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

History of Solar Discoveries

Sunspots The first recorded observations of sunspots were made by Chinese astronomers. The two earliest records of sunspot observations are found in a Chinese book, the Book of Changes, dating back to about 800BC or earlier. Much more complete Chinese 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 the Chronicles of John of Worcester. The observation dates to 8 December 1128, and shows the sunspot's umbra and penumbra. The accompanying text reads, "...from morning to evening, appeared something like two black circles within the disk of the Sun, 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 these spots 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 solar surface. (Sun Kings, p. 28)

But what were sunspots? Christoph Scheiner from Germany thought that they were silhouettes of undiscovered planets. Galileo showed that this could not be true because of the strange way that sunspots moved. Galileo reasoned that this was the behavior of something affixed to the surface of a spinning ball. He could also see sunspots growing and shrinking in size, behavior that could not be attributed to a planet. The issue of sunspots and what they were provoked a somewhat emotional response at the time, as the prevailing belief was that the heavens were perfect and unchanging, a reflection of God.

Galileo thought they were dark clouds in the solar atmosphere, other astronomers thought 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 volcanic eruptions, 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 an astronomer could go blind if not done properly. Larger telescope focused more light, even making it harder. William Herschel made an unpolished mirror that naturally scattered light, allowing for his solar telescopes to hold larger mirrors and see higher levels of detail. He saw that Sun’s surface looked more like an orange peel and that the black spots were actually depressions in the surface. This lead him to believe that they were openings, allowing astronomers to see the dark surface of the Sun below. (The Sun Kings, p. 30) He thought that the Sun’s atmosphere consisted of 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 richly stocked with inhabitants. He also used the fact that mountain climbers experienced a 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 equal quantities of heat, which was the prevailing belief at the time. Some scientists believed he was a rambling fool, but after a controlled experiment in which Herschel displayed the Sun’s light on the wall through a prism and measured the temperatures of the different colors (while another thermometer in the room measured 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 lines with 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 pure samples 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 will produce an emission line spectrum; and a hot object with a cooler tenuous gas around it will create a spectrum with dark absorption lines. Because they identified the elements on the Sun’s surface and it was well known that these metals could only 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 Variability Going through his records in 1801, Herschel noticed that the number of sunspots seemed to come and go. He looked through scientific journals and identified possible periods over which he saw this variability. (The Sun Kings, p. 36)

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

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

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

Solar Differential Rotation Richard Carrington of England set up his own observatory at Redhill, Surrey. He was an extremely careful observer whose northern star catalog was published by the British admiralty with public funds because of its importance for navigation. Starting in 1853, Carrington applied his detail‐oriented technique to observing sunspots, trying to determine the Sun’s rotation period once and for all, since astronomers could not seem to agree on a value, finding somewhere between 25 – 28 days (The Sun Kings, p. 77). In 1858, Carrington had enough information to show that spots at higher latitudes moved more slowly than those at the equator. This differential 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_CarringtonObsTN S.jpg.jpg)

Solar Flares and Solar Storms At 33 years old, Richard Carrington was observing sunspots through his telescope as usual on September 1, 1859. He projected the Sun’s image onto a wall. Wires in the eyepiece cast the shadow of a grid onto the wall. He sketched the sunspots, then timed them as they crossed the grid lines. At 11:18am, two bright beads of searing white light appeared over a monstrous sunspot group (The Sun Kings, p. 12). He was surprised, but then quickly noted the time, checked that this was not a stray reflection, then ran to find a witness. He returned 60 seconds later, but the lights were dimming. They completely vanished at 11:23am. He sketched the sunspot group and flare location (Image Source: http://astroguyz.com/wp‐ content/uploads/2009/08/Carrington_Richar d_sunspots_1859.jpg).

About 18 hours later, the skies around the world burst into massive auroras, telegraph wires became inoperable, giant sparks or streams of fire flowed from the lines into the equipment and even into one poor soul’s forehead (though he recovered shortly after) (The Sun Kings, p. 83). The New York Times reported that newsprint could be read by the auroral light. The aurora were seen almost everywhere across the globe, even as far south as Key West, the Bahamas, and Hawaii. (NY Times article source: http://query.nytimes.com/gst/abstract.html?res=9F05E6DB1638E033A25750C0A 96F9C946892D7CF&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 the Earth’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 and the Earth. It was an almost unthinkable thought that such a force could act over such a 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 the American Astronomical Society, was observing the Sun through his home observatory near Chicago. He managed to capture a solar flare over a particularly large group of sunspots on film. The next day, the sky burst into aurora and communications lines were disrupted. (The Sun Kings, p. 150)

In 1872, Edward Walter Maunder was hired as an assistant at the Royal Observatory in 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 the 1892 solar storm, Maunder decided that he would prove the link between magnetic storms and sunspots once and for all.

