Heliophysics Background - Solar Discoveries in History, Solar Activity, and the Sun's Effect on...

8/6/2019 Heliophysics Background - Solar Discoveries in History, Solar Activity, and the Sun's Effect on Humans

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Heliophysics BackgroundBy Katie Whitman

Center for Computational Heliophysics in Hawaii (C2H2)http://c2h2.ifa.hawaii.edu

History of Solar Discoveries

SunspotsThe first recorded observations of sunspots were made by Chinese astronomers.The two earliest records of sunspot observations are found in a Chinese book, theBook of Changes, dating back to about 800BC or earlier. Much more completeChinese 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 theChronicles of John of Worcester. The observation dates to 8 December 1128, andshows the sunspot's umbra and penumbra . The accompanying text reads, "...frommorning to evening, appeared something like two black circles within the disk of theSun, 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 thesespots 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 solarsurface. (Sun Kings, p. 28)

But what were sunspots? Christoph Scheiner from Germanythought that they were silhouettes of undiscovered planets.

Galileo showed that this could not be true because of thestrange way that sunspots moved. Galileo reasoned that thiswas the behavior of something affixed to the surface of aspinning ball. He could also see sunspots growing andshrinking in size, behavior that could not be attributed to aplanet. The issue of sunspots and what they were provokeda somewhat emotional response at the time, as the prevailing belief was that theheavens were perfect and unchanging, a reflection of God.

Galileo thought they were dark clouds in the solar atmosphere, other astronomersthought 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 volcaniceruptions, 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 anastronomer could go blind if not done properly. Larger telescope focused more light,even making it harder. William Herschel made an unpolished mirror that naturallyscattered light, allowing for his solar telescopes to hold larger mirrors and seehigher levels of detail. He saw that Sun’s surface looked more like an orange peeland that the black spots were actually depressions in the surface. This lead him tobelieve that they were openings, allowing astronomers to see the dark surface of theSun below. (The Sun Kings, p. 30) He thought that the Sun’s atmosphere consistedof 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 richlystocked with inhabitants. He also used the fact that mountain climbers experienceda 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 equalquantities of heat, which was the prevailing belief at the time. Some scientistsbelieved he was a rambling fool, but after a controlled experiment in which Herscheldisplayed the Sun’s light on the wall through a prism and measured thetemperatures of the different colors (while another thermometer in the roommeasured 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 lineswith 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 puresamples 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 willproduce an emission line spectrum; and a hot object with a cooler tenuous gasaround it will create a spectrum with dark absorption lines. Because they identifiedthe elements on the Sun’s surface and it was well known that these metals couldonly 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 VariabilityGoing through his records in 1801, Herschel noticed that the number of sunspotsseemed to come and go. He looked through scientific journals and identifiedpossible periods over which he saw this variability. (The Sun Kings, p. 36)



The Sun and Climate or How Sunspots Affect the Sun’s TemperatureHerschel became interested in the Sun’s affect on climate. He didn’t have long‐termsolar records, but he had a genius stroke to look at the price of wheat as a proxy fortemperatures. Around 1802, he thought that years with many sunspots (when theSun 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 theSun and the sunspots were the result of this emission. Those in the scientificcommunity either did not listen or chose to criticize and even ridicule him. TheEdinborough Review stated that Herschel’s speculations were “all eclipsed by thegrand absurdity which he has there committed; in his hasty and erroneous theoryconcerning the influence of the solar spots on the price of grain. Since thepublication of Gulliver’s voyage to Laputa, nothing so ridiculous has ever beenoffered to the world.”

The Beginning of the Sun Earth Magnetic Connection

In 1802, Alexander von Humboldt was traveling in Peru, concerned about the effectsthat land clearing and agriculture had on the climate. In Venezuela, he arrived at acommunity that was wondering why the water levels in their lake were dropping.Humboldt investigated and found that the forests they had been clearing helpedtrap moisture and increase rainfall. Humboldt was exploring, but he also had theposition of the Earth’s magnetic equator. He measured the orientation of the Earth’smagnetic equator at the geographic equator. Back in Berlin in 1806, Humboldt began monitoring the daily movement of magnetic needles. On the 21 st of December,Humboldt saw his magnetic needles varying wildly and noted that aurora were inthe 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. Hetirelessly fought for expeditions to the southern hemisphere. In 1840, magneticstations were founded at Greenwich, Dublin, Toronto, St. Helena in the SouthAtlantic, the Cape of Good Hope, Van Dieman’s Land, Madras, Simla, Bombay, andSingapore. (The Sun Kings p. 57)

Heinrich Schwabe began observing sunspots every day since 1825. He noticed apattern 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 andpublished it in his book Kosmos. Sabine’s wife was translating this book and GeorgeSabine noticed the patter immediately. He saw that magnetic storms and sunspot numbers moved in lockstep. (The Sun Kings, p. 69) In 1852, Edward Sabine showedthat global magnetic fluctuations are synchronized with the Sun’s 11 year cycle.(The Sun, Earth and Sky)



Solar Differential RotationRichard Carrington of England set up his own observatory at Redhill, Surrey. He wasan extremely careful observer whose northern star catalog was published by theBritish admiralty with public funds because of its importance for navigation.Starting in 1853, Carrington applied his

detail‐oriented technique to observingsunspots, trying to determine the Sun’srotation period once and for all, sinceastronomers could not seem to agree on avalue, finding somewhere between 25 – 28days (The Sun Kings, p. 77). In 1858,Carrington had enough information toshow that spots at higher latitudes movedmore slowly than those at the equator. Thisdifferential rotation was proof that the Sun was a gaseous body and not a solid one.(The Sun Kings p. 78)

(Image Source:http://www.thenakedscientists.com/HTML/uploads/RTEmagicC_CarringtonObsTNS.jpg.jpg)

Solar Flares and Solar StormsAt 33 years old, Richard Carrington was observing sunspots through his telescope asusual on September 1, 1859. He projected the Sun’s image onto a wall. Wires in theeyepiece cast the shadow of a grid onto the wall. He sketched the sunspots, thentimed them as they crossed the grid lines. At 11:18am, two bright beads of searingwhite light appeared over a monstrous sunspot group (The Sun Kings, p. 12). He wassurprised, but then quickly noted the time,checked that this was not a stray reflection,then ran to find a witness. He returned 60seconds later, but the lights were dimming.They completely vanished at 11:23am. Hesketched the sunspot group and flare location(Image Source: http://astroguyz.com/wp‐content/uploads/2009/08/Carrington_Richard_sunspots_1859.jpg ).

