Events at the Surface of the Sun

Introduction

President Ronald Reagan was airborne over the Pacific Ocean en route to the People's Republic of China on April 24, 1984 when an angry outburst from the sun severed all short-wave communications to and from Air Force One. As the plane's radio fell silent, the Commander in Chief - suspended seven miles high somewhere between Fiji and Midway - found himself unable to send or receive messages. For a period of several hours, the Great Communicator, as Reagan had been dubbed by the press, was cut off from the rest of the world by a star 93 million miles distant. Although this period of presidential isolation ended without mishap, it created some harrowing moments for Air Force specialists charges with keeping the president in constant communication with military commanders back home.


The solar radiation that blacked out communications to the president's plane had originated in a solar flare so intense that measurements of the event went right off the charts. On the specialized scale that solar scientists use to gauge flares, the outbreaks normally range from a low intensity of c-1 to a high of x-10, with the two extremes differing in magnitude by a factor of 1,000. This flare registered a phenomenal x-13, and the radiation storm it generated on Earth did not even begin to subside until the following day. The American military was made acutely aware of the effects of a nuclear pulse of radiation and took quick steps to protect our defense system from a Soviet attack, and then launched an intensive initiative to study solar phenomenon to prepare for further disruptions.

What You Will Learn Here

This page will help you learn about Sunspots, the Sunspot Cycle, Solar Flares, and the Northern Lights. All of these things are related to complexities in the Sun's magnetic field. There is even a little piece of information about how sunspots affected the perfect sound of the Stradivarius violin!

You Begin Here With Sunspots

The sun has been shown to have a remarkable history of flurries of eruptions alternating with long - sometimes remarkably long - periods of silence. Indeed, the record suggests that the only consistent aspect of this moody star may well be its inconsistency. Flares like the one that isolated the president's plane, for example, are associated with large sunspots, the dark blemishes whose numbers wax and wane in a cycle of approximately eleven years. Yet flares can erupt at any time during the sunspot cycle, and the customary eleven-year span between one solar maximum, or peak of solar activity, and the next can stretch to thirteen years or shrink to a mere eight. And although scientists have recently determined that the sun dims and brightens slightly, they as yet have no clear idea why, or how, or at what intervals it does so.


For quite a number of years, solar scientists attempted to measure the sun's luminosity. In 1980, NASA placed its Solar Maximum Mission satellite into orbit and measured accurately the "solar constant" at an average of 1,367.5 watts of solar radiation falling on a square meter outside the earth's atmosphere at one astronomical unit from the sun. We call it the "solar constant" but when accurate measurements are taken from the solar observatory atop Mauna Loa, the sun’s energy does not stream toward the Earth at a constant rate. There are subtle variations due to sunspots, flares, and energy flux.


Beyond the inconsistency of the "solar constant" the sun has shown itself to be a highly variable star in a number of other ways. Galileo demonstrated that dark spots occasionally appear on the surface of the sun. Clearly, the sun was not a "perfect" creation, but demonstrated flaws. Sunspots became an area of study late in the 1800’s and mid 1900’s. For many years, astronomers held to Aristotle's incorrect assertion that the sun was free of imperfections. However, in the nineteenth and twentieth centuries, evidence began to emerge that the sun may have gone through extended periods of inactivity in the past with an absence of sunspots - including during Aristotle's lifetime - which could account for an impression of solar serenity.

The German astronomer Heinrich Schwabe established in 1843 that sunspots come and go in a cycle of 11 years. However, another German astronomer, Gustav Sporer who was studying older records noticed that during the years from 1645 to 1715, the number of sunspots reported in the scientific literature from several European countries fell to nearly zero. Surprisingly, the lack of sunspots occurred at a time when global weather was significantly cooler than normal. Lithographs show mountain glaciers to be longer than ever before during these 70 years, and many cities experienced snow in winter when rainfall was typical. Although Sporer published two papers on his findings, they were largely ignored. In 1890, English astronomer Edward Maunder, the superintendent of the solar department at England's Greenwich Observatory, came across Sporer's findings and decided to make his own search through the same and other records. Maunder saw that Sporer was right: firsthand reports of solar observations noted not one spot on the sun's northern hemisphere between 1672 and 1704. Indeed, fewer sunspots had been sighted during the entire seventy year period examined by Sporer than were seen during a single average year of the era Maunder was living in. Once again, when Maunder published his findings in 1894, they were ignored, and again in 1922. Maunder wrote to Andrew Douglass who was measuring tree ring growth patterns to learn if there was any connection between ring patterns and the sunspot cycle. Although the data set was too small to be statistically significant, it was still sufficient to convince Douglass, but still no one else.

