere. To calculate the probability of seeing theaurora, you must also consider the percentage of clear nights andthe phase of the eleven-year sunspot cycle.

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    Figure 2. The aurora occurs within an oval region centered on theearth’s Geomagnetic Poles (both north and south). Shown here is adepiction of the northern auroral oval, extended further south than nor-mal due to modest auroral activity. During periods of quieter solar activi-ty, the oval’s southern edge retreats northward.

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  • The “bad side” of the auroraThe same electromagnetic processes that pro-duce wonderful auroral displays in the skycan create severe problems for human activi-ties ranging from radio communication toelectric power transmission. The incomingcharged particles that cause the auroral colorsalso modify the earth’s ionosphere (an atmos-pheric layer containing charged ions, existingbetween altitudes of approximately 80 and400 km). High-frequency (HF) radio com-munications can exploit the reflectivity of theionosphere to bounce radio waves over dis-tances that are much longer than the allow-able line-of-sight propagation. Indeed, beforecommunications satellites, this was the pri-mary method of long-distance communica-tion. Solar disturbances which lead to auroraalso greatly increase the ion density in thelower ionosphere, resulting in absorption ofradio waves before they can be reflected. Theprocess leading to auroras can also generatelow-frequency radio waves, which are reflect-ed by the ionosphere back into space, creatingan intense radio signal that could be detectedat long distances from earth much more easi-ly than the visible aurora.

    Another problem is what happens to elec-tric power systems as a result of the fluctuat-ing geomagnetic field associated with theaurora. Maxwell’s equations, the mathemati-cal basis of electromagnetic theory, tell usthat an electric field is created by a time-vary-ing magnetic field. Therefore, fluctuations inthe geomagnetic field can induce electric cur-rents to flow in any electrically conductive system thatis grounded at different locations: examples are long-distance electrical power transmission lines andpipelines. During periods of intense auroral activity,the induced quasi-direct current can trip breakers oreven burn out large electrical transformers, as well aspromote corrosion in pipelines. One of the mostfamous examples of this kind of incident took place inMarch 1989, when Hydro Québec lost a major trans-former during a particularly intense aurora: the resultwas a sweeping power outage throughout the regionand the need to replace a very expensive transformer.As an undergraduate student at the University of Alas-ka-Fairbanks, I helped build and install instrumenta-tion to measure geomagnetically induced currents inthe Anchorage-Fairbanks intertie system that allowsthe exchange of power between the two primary popu-lation centers of Alaska. Today, this remains an activearea of research.6

    Teaching about the auroraIf you wish to teach students some basic principles, beginby demonstrating electricity generation with a conductormoving back and forth through a magnetic field. Pointout that in similar fashion, auroral power is generated by

    the motion of charged particles carried by the solar windthrough a magnetic field. In this case, the magnetic fieldarises from the interaction of the earth’s magnetic fieldwith the magnetic field carried from the sun by the plas-ma flow of the solar wind. The main point is that thecharged particles flow across magnetic field lines, therebygenerating electrical energy. Some of this energy thenflows in a great circuit and discharges in the upper por-tions of earth’s atmosphere, creating light.

    The creation of different colors through electric dis-charge can be demonstrated simply by connecting gasdischarge tubes (e.g., neon lights) to a power supply: thecolor produced depends on the type of gas the tube con-tains. Finally, if you have one to spare, an old televisionor computer monitor and a handheld magnet can beused to demonstrate how a moving magnetic fielddeflects the electron beam, resulting in a warped imageon the screen.

    ConclusionThese are just a few examples of what can be seen in thenorthern skies if you brave the cold. If you have alwayswanted to visit the far north or south, I recommenddoing so in the next few years (or 11 years later). Andavoid going in the summer! If you live at the northern

    O p t i c s & P h o t o n i c s N e w s / N o v e m b e r 1 9 9 9 23

    Figure 5. Active green and red aurora in 1990 at Snow Lake, Manitoba, Canada.

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    Figure 4. Mild green aurora in 1998 at Fairbanks, Alaska.

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  • end of the mid-latitudes, keep your eyes on the sky dur-ing clear winter nights, especially over the next few years.Long-term auroral predictions are not practical, so if youplan a vacation to view the aurora, be sure to allow plentyof time. A strong solar wind can reach the earth in abouttwo days, so periods of intense sunspot activity suggestgood auroras within a few days time. If you want towatch for auroras where you live, keep an eye on the solaractivity forecast web page posted by the National Oceanicand Atmospheric Administration’s Space EnvironmentCenter (http://www.sec.noaa.gov /forecast.html).

    AcknowledgmentsI gratefully acknowledge assistance from Professor Syun-Ichi Akasofu, Director of the International ArcticResearch Center at the University of Alaska-Fairbanks,Professor Glenn Shaw of the Geophysical Institute, Uni-versity of Alaska-Fairbanks, and Paul Neiman of theNOAA Environmental Technology Laboratory, Boulder.The graphics in Figures 1–3 were provided by the Univer-sity of Alaska’s Geophysical Institute (Figures 2 and 3 byDon Rice), and the photographs in Figures 5 and 6 wereprovided by Paul Neiman of NOAA/ETL in Boulder.

    I dedicate this article to the memory of ProfessorRobert P. Merritt (1924–1999), who first taught me atthe University of Alaska how the aurora affects electri-cal systems. Any errors in this article are mine, not his.

    References1. S.I. Akasofu, “Why do auroras look the way they do?” EOS

    (American Geophysical Union) 80 (17), 193, 198-99 (1999).2. S.-I. Akasofu, “The Dynamic Aurora,” Scientific American, 260

    (5), 90-97 (1989).3. S.-I. Akasofu, “The Aurora: new light on an old subject,” Sky

    and Telescope, 534-37 (1982).4. S.-I. Akasofu, Aurora Borealis: The Amazing Northern Lights.

    Alaska Geographic 6 (2), (1979; 1994 reprint). ISBN 0-88240-124-6.

    5. T.N. Davis, The Aurora Watcher’s Handbook. University of Alas-ka Press (1992). ISBN 0-912006-59-5 cloth, 0-912006-60-9 paper.

    6. J.D. Aspnes, R.P. Merritt, and B.D. Spell, “Geomagnetic distur-bances and their effect on electric power systems,” IEEE Pow-er Engineering Review 9 (7), 10–13 (1989).

    Web ResourcesAmong the many great web sites for learning about auroras arethose maintained by the Geophysical Institute at the Universityof Alaska-Fairbanks (http://www.gi.alaska.edu/ pfrr/aurora;http://gedds.pfrr.alaska.edu/aurora/); NOAA’s Space Environ-ment Center in Boulder (http://www.sec. noaa.gov/); the Pub-lic Northern Lights Planetarium in Norway (http://www.uit.no/npt/homepage-npt.en.html); and Michigan Technological Univer-sity (http://www.geo.mtu. edu/weather/aurora). Be sure not tomiss the great photographs by Jan Curtis at the University ofAlaska’s Geophysical Institute (http://climate.gi.alaska.edu/Curtis/ curtis.html). Photographs taken by the author andhis colleagues will also be posted soon on the education/out-reach portion of the NOAA Environmental Technology Laboratoryweb page (http://www.etl.noaa. gov).

    Joseph A. Shaw ([email protected]) is an electro-optical engineer for the NOAAEnvironmental Technology Laboratory in Boulder, CO.

    24 O p t i c s & P h o t o n i c s N e w s / N o v e m b e r 1 9 9 9

    Figure 6. Aurora with a large amount of red emission in 1991 at Nederland, Colorado.

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