About 18 hours later, the skies around the world burst into massive auroras,telegraph wires became inoperable, giant sparksor streams of fire flowed from the lines into theequipment and even into one poor soul’s forehead(though he recovered shortly after) (The SunKings, p. 83). The New York Times reported that newsprint could be read by the auroral light. Theaurora were seen almost everywhere across theglobe, even as far south as Key West, the Bahamas,and Hawaii. (NY Times article source:



http://query.nytimes.com/gst/abstract.html?res=9F05E6DB1638E033A25750C0A96F9C946892D7CF&scp=27&sq=september+3%2C+1859&st=p )

Up until now, scientists knew that aurora caused magnetic disturbances, but not much more. The magnetic instruments at the Kew Observatory captured the

disturbance of the auroras. September 1st

and 2nd

showed huge deviations in theEarth’s magnetic field (pictured below). (The Sun Kings, p. 19)

Image Source:http://www.geomag.bgs.ac.uk/images/carrington_images/hex572.jpg

Despite Carrington’s observation and the response of Kew’s magnetic instruments,astronomers continued to be wary of the magnetic connection between the Sun andthe Earth. It was an almost unthinkable thought that such a force could act over sucha large distance so quickly. Astronomers of high standing, such as George Airy,discounted this connection, keeping doubt within the community.

In February of 1892, George Ellery Hale, a graduate of MIT and later founder of theAmerican Astronomical Society, was observing the Sun through his homeobservatory near Chicago. He managed to capture a solar flare over a particularlylarge group of sunspots on film. The next day, the sky burst into aurora andcommunications lines were disrupted. (The Sun Kings, p. 150)

In 1872, Edward Walter Maunder was hired as an assistant at the Royal Observatoryin England. Working under the disapproving eye of Airy, Maunder, who was not university educated, was fascinated with the Sun. (The Sun Kings, p. 129) After the1892 solar storm, Maunder decided that he would prove the link between magneticstorms and sunspots once and for all.

Lord Kelvin also decided to bring an end to this controversy onceand for all. Using James Clerk Maxwell’s new laws, Kelvincalculated the amount of energy that must be required by anexplosion on the Sun to cause the magnetic effects we experience



on Earth, 93 million miles away. The amount of energy required in these flares wascompletely impossible, in Kelvin’s reasoning, as it was equivalent to four months of regular radiation coming from the Sun. (The Sun Kings, p. 153) In 1892, Kelvinurged scientists to search for the real connection between auroras and magneticstorms, as it was impossible that they were caused by the Sun.

(Image Source: http://en.wikipedia.org/wiki/File:Lord_Kelvin_photograph.jpg )

Maunder and other, however, knew that the statistical connection was irrefutable(The Sun Kings, p. 154).

In 1860, Elias Loomis (Yale) mapped out the geographic distribution of aurora,showing that they occurred in an oval that was centered on the North Pole. (Sun,Earth, and Sky p. 178)

Birkeland’s experiment 1896 (above, source:http://upload.wikimedia.org/wikipedia/commons/a/a0/Birkeland‐terrella.jpg ):Birkeland demonstrated the theory that electrons could flow along magnetic field topoles by sending an electron beam towards a magnetized sphere painted withphosphorescent light to show where the electrons struck. The sphere was enclosedin a vacuum to simulate space. The glowing shapes reproduced many features of theaurora. (Sun, Earth, and Sky, p. 178)

In 1903, Maunder and his wife, Annie, began digging through sunspot records andmagnetic storm data. Maunder finally found the evidence he was looking for. In1886, four storms separated by 27 days had been recorded. This was the same

amount of time it took for the Sun to make a complete rotation, sweeping the samesolar storm past Earth over and over again. On November 11, 1904, Maunderprovided his indisputable proof. Some astronomers questioned what mechanismscould make this possible and others came to Maunder’s defense (The Sun Kings, p.162). Larmour came to Maunder’s defense and said that his mathematics werestrong and the the tiny charged particles (electrons) he had been studying couldlikely be the carriers of force between the Sun and the Earth.



Finally, the controversy began to shift. In 1908, Hale made the first detection of strong magnetic fields in sunspots and observers became familiar with the type of behavior to look for and the impending magnetic storms that would come thefollowing day. Kelvin and Airy’s naysaying delayed progress on this front fordecades, but finally the evidence made the Sun‐Earth magnetic connection clear.

The Corona and CMEsThe first photograph taken of a totally eclipsed Sun was WarrenDe La Rue (who had been making photographic observations of the Sun for some time) from the Italian city of Rivabellosa onJuly 18, 1860. The photograph clearly shows solar prominencesarching over the solar disk (The Sun Kings, p. 104 ‐ 108).(Image Source: http://www.britannica.com/bps/media‐view/141531/1/0/0 )

In Labrador, astronomer R. N. Ashe observed the same eclipse. He was a bright flashfrom one side of the disk before the clouds closed in (The Sun Kings, p. 109). Twohours later, observers in Spain saw what looked like a bubble extending from theSun. Some astronomers did not notice this bubble, bringing its existence intoquestion, and what it might be was not further investigated.