Lord Kelvin also decided to bring an end to this controversy once and for all. Using James Clerk Maxwell’s new laws, Kelvin calculated the amount of energy that must be required by an explosion on the Sun to cause the magnetic effects we experience on Earth, 93 million miles away. The amount of energy required in these flares was completely 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, Kelvin urged scientists to search for the real connection between auroras and magnetic storms, 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 to poles by sending an electron beam towards a magnetized sphere painted with phosphorescent light to show where the electrons struck. The sphere was enclosed in a vacuum to simulate space. The glowing shapes reproduced many features of the aurora. (Sun, Earth, and Sky, p. 178)

In 1903, Maunder and his wife, Annie, began digging through sunspot records and magnetic storm data. Maunder finally found the evidence he was looking for. In 1886, 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 same solar storm past Earth over and over again. On November 11, 1904, Maunder provided his indisputable proof. Some astronomers questioned what mechanisms could 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 were strong and the the tiny charged particles (electrons) he had been studying could likely 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 the following day. Kelvin and Airy’s naysaying delayed progress on this front for decades, but finally the evidence made the Sun‐Earth magnetic connection clear.

The Corona and CMEs The first photograph taken of a totally eclipsed Sun was Warren De La Rue (who had been making photographic observations of the Sun for some time) from the Italian city of Rivabellosa on July 18, 1860. The photograph clearly shows solar prominences arching 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 flash from one side of the disk before the clouds closed in (The Sun Kings, p. 109). Two hours later, observers in Spain saw what looked like a bubble extending from the Sun. Some astronomers did not notice this bubble, bringing its existence into question, and what it might be was not further investigated.

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

Sunspots The strongest magnetic fields on the solar surface are found in sunspots, which appear 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 the motion 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 a lot 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. In these areas, the hot convective bubbles are blocked from reaching the surface, causing that area to cool and look dark compared to surroundings. However, above sunspots is where flares and CMEs are born. The localized strong magnetic fields that 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. The places where the sunspots form are called the footpoints of the magnetic loops. Because the sunspots are created from magnetic loops, sunspots always come in pairs – 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 visible light. 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 the right (Image Source: JHelioviewer image of the Sun from Feb. 15th, 2011), shows the corona 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 a solar 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 minimum temperature in the photosphere, the visible surface of the Sun. The average temperature in the photosphere is 5777 Kelvin (or about 6000 degrees Celsius). Above the photosphere, the Sun has three more important layers in its atmosphere. The layer just above the photosphere is called the chromosphere and it is very, very thin ‐ only about 2000 km in height. During a solar eclipse, the chromosphere is seen as a very thin bright red circle around the Sun, giving it its name which literally means “colored sphere.” The chromosphere actually increases in temperature, going from 6000 at the bottom to 20,000 Kelvin near the top. Above the chromosphere is another very thin layer called the transition region. Here, the temperature rapidly increases from 20,000 K to 1 million K in only 200 km. The corona begins at the top of the transition region and stretches millions of miles out into space. Here the temperature ranges between 1 million and 2 million degrees. Image Source: http://imageshack.us/photo/my‐ images/265/sunpartsfull.jpg/sr=1

Solar Variability The Sun experiences an 11 year activity solar cycle and a 22 year magnetic solar cycle. Over 11 years, on average, the Sun changes from being a very quiet star exhibiting very little activity and no sunspots to a dramatically active star covered in sunspots and experiencing multiple explosions every day. This cycle can be tracked by counting the number of sunspots on the Sun over time (see plot below). At the heart 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 field looks very much like a simple dipole, like a bar magnet. Over the next five or six year period, the magnetic field gets twisted up (due to differential rotation), causing it to become more and more complicated. As the magnetic field twists, the Sun exhibits more sunspots and more activity. Finally, the Sun reaches solar maximum. At this point, the magnetic field looks almost nothing like a simple bar magnet and the Sun is experiencing many flares, CMEs, sunspots, and more every day. At solar maximum, the Sun's magnetic field begins to flip. Over the following five or six years, the Sun begins to quiet again until it has reached solar minimum and its magnetic field has flipped.