This image was taken July 29, 2002, and current images can be seen at SpaceWeather.

In the mid 1970's, John Eddy, in Boulder, Colorado set out to prove Maunder wrong. After much study, Eddy was convinced that Maunder had been correct all along, and he dubbed the period of solar inactivity the "Maunder Minimum". Eddy discovered that not one aurora had been witnessed during a 37 year period of the 70 year Maunder minimum. When Eddy consulted the records of the Japanese and Chinese, which dated back to the third century AD, he noticed that not one sunspot was recorded between 1639 and 1720. When Eddy studied tree growth rings more carefully, he even found a prolonged minimum from 1420 to 1530, and using the Bristlecone Pine, he traced quiescent periods back to the Bronze Age. He concluded that the sun spends 1/3rd of its lifetime relatively quiet.


"The reality of the Maunder Minimum and its implication of basic solar change," Eddy mused before a packed audience in Boston in February 1975, "may be but one more defeat in our long and losing battle of wanting to keep the sun perfect, and if not perfect, constant, and if not constant, regular. Why the sun should be any of these when other stars are not is probably more a question for social than for physical science." Today we know that the sunspot cycle occurs every 11 years on average, but with a complete reversal of the polarity of the sun’s magnetic field. After 11 more years, the field is reversed once again to its starting point and the 22 year period is completed. Additionally, the dark spots appear to emit radiation in greater quantities than when the sun is without them. Thus, the quiescent sun at the surface during a solar minimum will lead to globally cooler temperatures while solar maximums will lead to typically warmer temperatures due to the increase in energy output. The Sunspot Cycle is seen in the image below, which records the number of sunspots visible on the Sun over time.

This close-up image of a sunspot group shows the center umbral region, less dark surrounding penumbra, and granulation of the photosphere. The granulation is the result of massive upheavals of solar material, convection from deeper inside much as boiling water convects from the bottom of the pot to the turmoil of the top. The interior temperature of the umbra may be less than 4000K. While this is significantly hot and very bright, it is 2000K less than the surrounding photosphere temperature of 6000K, and therefore appears dark in contrast. The size of the particular sunspot is greater than the diameter of two Earths!

Check out the UCAR page to learn a simple explanation of the development of sunspots. Basically, since the Sun's gas rotates at different speeds in different latitudes, the magnetic field lines get mixed and twisted, and the sunspots seem to develop at these cross-over regions on the photosphere, and you will learn in a bit more detail below.

 

 

The Sun has a massive magnetic field with north and south poles. Since the Sun is a huge ball of hot gases, it does not rotate as a solid would. The equatorial regions complete a rotation in 25 days while the polar regions require 33 days to spin once. As a result of this differential rotation, the lines of the magnetic field become twisted out of nice north-south orientations with successive rotations. When the magnetic field becomes sufficiently contorted, a weakening develops at the photosphere and sunspots develop. The more contorted the magnetic field, the more sunspots will develop. In time, the entire magnetic field of the Sun flips and the north magnetic pole will occupy the south heliologic pole. After an other 11 years, the poles flip again.

The sunspot cycle at its maximum during this time when the magnetic field is ready to flip. This time is also associated with dangerous solar events like flares and prominences. Of the many questions about the sun that remain firmly rooted in the domain of physical science, perhaps the most pressing is whether short-term solar behavior can be predicted with any accuracy If it can, astrophysicists and climatologists will be able to anticipate - and adequately prepare for - the sun's most dramatic and damaging effects.

 

The Swedish Solar Observatory, located on the Canary Islands, has a large telescope devoted to study of the Sun. Astronomers there have taken some of the most spectacular images ever of the surface of the Sun and some prominent sunspots. Click on the sunspot image below to learn more.

A solar prominence occurs when the Sun erupts material but that material is caught within magnetic fields, forming beautiful arcs. As the magnetic field "sews up" the tear in the photosphere, the prominence is returned to the photosphere and seeming order is somewhat returned. The image to your left shows such a solar prominence as photographed by one of our solar satellites TRACE in June 9, 2002. To give you a perspective of the immensity of this eruption, remember that 110 Earths would fit across the diameter of the Sun. This prominence therefore has a width of over 500,000 km!