In 1898, Maunder and his wife,Annie, a female computer hiredto work at the Royal Observatory,went to Masur, India tophotograph an eclipse withAnnie’s camera. On January 22 nd ,Annie Maunder captured the first image of the Sun’s corona on film(The Sun Kings, p. 157). Anniecalculated that the streamers in

the pictures extended 6 million miles into space. (Image Source:http://en.wikipedia.org/wiki/File:Solar_eclipse_1898Jan22‐photo_wide.png )

Modern View of the Sun

SunspotsThe strongest magnetic fields on the solar surface are found in sunspots, whichappear as dark spots on the solar surface when observing in visible wavelengths.Inside the Sun’s convection zone, magnetic fields are kinked and twisted up by themotion of convective cells rising to the surface of the Sun and falling down again.Twisting causes the magnetic fields to become very concentrated, strong and hold alot of energy. At the location of sunspots, these twisted magnetic fields have actually



risen to the visible surface of the Sun ( i.e. photosphere) and are poking through. Inthese areas, the hot convective bubbles are blocked from reaching the surface,causing that area to cool and look dark compared to surroundings. However, abovesunspots is where flares and CMEs are born. The localized strong magnetic fieldsthat create sunspots interact with plasma in the Sun’s corona to create explosive

events. ( http://c2h2.ifa.hawaii.edu/Pages/Education/sun_activity.php )

Sunspots are created by magnetic loops that extend above the photosphere. Theplaces where the sunspots form are called the footpoints of the magnetic loops.Because the sunspots are created from magnetic loops, sunspots always come inpairs – a spot where the magnetic field is going in an upward and outward direction(a magnetic north pole), and a spot where the magnetic fields loops downwards,back into the solar surface (a magnetic south pole).

In the images below, the left image contains a sunspot group as viewed in visiblelight. At this level in the Sun’s atmosphere, the magnetic fields hinder convection,causing the region inside the sunspot to appear cooler and darker. The image on theright (Image Source: JHelioviewer image of the Sun from Feb. 15 th , 2011), shows thecorona above a sunspot. The magnetic loops above the sunspot heat the plasma of the corona, causing this part of the solar atmosphere to become very hot and bright.When the Sun has a lot of sunspots, it is actually a little bit hotter compared to asolar disk devoid of sunspots.(Image Source: www.solarphysics.kva.se/gallery/images/2002/c4877_color.gif )



The Sun is Hot In the center of the Sun, there is ongoing nuclear fusion of Hydrogen into Helium

that gives the Sun all of its energy. In the core,the temperature is about 13.6 million Kelvin.As expected, the temperature decreases in the

outer parts of the Sun, reaching its minimumtemperature in the photosphere, the visiblesurface of the Sun. The average temperature inthe photosphere is 5777 Kelvin (or about 6000degrees Celsius). Above the photosphere, theSun has three more important layers in itsatmosphere. The layer just above thephotosphere is called the chromosphere and it is very, very thin ‐ only about 2000 km inheight. During a solar eclipse, thechromosphere is seen as a very thin bright red

circle around the Sun, giving it its name which literally means “colored sphere.” Thechromosphere actually increases in temperature, going from 6000 at the bottom to20,000 Kelvin near the top. Above the chromosphere is another very thin layercalled the transition region. Here, the temperature rapidly increases from 20,000 Kto 1 million K in only 200 km. The corona begins at the top of the transition regionand stretches millions of miles out into space. Here the temperature ranges between1 million and 2 million degrees. Image Source: http://imageshack.us/photo/my‐images/265/sunpartsfull.jpg/sr=1

Solar VariabilityThe Sun experiences an 11 year activity solar cycle and a 22 year magnetic solarcycle. Over 11 years, on average, the Sun changes from being a very quiet starexhibiting very little activity and no sunspots to a dramatically active star covered insunspots and experiencing multiple explosions every day. This cycle can be trackedby counting the number of sunspots on the Sun over time (see plot below). At theheart of this cycle is the Sun's magnetic field.

Image Source: NASA/SOHO (left),http://www.ucar.edu/news/releases/2006/images/figpredic24‐1.jpg (right)



The magnetic field guides and controls all of the solar activity described above. At the quiet point in the solar cycle, called solar minimum, the Sun's magnetic fieldlooks very much like a simple dipole, like a bar magnet. Over the next five or six yearperiod, the magnetic field gets twisted up (due to differential rotation), causing it tobecome more and more complicated. As the magnetic field twists, the Sun exhibits

more sunspots and more activity. Finally, the Sun reaches solar maximum. At thispoint, the magnetic field looks almost nothing like a simple bar magnet and the Sunis experiencing many flares, CMEs, sunspots, and more every day. At solarmaximum, the Sun's magnetic field begins to flip. Over the following five or sixyears, the Sun begins to quiet again until it has reached solar minimum and itsmagnetic field has flipped.

At the start of a solar cycle, the Sun's north pole may be at the "top" and the southpole at the "bottom." By the end of the solar cycle, the north pole will be at the"bottom" and the south pole at the "top." After a second solar cycle, the north polewill be at the "top" again. That is why the magnetic cycle is 22 years ‐ because it

takes 22 years for a magnetic pole on the Sun to return to its starting location.

The Sun and Climate or How Sunspots Affect the Sun’s TemperatureIn 1801, Herschel looked for a link between sunspot number and the price of wheat,reasoning that a raise in the Sun’s temperature would result in warmertemperatures on Earth, more wheat grown, and lower wheat prices. He did findwhat appeared to be a link, though other studies have found that the relationshipdepends on which crops are chosen for comparison. It is now known that there arevariations in solar energy output on timescales of 20 years and more (The Sun, SolarAnalogs, and the Climate, p. 248).