At the start of a solar cycle, the Sun's north pole may be at the "top" and the south pole 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 pole will 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 Temperature In 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 warmer temperatures on Earth, more wheat grown, and lower wheat prices. He did find what appeared to be a link, though other studies have found that the relationship depends on which crops are chosen for comparison. It is now known that there are variations in solar energy output on timescales of 20 years and more (The Sun, Solar Analogs, and the Climate, p. 248).

Solar Differential Rotation It is now known that differential rotation is at the heart of the cyclic behavior exhibited by the Sun. The Sun is made up primarily of moving, rotating plasma in magnetic fields. Magnetic fields can guide the course of the moving plasma, as is often seen in close‐up images of solar activity where the plasma 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 its magnetic field along with it. This property leads to very complicated behavior by the solar magnetic field, leading to an explanation of why the Sun experiences a regular solar cycle and provides a mechanism for all solar activity. 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 deep inside the Sun flows from the poles towards the equator. This flow drags the twisted and fragmented southern magnetic fields toward the north pole and north facing magnetic fields towards the south pole, causing the Sun's magnetic field to eventually flip and reform. Then it starts all over again. The solar magnetic poles flip approximately every 11 years. At solar maximum, right before the magnetic poles flip, the magnetic field is in its most stressed state and the Sun exhibits lots of sunspots, flares, and CME activity. Just after the poles flip, the Sun experiences solar minimum with no sunspots, very little activity, and its magnetic field looks most like a simple bipole.

Solar Flares, Coronal Mass Ejections, and the Magnetic Sun­Earth Connection Solar flares are the most energetic explosions observed on the surface of the Sun. To make 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 atmosphere that 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 solar energetic 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 on the Sun. Unlike flares, CMEs result in the ejection of a huge amount of matter from the Sun's atmosphere into space. In fact, each CME ejects about the mass of 500 million Hummer H2’s into space. If a flare may be compared to a tornado, then a CME is most like a hurricane; overall, CMEs are usually less energetic than flares, however their extraordinary size and ejected mass makes them the most threatening solar space weather events the Earth can experience.

A CME is a huge bubble of plasma that strikes the Earth. Much of this plasma (98%) is deflected by the Earth's magnetosphere, but 2% of the plasma enters the Earth's magnetosphere. Some of these streaming charged particles generate electric currents in the atmosphere. These fluctuating electric currents create magnetic fields that are felt on the ground. The magnetic fields then generate electric 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 the electric currents are strong enough, they can cause protective relays in power stations to trip or even destroy transformers, leading to wide‐scale power loss. This happened 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 on Earth is very clear.

Aurora and Magnetic Storms The Northern Lights or aurora borealis are some of the most obvious and beautiful effects of space weather. Aurora are the result of charged particles from the solar wind interacting with the Earth's magnetic field and atmosphere. Most of the electrons, protons, and ions in the solar wind are deflected by the Earth's magnetosphere, however some of them actually get trapped in the magnetic field surrounding 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 magnetic field exerts on the particle causes it to spiral around and follow the magnetic field lines. When a CME hits the Earth, the magnetosphere is stretched and pulled, compressed and released, like a rubber band. The charged particles (electrons and protons) caught in the magnetosphere get energized by this activity and go shooting along the magnetic field lines to the poles where they are brought down toward the ground. This is why aurora occur at the poles ‐ because that is where the Earth's magnetic field approaches the ground!

The high‐speed charged particles stream along the magnetic field lines and slam into atoms and molecules in the atmosphere. The impact puts energy into the atmospheric atoms, causing their electrons to jump to higher energy levels temporarily. When the electrons fall back to the ground state in the atom, they release the extra energy they gained from the collision in the form of photons, i.e. light. Oxygen emits green and brownish‐red light, while nitrogen emits 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 experiencing electron transitions from a range of energy levels. This mechanism is that same one that lights up the fluorescent and neon lights that you experience every day. Image Source: http://en.wikipedia.org/wiki/File:Red_and_green_aurora.jpg

When the Earth and Sun Were Young

The Early Earth The Earth is currently estimated to be 4.5 billion years old. The earliest rocks formed 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 in which 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 the portion of the solar system where the Earth formed to generate an atmosphere at formation. The atmosphere and water on Earth appear to be secondary in origin. The atmosphere is thought to have been formed from gas released by the mantle during the first half billion years (4 billion years ago) after accretion (consistent with a molten mantle from the strikes 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 only 30% – 50% as strong as it is in the present. This has serious implications for the effect 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 Sun Stellar evolution models clearly indicate that the early Sun was only about 70% as bright as it is today. In fact, the Sun will increase in brightness throughout its Main Sequence lifetime. This effect is due to the nuclear fusion of hydrogen into helium in the Sun’s core. As more and more hydrogen are converted to helium, the average density of the Sun’s core increases. The pressure inside the star must increase to hold up the star’s heavier mass and the mechanism to make this happen is an increase 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 the section entitled Faint Young Sun Paradox.