Solar Differential RotationIt is now known that differential rotation is at the heart of the cyclic behaviorexhibited by the Sun. The Sun is made up primarily of moving, rotating plasma in magnetic fields. Magneticfields can guide the course of the moving plasma, as isoften seen in close‐up images of solar activity where theplasma very clearly follows magnetic field lines.However, the reverse is also true. If a large amount of plasma moves from place to place, it will drag itsmagnetic field along with it. This property leads to verycomplicated behavior by the solar magnetic field, leadingto an explanation of why the Sun experiences a regularsolar cycle and provides a mechanism for all solaractivity.Image Source: http://solarscience.msfc.nasa.gov/dynamo.shtml

The Sun experiences differential rotation (equator rotates faster than the poles),which causes the magnetic field to become wound up and break into fragments.Plasma also flows poleward and vice versa, denoted the meridional flow. Plasma at



the solar surface travels from the equator towards the poles while plasma deepinside the Sun flows from the poles towards the equator.This flow drags the twisted and fragmented southernmagnetic fields toward the north pole and north facingmagnetic fields towards the south pole, causing the Sun's

magnetic field to eventually flip and reform. Then it starts allover again. The solar magnetic poles flip approximatelyevery 11 years. At solar maximum, right before the magneticpoles flip, the magnetic field is in its most stressed state andthe Sun exhibits lots of sunspots, flares, and CME activity.Just after the poles flip, the Sun experiences solar minimum with no sunspots, verylittle activity, and its magnetic field looks most like a simple bipole.

Solar Flares, Coronal Mass Ejections, and the Magnetic Sun Earth ConnectionSolar flares are the most energetic explosions observed on the surface of the Sun. Tomake a comparison with weather on Earth, solar flares are analogous to tornadoes ‐

small in size but incredibly powerful and energetic. On images,flares appear as extremely bright spots in the Sun’s atmospherethat last for only a few minutes. Flares release a huge amount of electromagnetic energy in the form of light over a broad range of wavelengths, including X‐rays. They can also accelerate solarenergetic particles (SEPs), i.e. potentially damaging, quick‐moving, highly energetic electrons, protons, and ions. The total

energy released during a flare is equivalent to 200 million nuclear warheads.

Like flares, coronal mass ejections (CMEs) are explosions onthe Sun. Unlike flares, CMEs result in the ejection of a hugeamount of matter from the Sun's atmosphere into space. Infact, each CME ejects about the mass of 500 million HummerH2’s into space. If a flare may be compared to a tornado, then aCME is most like a hurricane; overall, CMEs are usually lessenergetic than flares, however their extraordinary size andejected mass makes them the most threatening solar space weather events the Earthcan experience.

A CME is a huge bubble of plasma that strikesthe Earth. Much of this plasma (98%) isdeflected by the Earth's magnetosphere, but 2% of the plasma enters the Earth'smagnetosphere. Some of these streamingcharged particles generate electric currents inthe atmosphere. These fluctuating electriccurrents create magnetic fields that are felt onthe ground. The magnetic fields then generateelectric currents in the rocks in the ground or

in the even more conductive man‐made power lines, power grids, and even oil



pipelines, that extend for very long distances (depicted in the image below). If theelectric currents are strong enough, they can cause protective relays in powerstations to trip or even destroy transformers, leading to wide‐scale power loss. Thishappened to the Quebec power grid in 1989, leaving six million people without power for 9 hours. A severe CME‐induced geomagnetic storm has the potential to

take out multiple power grids, potentially leaving the majority of a country without power.

Image Source: http://en.wikipedia.org/wiki/File:GIC_generation.jpg

In the modern era, the connection between solar storms and magnetic storms onEarth is very clear.

Aurora and Magnetic StormsThe Northern Lights or aurora borealis are some of the most obvious and beautifuleffects of space weather. Aurora are the result of charged particles from the solarwind interacting with the Earth's magnetic field and atmosphere. Most of theelectrons, protons, and ions in the solar wind are deflected by the Earth'smagnetosphere, however some of them actually get trapped in the magnetic fieldsurrounding the Earth, called the magnetosphere.

Image Source:http://news.bbc.co.uk/nol/shared/spl/hi/sci_nat/10/aurora_borealis/img/aurora_

borealis_624in.gif

When a charged particle encounters a magnetic field, the force that the magneticfield exerts on the particle causes it to spiral around and follow the magnetic fieldlines. When a CME hits the Earth, the magnetosphere is stretched and pulled,



compressed and released, like a rubber band. The charged particles (electrons andprotons) caught in the magnetosphere get energized by this activity and go shootingalong the magnetic field lines to the poles where they are brought down toward theground. This is why aurora occur at the poles ‐ because that is where the Earth'smagnetic field approaches the ground!

The high‐speed charged particles stream along themagnetic field lines and slam into atoms andmolecules in the atmosphere. The impact puts energyinto the atmospheric atoms, causing their electrons tojump to higher energy levels temporarily. When theelectrons fall back to the ground state in the atom,they release the extra energy they gained from thecollision in the form of photons, i.e. light. Oxygenemits green and brownish‐red light, while nitrogenemits bluish and bright red light. That is why the aurora glow in different colors; the

light that makes the aurora glow comes from different types of atoms experiencingelectron transitions from a range of energy levels. This mechanism is that same onethat lights up the fluorescent and neon lights that you experience every day. ImageSource: http://en.wikipedia.org/wiki/File:Red_and_green_aurora.jpg

When the Earth and Sun Were Young

The Early EarthThe Earth is currently estimated to be 4.5 billion years old. The earliest rocksformed about 3.6 ‐ 3.9 billion years, when the molten Earth cooled enough for rock formation. This crust formation occurred in an extremely turbulent environment inwhich the Earth was experiencing a continued heavy bombardment of planitessimals (Solar System Evolution, p. 345).

The early Earth did not form with an atmosphere.Evidence indicates that there was not gas available in theportion of the solar system where the Earth formed togenerate an atmosphere at formation. The atmosphereand water on Earth appear to be secondary in origin. Theatmosphere is thought to have been formed from gasreleased by the mantle during the first half billion years

(4 billion years ago) after accretion (consistent with a molten mantle from thestrikes of planitessimals and the formation of the Moon by a massive collision)(Solar System Evolution, p. 362 – 363).(Image Source: http://www.phenomenica.com/2011/02/rare‐sulphur‐could‐alter‐theories‐of.html )



The Earth’s magnetic field existed by 3.45 billion years ago, as was found in a recent study using very old quartz crystals from Australia. The field, however, was only30% – 50% as strong as it is in the present. This has serious implications for theeffect of the Sun’s solar wind on Earth. (Tarduno et al. , 2010,http://www.space.com/8006‐early‐earth‐magnetic‐field‐weakling.html ,

http://www.wired.com/wiredscience/2010/03/earths‐magnetic‐field‐is‐35‐billion‐years‐old/ ).

The Early SunStellar evolution models clearly indicate that the early Sun was only about 70% asbright as it is today. In fact, the Sun will increase in brightness throughout its MainSequence lifetime. This effect is due to the nuclear fusion of hydrogen into helium inthe Sun’s core. As more and more hydrogen are converted to helium, the averagedensity of the Sun’s core increases. The pressure inside the star must increase tohold up the star’s heavier mass and the mechanism to make this happen is anincrease in overall temperature, hence brightness, of the star. If the Sun was so

much cooler, shouldn’t the Earth have been as well? This will be discussed in thesection entitled Faint Young Sun Paradox.

It has been observed that younger stars have stronger magnetic fields and spinmore quickly than older stars. The Sun has and has always had a solar wind, i.e. plasma escaping into the solar system. The solar wind carries some of the Sun’sangular momentum away with it, causing the Sun to slow in rotation period overtime (Keppens, MacGregor, and Charbonneau, 1995). The overall magnetic field of stars is seen to decrease over time, as well. This paints the picture of a young Sunwith a fast rotation period (possibly just a few days compared to the current 27 dayrotation period), a stronger magnetic field, and a more intense solar wind(http://www.wired.com/wiredscience/2010/03/earths‐magnetic‐field‐is‐35‐billion‐years‐old/ ). The record of ion implantation in lunar rocks and meteoritesdoes indicate a more intense ancient solar wind (Gaidos, Gudel, and Blake).

The Impact of the Early Sun on the Early EarthThe fact that the Earth had a magnetic field so early on is crucial to the development of its atmosphere. With no protection from magnetic fields, the solar winds wouldhave blown much of the atmosphere away, as is believed to have happened on Mars.

The amount of radiation the Earth likely experienced from the early solar wind on adaily basis is what the Earth only experiences during the strongest solar stormstoday. We now know that the Earth’s magnetic field was much weaker and the earlymagnetosphere only extended about half as far above the Earth’s surface as it doestoday. The strong solar winds combined with the Earth’s weaker magnetospherewould have lead to a constant display of auroras glowing in the Earth’s youngatmosphere regularly extending down New York City level latitudes, an event that isrelatively rare today. ( http://www.space.com/8006‐early‐earth‐magnetic‐field‐weakling.html )



An additional consequence of the situation outlined above would be a higher lossrate of volatile molecules, like hydrogen, from the Earth’s atmosphere.

The oldest evidence for life dates back to about 3.5 billion years ago (and it is certainthat life had formed by 2.7 billion years ago), indicating that the Earth likely had a

magnetic field by the time that life formed.(http://paleobiology.si.edu/geotime/main/htmlversion/archean3.html )

The Faint Young Sun ParadoxAs explained above, the early sun was only 70% as bright as it is today. Acalculation shows that, at that brightness, the Earth should have been frozen over,however evidence for running water abounds from times as early as 3.2 billionyears ago and earlier.(http://www.astrosociety.org/pubs/mercury/35_06/paradox.html )

Carl Sagan and George Mullen first pointed out this paradox in 1972. They felt that

the best solution was an increase in greenhouse gases in the atmosphere, whichwould hold heat much more efficiently than today. Since they posed this problem,scientists have been trying to come up with answers, but with little success.

There are three ways to solve this puzzle. 1) The Earth must be able to moreefficiently hold on to heat; 2) The Earth must be able to absorb heat more efficiently;3) The Sun must not have been as dim as models indicate.

Some proposed, as Sagan and Mullen did, that there was more CO 2 and methane inthe early atmosphere, however recent studies of early atmospheric levels show noevidence of the high levels of carbon dioxide needed to solve the paradox (Kasting,2010). Other scientists proposed that the early atmosphere had less cloud cover or adifferent distribution of clouds, causing less of the Sun’s light to be reflected andmore heat to stay within the atmosphere, but these studies typically fell short of asolution. Some attempted to propose a different evolutionary history for the Sun,calling the its 30% reduction in brightness into question, however surveying solartype stars did not support this possibility (Gaidos, Gudel, and Blake). It should alsobe emphasized that solar evolution models are tested on a data set of thousands of stars, returning a very robust result.

In 2010, geologists Minik Rosing and Allan Cox believed that they had solved theproblem. The continents were smaller in the past, somore of the Earth was covered with dark oceans that could absorb more heat. Their model also depends onfewer clouds, which may have very well been the case asclouds today form around biogenic sulphur gases andplants had not yet flourished back then. “We put togethersome models that demonstrate, with the slow continentalgrowth and with a limited amount of clouds, you could



keep water above freezing throughout geologic history.” (Stanford Report, 2010)(Image Source:http://www.cotf.edu/ete/modules/msese/earthsysflr/cambrian.html )

Perhaps the Young Sun Paradox has been resolved, or perhaps not. A Nature article

claims that there are potential issues with Rosing and Cox’s theory, pointing out that the albedo of ice was not taken into account and the cloud feedback mechanismsused in Rosing and Allen’s research fail to work in some conditions (Kasting, 2010).

The Sun, Modern Society, and Human Health

The HeliosphereThe same way that the Earth’smagnetosphere forms a protectivebubble around the Earth from the solarwind, the Sun’s heliosphere forms aprotective bubble around the solarsystem, protecting it from theinterstellar medium. The same way that the solar wind buffets the Earth’smagnetosphere and compresses it onthe “upwind” side, the interstellar wind(due to our motion around the center of the galaxy) compresses the heliosphere on the “upwind” size. Where does this edgeof the heliosphere occur? Ultimately, at some point, the solar wind expands farenough that it does not have enough pressure to repel the interstellar medium. Theplace where this occurs is named the heliopause. The heliopause ends in a turbulent termination shock. The location of the heliopause has been inferred thanks to theVoyager 1 & 2 spacecraft, launched in 1977 and have been traveling outwards intospace ever since. In the past decade, the Sun experienced intense explosions. Theseexplosions traveled outwards to the heliopause, creating a radio hiss 13 monthslater that was detected by the Voyagers. The speed of the disturbances wasmeasured, allowing scientists to estimate that the heliopause is located between 110– 160 AU. In 2004, Voyager 1 was located at 94 AU and is expected to cross theheliopause in the next decade. The Voyagers have enough electrical power and fuelto operate until 2020.