It has been observed that younger stars have stronger magnetic fields and spin more 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’s angular momentum away with it, causing the Sun to slow in rotation period over time (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 Sun with a fast rotation period (possibly just a few days compared to the current 27 day rotation 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 meteorites does indicate a more intense ancient solar wind (Gaidos, Gudel, and Blake).

The Impact of the Early Sun on the Early Earth The 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 would have 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 a daily basis is what the Earth only experiences during the strongest solar storms today. We now know that the Earth’s magnetic field was much weaker and the early magnetosphere only extended about half as far above the Earth’s surface as it does today. The strong solar winds combined with the Earth’s weaker magnetosphere would have lead to a constant display of auroras glowing in the Earth’s young atmosphere regularly extending down New York City level latitudes, an event that is relatively 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 loss rate 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 certain that 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 Paradox As explained above, the early sun was only 70% as bright as it is today. A calculation 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 billion years 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, which would 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 more efficiently 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 CO2 and methane in the early atmosphere, however recent studies of early atmospheric levels show no evidence 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 a different distribution of clouds, causing less of the Sun’s light to be reflected and more heat to stay within the atmosphere, but these studies typically fell short of a solution. Some attempted to propose a different evolutionary history for the Sun, calling the its 30% reduction in brightness into question, however surveying solar type stars did not support this possibility (Gaidos, Gudel, and Blake). It should also be 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 the problem. The continents were smaller in the past, so more of the Earth was covered with dark oceans that could absorb more heat. Their model also depends on fewer clouds, which may have very well been the case as clouds today form around biogenic sulphur gases and plants had not yet flourished back then. “We put together some models that demonstrate, with the slow continental growth 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 mechanisms used in Rosing and Allen’s research fail to work in some conditions (Kasting, 2010).

The Sun, Modern Society, and Human Health

The Heliosphere The same way that the Earth’s magnetosphere forms a protective bubble around the Earth from the solar wind, the Sun’s heliosphere forms a protective bubble around the solar system, protecting it from the interstellar medium. The same way that the solar wind buffets the Earth’s magnetosphere and compresses it on the “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 edge of the heliosphere occur? Ultimately, at some point, the solar wind expands far enough that it does not have enough pressure to repel the interstellar medium. The place 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 the Voyager 1 & 2 spacecraft, launched in 1977 and have been traveling outwards into space ever since. In the past decade, the Sun experienced intense explosions. These explosions traveled outwards to the heliopause, creating a radio hiss 13 months later that was detected by the Voyagers. The speed of the disturbances was measured, 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 the heliopause in the next decade. The Voyagers have enough electrical power and fuel to operate until 2020.

On June 20th, 2011, Scientific American reports that Voyager 1 is now over 17 billion km at Earth and is either approaching the heliopause or may have passed through into interstellar space. It no longer measures the solar wind at its back and it is a tranquil environment. If the Voyager has crossed the heliopause, it has become the first 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 Radiation The Sun emits two different distinct types of radiation – electromagnetic radiation and particle radiation. We also use radiation to describe emissions from radioactive particles. 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 Radiation Electromagnetic radiation, or EM radiation, refers to waves in the entire EM spectrum. This includes radio waves, microwaves, infrared waves, visible light, ultraviolet light, X‐rays, and gamma rays. The shorter the wavelength, the more energy 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 a person’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 DNA molecule, 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 Radiation Particle radiation is caused by subatomic particles moving at high speeds, often close to the speed of light. Because they move so fast, they carry a lot of energy. The most common particles are electrons (also called beta particles), protons, helium nuclei (also called alpha particles), and neutrons.