On June 20 th , 2011, Scientific American reports that Voyager 1 is now over 17 billionkm at Earth and is either approaching the heliopause or may have passed throughinto interstellar space. It no longer measures the solar wind at its back and it is atranquil environment. If the Voyager has crossed the heliopause, it has become thefirst human object ever to enter interstellar space.(http://www.scientificamerican.com/podcast/episode.cfm?id=voyager‐1‐may‐have‐reached‐the‐heli‐11‐06‐20_ ; Image Source:http://www.nasa.gov/centers/goddard/news/topstory/2007/dragon_fire_prt.htm )



Types of RadiationThe Sun emits two different distinct types of radiation – electromagnetic radiationand particle radiation. We also use radiation to describe emissions from radioactiveparticles. The term “radiation” is ambiguous and it’s important to understand what kind of radiation is being discussed and the dangers of each kind.

Electromagnetic RadiationElectromagnetic radiation, or EM radiation, refers to waves in the entire EMspectrum. This includes radio waves, microwaves, infrared waves, visible light,ultraviolet light, X‐rays, and gamma rays. The shorter the wavelength, the moreenergy an electromagnetic wave carries, thus X‐rays and gamma rays are the most energetic forms of EM radiation.

X‐rays (and shorter wavelength EM radiation) have enough energy to ionize atoms,hence they are referred to as ionizing radiation. If this EM radiation enters aperson’s body, electrons may be ejected from atoms or molecules, leaving a charged

free ion, called a free radical. These free radicals may then interact with a DNAmolecule, creating an abnormal cell that may be able to divide, and in some cases,cause cancer.

Image Source: http://www.andor.com/learning/light/ Particle RadiationParticle radiation is caused by subatomic particles moving at high speeds, oftenclose to the speed of light. Because they move so fast, they carry a lot of energy. Themost common particles are electrons (also called beta particles), protons, heliumnuclei (also called alpha particles), and neutrons.

Energetic particles can be emitted by radioactive atomshere on Earth or they can come from space. The Sun emitsenergetic particles that are called solar energetic particles(SEPs). These electrons, protons, and alpha particles arenot very energetic compared to galactic cosmic rays



(GCRs) that come from the galaxy and the universe. GCRs move at nearly the speedof light and are created in extremely energetic physical processes like supernovae.The most energetic cosmic rays are millions of times more energetic than thehighest energy particles we can produce in our most advanced particle accelerators,such as the Large Hadron Collider (LHC) at CERN in Geneva.

(http://www.esa.int/esaMI/Lessons_online/SEM8V1V7D7F_0.html ) (Image Source:http://astronet.ru/db/xware/msg/1215207/crshower2_nasa_big.jpg.html )

Radioactive AtomsRadioactive atoms are atoms with a very large number of neutrons compared toprotons, making their nuclei unstable. The decay rate of a radioactive atom iscompletely independent from all outside influences, such as temperature, pressure,or chemical compound in which the radioactive nucleus is found. Radioactive decayis also completely random.(http://hyperphysics.phy‐astr.gsu.edu/hbase/nuclear/halfli2.html ).

When a radioactive atom decays, it can emit an alpha particle (2 protons and 2neutrons, also called a helium nucleus), a beta particle (electron), and/or gammarays (electromagnetic radiation).

Alpha particles are heavy and move veryslowly. They do not penetrate very deeplyinto materials and can be stopped by a pieceof paper or even the outer dead layers of skin,however eyes and open wounds are at risk.Because they are heavy, however, they arevery good at ionizing atoms

(http://darvill.clara.net/nucrad/types.htm ). If someone happened to inhale analpha emitting radioactive atom, it could do damage to internal tissues, potentiallycausing cancer. Some alpha emitters include plutonium‐236, uranium‐238, radium‐226, and radon‐222 ( http://www.epa.gov/radiation/understand/alpha.html ).(Image Source: http://www.vae.lt/en/pages/about_radioactive_waste )

Beta particles are emitted when a neutron in an unstable nucleus decays to a protonand electron. The proton stays in the nucleus and the electron is ejected. Betaparticles are very light and fast. They can penetrate deeper into materials than alphaparticles, but are stopped by solid objects. Beta particles can cause reddening orburning of the skin. Inhaled beta particles are even more dangerous as they canpenetrate deeper into tissues and disrupt cell function. Some beta emitters arecobalt‐60, iodine‐129 and ‐131, and cesium‐137.(http://epa.gov/radiation/understand/beta.html )

The emission of gamma radiation (a gamma ray) often follows the emission of a betaparticle. Gamma rays have enough energy to penetrate into the body and evencompletely pass through it. They can potentially cause a lot of damage by exposingall organs. A gamma ray may energize an electron inside tissues, which could then



ionize a molecule or atom. A gamma ray may ionize an atom or molecule insidetissue directly. ( http://epa.gov/radiation/understand/gamma.html )

Types and Effects of Radiation from the SunFlares generate light at all wavelengths, but the increase in X‐ray radiation is

particularly important. Energetic X‐rays can ionize atoms in the Earth's upperatmosphere, called the ionosphere. We use the ionosphere to bounce short‐waveradio signals from one part of the Earth to the other, so a change in the ionospherecauses a disruption in short‐wave radio communications.

X‐rays also heat up the Earth's upper atmosphere. This causes the atmosphere toexpand or puff up. Usually, satellites are placed in orbits high above the Earth wherethe atmosphere is very thin or almost nonexistent. If the atmosphere expands,satellites suddenly find themselves surrounded by air, which causes a frictionaldrag. This drag will cause the satellites to lose energy and possibly fall out of orbit if corrective actions aren't taken. Satellites may also burn up in the temporarily higher

density atmosphere. Another possibility is that atmospheric friction may put atorque on the satellite, causing it to spin out of control.

Image Source: http://solarb.msfc.nasa.gov/science/space_weather/



Flares eject a huge amount of solar energetic particles (SEPs) from the surface of theSun. SEPs are charged particles, like protons, electrons, helium nuclei, and otherions, that are ejected and accelerated by really energetic activity on the Sun. SEPscan travel at speeds close to the speed of light. At these speeds, SEPs carry a lot of energy that can have damaging effects. These charged particles can cause satellite

detectors to malfunction or even break. The small white dots and streaks on theimage below are "snow" caused by SEPs. The image was taken with a coronograph,so the Sun is behind the white circle.

Lastly, and most importantly, a blast of SEPs is really a huge dose of radiation. OnEarth, we are protected by our magnetic field. Inside the space station, astronautsare protected, though an astronaut on a space walk during a burst of SEPs may be introuble. In the future, humans traveling to Mars would be in great danger from SEPradiation. In any mission where people would spend a long time in space far fromthe protection of Earth's magnetic field, SEPs would be a serious problem.

Like in flares, CMEs send a huge number of energetic charged particles towardsEarth. These particles can cause damage to satellites in space or expose astronautsto radiation

Unlike flares, a CME is a huge bubble of plasma that strikes the Earth. Much of thisplasma (98%) is deflected by the Earth's magnetosphere, but 2% of the plasmaenters the Earth's magnetosphere. The streaming charged particles generate electriccurrents in the atmosphere. These fluctuating electric currents create magneticfields that are felt on the ground. The magnetic fields then generate electric currentsin the rocks in the ground or in the even more conductive man‐made power lines,power grids, and even oil pipelines that extend for very long distances. If the electriccurrents are strong enough, they can cause protective relays in power stations totrip or even destroy transformers, leading to wide‐scale power loss. This happenedto the Quebec power grid in 1989, leaving six million people without power for 9hours. A severe CME‐induced geomagnetic storm has the potential to take out multiple power grids, potentially leaving the majority of a country without power.

Solar Radiation and Human Health RisksFirst it should be pointed out that humans experience radiation every day, evenfrom inside our own bodies. The Earth, cosmic rays from space, and even livingthings are natural sources of radiation that our bodies have evolved to withstand.

There are occasions when the Sun releases a large number of high‐energy protons,termed solar proton events, which can last several hours. These protons carry farless energy than galactic cosmic rays, but they do have enough energy to be apotential health risk to people in space or at high altitudes. Solar protons are alsocalled cosmic rays, but the term galactic cosmic rays (GCRs) is reserved for thoseextremely high energy particles that are clearly not generated inside of our solarsystem.



So how high energy is high energy? Particles from the Sun tend to be what is consideredon the lower energy end in the 10 – 100 MeV (million electron volts) range. Particlesneed to have energies of greater than 450 MeV to be detected on the ground. Near theequator, only particles with energies of about 15 GeV (that’s giga electron volts; 1 GeV =1000 MeV) can get through the Earth’s magnetic field to the upper atmosphere. Particles

of all energies can enter the atmosphere near the poles.

To put these energies into perspective, a proton with an energy of 100 MeV is traveling43% the speed of light. A proton with an energy of 10 GeV is traveling 99.6% the speedof light! Most galactic cosmic rays have energies between 100 MeV and 10 GeV.

Image Source: http://blogs.agu.org/wildwildscience/2009/09/01/how‐much‐

radiation‐does‐it‐take‐to‐kill‐you/

The Earth’s atmosphere provides a very effective shieldagainst solar proton events and only about 15% of theprotons in these events have enough energy (> 450 MeV)to create a particle cascade that is detectable on theground. The Earth’s magnetosphere is the primarybarrier to cosmic rays from the Sun. It is easiest for thesecosmic rays to reach the Earth’s atmosphere near thepoles where the magnetic field heads down into theground. Farther from the poles, the magnetic field raisesincreasingly higher above the ground, requiring protonsof higher and higher energies to penetrate to theatmosphere. Thus, the amount of radiation that penetrates to the atmosphere depends on magneticlatitude. People that live above 50 degrees geomagneticlatitude experience about twice as much radiation aspeople that live below this latitude. (Image Source:http://physik.uibk.ac.at/hephy/Hess/Cosmic_Rays‐Cosmo_ALEPH.gif )



Increasing in altitude also increases the risk of radiation. For example, people livingin Denver, Colorado experience about twice as much cosmic radiation as peopleliving at sea level. This added exposure to cosmic rays does not appear to have asignificant health impact as medical studies show that people living at mountain

altitudes are generally healthier and have longer life spans than those living at sealevel. However, increased exposure to cosmic rays is a concern for people travelingat aircraft altitudes (especially above 40,000 feet), particularly for pilots, flight attendants, and passengers traveling circumpolar routes. Accumulated radiationfrom a flight schedule of 70 hours per month at high latitude routes could reach theInternational Council on Radiological Protection (ICRP) recommended limit for apregnant female in just five months (2 mSv). However, it is difficult to accumulatethe annual IRCP recommended limit of 20 mSv for everyone else.

The radiation exposure of those in flight during solar proton events can beworrisome, though still not at dangerous levels. The September 29, 1989 event was

recorded by instruments on the ground and mounted in airplanes. It was found that a 7 hour flight over the North Atlantic would have resulted in a dosage of about .05mSv. At an altitude of 50,000 feet, the accumulated dosage would have been about .075 mSv in just three hours. While not dangerous in and of itself, this does pose aradiation concern and circumpolar flights are generally rerouted during solarstorms.