Energetic particles can be emitted by radioactive atoms here on Earth or they can come from space. The Sun emits energetic particles that are called solar energetic particles (SEPs). These electrons, protons, and alpha particles are not very energetic compared to galactic cosmic rays (GCRs) that come from the galaxy and the universe. GCRs move at nearly the speed of light and are created in extremely energetic physical processes like supernovae. The most energetic cosmic rays are millions of times more energetic than the highest 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 Atoms Radioactive atoms are atoms with a very large number of neutrons compared to protons, making their nuclei unstable. The decay rate of a radioactive atom is completely independent from all outside influences, such as temperature, pressure, or chemical compound in which the radioactive nucleus is found. Radioactive decay is 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 2 neutrons, also called a helium nucleus), a beta particle (electron), and/or gamma rays (electromagnetic radiation).

Alpha particles are heavy and move very slowly. They do not penetrate very deeply into materials and can be stopped by a piece of paper or even the outer dead layers of skin, however eyes and open wounds are at risk. Because they are heavy, however, they are very good at ionizing atoms (http://darvill.clara.net/nucrad/types.htm). If someone happened to inhale an alpha emitting radioactive atom, it could do damage to internal tissues, potentially causing 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 proton and electron. The proton stays in the nucleus and the electron is ejected. Beta particles are very light and fast. They can penetrate deeper into materials than alpha particles, but are stopped by solid objects. Beta particles can cause reddening or burning of the skin. Inhaled beta particles are even more dangerous as they can penetrate deeper into tissues and disrupt cell function. Some beta emitters are cobalt‐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 beta particle. Gamma rays have enough energy to penetrate into the body and even completely pass through it. They can potentially cause a lot of damage by exposing all 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 inside tissue directly. (http://epa.gov/radiation/understand/gamma.html)

Types and Effects of Radiation from the Sun Flares 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 upper atmosphere, called the ionosphere. We use the ionosphere to bounce short‐wave radio signals from one part of the Earth to the other, so a change in the ionosphere causes a disruption in short‐wave radio communications.

X‐rays also heat up the Earth's upper atmosphere. This causes the atmosphere to expand or puff up. Usually, satellites are placed in orbits high above the Earth where the atmosphere is very thin or almost nonexistent. If the atmosphere expands, satellites suddenly find themselves surrounded by air, which causes a frictional drag. 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 a torque 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 the Sun. SEPs are charged particles, like protons, electrons, helium nuclei, and other ions, that are ejected and accelerated by really energetic activity on the Sun. SEPs can 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 the image 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. On Earth, we are protected by our magnetic field. Inside the space station, astronauts are protected, though an astronaut on a space walk during a burst of SEPs may be in trouble. In the future, humans traveling to Mars would be in great danger from SEP radiation. In any mission where people would spend a long time in space far from the protection of Earth's magnetic field, SEPs would be a serious problem.

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

Unlike flares, a CME is a huge bubble of plasma that strikes the Earth. Much of this plasma (98%) is deflected by the Earth's magnetosphere, but 2% of the plasma enters the Earth's magnetosphere. The streaming charged particles generate electric currents in the atmosphere. These fluctuating electric currents create magnetic fields that are felt on the ground. The magnetic fields then generate electric 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. If the electric currents are strong enough, they can cause protective relays in power stations to trip or even destroy transformers, leading to wide‐scale power loss. This happened 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.

Solar Radiation and Human Health Risks First it should be pointed out that humans experience radiation every day, even from inside our own bodies. The Earth, cosmic rays from space, and even living things 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 far less energy than galactic cosmic rays, but they do have enough energy to be a potential health risk to people in space or at high altitudes. Solar protons are also called cosmic rays, but the term galactic cosmic rays (GCRs) is reserved for those extremely high energy particles that are clearly not generated inside of our solar system.

So how high energy is high energy? Particles from the Sun tend to be what is considered on the lower energy end in the 10 – 100 MeV (million electron volts) range. Particles need to have energies of greater than 450 MeV to be detected on the ground. Near the equator, 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 traveling 43% the speed of light. A proton with an energy of 10 GeV is traveling 99.6% the speed of 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 shield against solar proton events and only about 15% of the protons in these events have enough energy (> 450 MeV) to create a particle cascade that is detectable on the ground. The Earth’s magnetosphere is the primary barrier to cosmic rays from the Sun. It is easiest for these cosmic rays to reach the Earth’s atmosphere near the poles where the magnetic field heads down into the ground. Farther from the poles, the magnetic field raises increasingly higher above the ground, requiring protons of higher and higher energies to penetrate to the atmosphere. Thus, the amount of radiation that penetrates to the atmosphere depends on magnetic latitude. People that live above 50 degrees geomagnetic latitude experience about twice as much radiation as people 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 living in Denver, Colorado experience about twice as much cosmic radiation as people living at sea level. This added exposure to cosmic rays does not appear to have a significant health impact as medical studies show that people living at mountain altitudes are generally healthier and have longer life spans than those living at sea level. However, increased exposure to cosmic rays is a concern for people traveling at aircraft altitudes (especially above 40,000 feet), particularly for pilots, flight attendants, and passengers traveling circumpolar routes. Accumulated radiation from a flight schedule of 70 hours per month at high latitude routes could reach the International Council on Radiological Protection (ICRP) recommended limit for a pregnant female in just five months (2 mSv). However, it is difficult to accumulate the annual IRCP recommended limit of 20 mSv for everyone else.