For astronauts in space, radiation exposure and solarproton events are a serious concern. Inside the most heavily shielded parts of the space station, astronauts arereasonably well protected from solar storms, but if theradiation from a storm happened to strike an astronaut during a space walk, he or she could suffer a large,potentially lethal dose of radiation. Additionally, NASA

enforces career radiation dose limits, so getting caught in a solar storm could end anastronaut’s career prematurely. (Image Source: http://www.nasa.gov/astronauts/ )

Luckily, NASA and other space agencies around the world have many satellites inorbit around the Earth and at the Lagrangian point, L1, located about a million milesfrom the Earth towards the Sun. These satellites provide an early warning system

for solar storms, alerting scientists topotentially dangerous solar activity a fewhours to a few days before it would reach theEarth.

Solar radiation is one of the largest problemsscientists must face when considering mannedspace flight. So far, NASA has been lucky. OnAugust 7, 1972, in between Apollo 16, which

was carried out during April of 1972, and Apollo 17, which was carried out in



December of 1972, there was a large solar storm that would have sickened or killedastronauts on the Moon. Some of the less optimistic studies indicate that a six‐month trip to Mars with a six‐month return trip could possibly increase the lifetimechance of getting cancer to 45%. Some scientists estimate that every third humancell would be damaged by energetic particles during the flight. (Sun, Earth and Sky,

P.185‐186) (Image Source: http://en.wikipedia.org/wiki/File:Mars_mission.jpg )

People who are interested in learning their estimated radiation dose from theenvironment can do so using a webpage on the Environmental Protection Agency’s(EPA) webpage at the following address:http://www.epa.gov/rpdweb00/understand/calculate.html .

The FAA provides a similar website to estimate the radiation dose from flying,which can be found at: http://jag.cami.jccbi.gov/cariprofile.asp .

The Effects of a Solar Storm in Modern Times

(Summarized from the book Storms from the Sun , pp. 93 ‐ )From March 6 to 19, an enormous sunspot group 54 times the size of the Earthexploded with over 195 flares, 11 of them classified with the label X‐class, reservedfor the most intense of solar storms. As the sunspot group rotated into view onMarch 6 th , it released one of the most powerful flares ever observed and stream of radiation that lasted 10 hours (the norm is about 30 minutes). “Solar physicistsestimated that the temperature inside the flar reached 20 million degrees Celsiusand more energy was released in those moments than humans have consumed inthe entire history of civilization.” (p. 94) The readings on the NOAA GOES‐7satellite’s X‐ray detector went off the scale for 27 minutes.

Image Source:http://www.redorbit.com/modules/imglib/download.php?Url=/modules/imagegal

lery/gallery_images/6_e8d8201fae6310c288a1d298b3e1a402.jpg



The active region continued to send energized streams of protons towards theEarth. On March 9 th , another enormous flare exploded with such brightness that it exceeded the maximum values on the scales used to record flare brightness. On the10 th , observers saw a white light flare like the one Carrington had seen in 1859, anextremely rare event. Following this flare, a halo CME was observed, meaning that it

was heading straight for the Earth.

On the night of March 12 th , the front edge of the CME reached the Earth and bymidday on the 13 th , the Earth’s magnetosphere was compressed from a normal34,000 miles above the Earth to possibly as low at 14,000 miles. This meant that satellites that were normally protected by theEarth’s magnetic field were suddenly laid bare inopen space.

Magnetic observatories had readings at the top of their charts for 5 or 6 hours, even those on the

American South. Auroras were seen in Mississippi,Arizona, Southern California, and Texas. Theaurora continued south and was seen in Florida,Cancun, and Honduras. (Image Source:http://www.scientificamerican.com/slideshow.cf m?id=geomagnetic‐storm‐march‐13‐1989‐extreme‐space‐weather&photo_id=FCC3D702‐EC6F‐977D‐26727B186E45F446 )

On the ground and in space, satellite, power, andelectronics companies were struggling. In the northeast US, a computer microchipmanufacturer shut down because the magnetic storm was disturbing the sensitiveequipment. Navigators saw their compasses distort to as far as 10 degrees. NorthSea oil companies had to stop drilling because the magnetic instruments that guidethe drills were way off course.

Satellites were being bombarded with streams of energetic charged particles,unprotected by the magnetosphere. At the same time, the Earth’s upper atmospherewas expanded due to heating from the bombardment of the particles.

Satellites that should not be were suddenly feeling thefrictional drag force of the atmosphere. The CheyenneMountain Operations Center which tracks about 8000pieces of space junk lost track of 1300 objects. NASA’sSolar Maximum Mission satellite (pictured) dropped 3miles in orbit over a course of a couple of days (ultimatelyre‐entering the Earth’s atmosphere and burning up inDecember of 1989). A classified US military satellite

tumbled uncontrollably through space. NOAA’s GOES‐7 weather satellite sufferedoutages and communication problems. The added electric currents due to charged



particles from the CME caused phantom switching and tripping of circuits. Somegeostationary satellites had trouble staying in place. The excited ionosphere lead totroubles communicating with GPS satellites, which returned poor or incorrect locations. (Image Source: http://solarscience.msfc.nasa.gov/images/SMM.jpg )

The geomagnetically induced currents (GICs) from the streaming charged particlesin the atmosphere lead to disturbed power companies in Maryland, California, NewYork, New Mexico, Arizona, and Pennsylvania. In New Jersey, a $10 milliontransformer was damaged beyond repair. Usually these type of transformers take ayear to build, but the power company was able to find one and get back up andrunning in six weeks.(Below, Image Source: http://spacefellowship.com/news/art23374/solar‐shield‐protecting‐the‐north‐american‐power‐grid.html )

This storm was also the one that caused thecollapse of the Hydro‐Quebec power plant and

its neighboring plants. Once one plant experienced a loss of power, the addeddemand on the other plants caused them tocollapse as well. All of this happened withinseconds. Six million people in Quebec City,Montreal, and the surrounding areas were left without power over the freezing CanadianMarch night. It took nine hours to restore power by channeling it from otherutilities. After a study of the system collapse, scientists reported that the entirenortheastern US, which depended partly on Hydro‐Quebec power, almost fell intoblackout, as well.

Image Source: http://www.theweatherspace.com/news/images/31311a.jpg



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