The radiation exposure of those in flight during solar proton events can be worrisome, 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 .05 mSv. 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 a radiation concern and circumpolar flights are generally rerouted during solar storms.

For astronauts in space, radiation exposure and solar proton events are a serious concern. Inside the most heavily shielded parts of the space station, astronauts are reasonably well protected from solar storms, but if the radiation 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 an astronaut’s career prematurely. (Image Source: http://www.nasa.gov/astronauts/)

Luckily, NASA and other space agencies around the world have many satellites in orbit around the Earth and at the Lagrangian point, L1, located about a million miles from the Earth towards the Sun. These satellites provide an early warning system for solar storms, alerting scientists to potentially dangerous solar activity a few hours to a few days before it would reach the Earth.

Solar radiation is one of the largest problems scientists must face when considering manned space flight. So far, NASA has been lucky. On August 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 killed astronauts 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 lifetime chance of getting cancer to 45%. Some scientists estimate that every third human cell 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 the environment 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 Earth exploded with over 195 flares, 11 of them classified with the label X‐class, reserved for the most intense of solar storms. As the sunspot group rotated into view on March 6th, 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 physicists estimated that the temperature inside the flar reached 20 million degrees Celsius and more energy was released in those moments than humans have consumed in the entire history of civilization.” (p. 94) The readings on the NOAA GOES‐7 satellite’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 the Earth. On March 9th, another enormous flare exploded with such brightness that it exceeded the maximum values on the scales used to record flare brightness. On the 10th, observers saw a white light flare like the one Carrington had seen in 1859, an extremely rare event. Following this flare, a halo CME was observed, meaning that it was heading straight for the Earth.

On the night of March 12th, the front edge of the CME reached the Earth and by midday on the 13th, the Earth’s magnetosphere was compressed from a normal 34,000 miles above the Earth to possibly as low at 14,000 miles. This meant that satellites that were normally protected by the Earth’s magnetic field were suddenly laid bare in open 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. The aurora 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, and electronics companies were struggling. In the northeast US, a computer microchip manufacturer shut down because the magnetic storm was disturbing the sensitive equipment. Navigators saw their compasses distort to as far as 10 degrees. North Sea oil companies had to stop drilling because the magnetic instruments that guide the 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 atmosphere was expanded due to heating from the bombardment of the particles.

Satellites that should not be were suddenly feeling the frictional drag force of the atmosphere. The Cheyenne Mountain Operations Center which tracks about 8000 pieces of space junk lost track of 1300 objects. NASA’s Solar Maximum Mission satellite (pictured) dropped 3 miles in orbit over a course of a couple of days (ultimately re‐entering the Earth’s atmosphere and burning up in December of 1989). A classified US military satellite tumbled uncontrollably through space. NOAA’s GOES‐7 weather satellite suffered outages and communication problems. The added electric currents due to charged particles from the CME caused phantom switching and tripping of circuits. Some geostationary satellites had trouble staying in place. The excited ionosphere lead to troubles 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 particles in the atmosphere lead to disturbed power companies in Maryland, California, New York, New Mexico, Arizona, and Pennsylvania. In New Jersey, a $10 million transformer was damaged beyond repair. Usually these type of transformers take a year to build, but the power company was able to find one and get back up and running 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 the collapse of the Hydro‐Quebec power plant and its neighboring plants. Once one plant experienced a loss of power, the added demand on the other plants caused them to collapse as well. All of this happened within seconds. Six million people in Quebec City, Montreal, and the surrounding areas were left without power over the freezing Canadian March night. It took nine hours to restore power by channeling it from other utilities. After a study of the system collapse, scientists reported that the entire northeastern US, which depended partly on Hydro‐Quebec power, almost fell into blackout, as well.

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

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