 

 

 

 

The most extensive precautions would likely be taken for a solar flare. A flare occurs when the photosphere tears and solar material gushes to the surface and bursts tremendous amounts of radiation. As Donald Neidig, one of a team of six Air Force astronomers detailed to the Sacramento Peak Solar Observatory, "A really big flare can produce enough energy to supply a major city with electricity for 200 million years. A series of photographs are shown below to give you a look at solar flares and how they are different from prominences. The upper left image is a series of pictures of the flare erupting. Unlike the prominence, there is enough energy for the material to actually escape from the Sun. The upper right image is a close up of such an eruption. The bottom left image is a flare as it appears at the photosphere ... a tear in the very "fabric" of the photosphere with the release of huge amounts of energy and material. The bottom right image is of a coronal hole, a region where the magnetic field of the Sun has opened up, and a potential source of the ejection of solar material in an event called a Coronal Mass Ejection (CME). This is what is happening in the upper right image, and is the greatest source of concern for astronauts.

Coronal Mass Ejection CME

Huge CME video

Typically, these monumental flares on the sun begin as an almost imperceptible bright loop. Within just a few hours, however, the loop suddenly explodes, spewing billions of tons of gaseous material - as well as a wide range of electromagnetic radiation and energetic nuclear particles - into space. Eight minutes later, a strong blast of x-rays and ultraviolet rays reaches earth and radically alters the ionization structure of the planet's upper atmosphere; this in turn wreaks havoc with the way radio waves are reflected from that layer. Although only a fraction of the flare's ejecta ever reaches earth, it can be enough to disrupt communications and electrical power systems all over the world. Twenty four minutes or so later, earth's neighborhood is bombarded by potentially dangerous high-energy protons traveling at one-fourth the speed of light. Astronauts en route to or from the Moon or Mars, for example, and thus beyond the protective sheath of earth's magnetosphere gases, could suffer lethal radiation poisoning. The last assault is a magnetic shock wave, a fast-moving magnetic disturbance created when matter is expelled from the sun sometimes at more than 600 miles per second; it washed over the earth two days after the eruption.


As this shock wave interacts with the ionosphere, electrons in these ions are excited to higher energy levels. As the electrons spontaneously fall to their resting state, a photon is released in a color related to the atomic structure. Excited negatively charged ions falling to the positively charged north magnetic pole and releasing colorful photons create the beautiful Aurora Borealis. The more energetic the flare eruption, the more magnificent the auroral display and the father south the display becomes visible. You are asked now to go the the Aurora page highlighted above and learn what the cause of these Northern Lights is, as well as to discover ways to look and photograph them yourself from your backyard.

In March of 1989, a particularly violent series of flares erupted during a ten day period, knocking out all electricity across Quebec, rendering normal radio frequencies unusable, and draping the night sky with aurora as far south as Key West, Florida. As the flare's extreme ultraviolet flux heated and expanded the earth's upper atmosphere, the increased atmospheric drag reduced the orbital energy of hundreds of satellites in low earth orbit. This knocked the spacecraft into lower and faster orbits, causing ground controllers to temporarily lose contact with them. Many of the 7,000 objects which are tracked by the US were lost from view, and some burned up in reentry. The Hubble Space Telescope was launched during a solar maximum to ensure that it was orbiting when the flare activity was highest since there is difficulty in moving it to a higher orbit without the shuttle aiding. Indeed, this catastrophe happened to the Solar Maximum Mission satellite which was designed to study flares. After blowing three fuses, the shuttle team made repairs, but the flare activity of 1989 caused it to burn up on premature reentry.

Even more incredible was the Perfect Solar Storm of September 1-2, 1859. Events such as this happen perhaps once every 100 years or so. If one were to occur today like the storm of 1859, it is estimated that 1/2 of the Ozone layer would be destroyed and the entire space satellite fleet would be rendered useless.

According to the leading hypothesis of solar flare origin, the flare results from a fast, catastrophic rearrangement of magnetic fields in an active region of the sun's corona. Somehow, this instability in the magnetic fields accelerates beams of protons or electrons, which follow the corona's arcing magnetic field lines down into the denser regions of the chromosphere. There, the particle beams are slowed by collisions with protons and free electrons, a process that releases high energy x-rays and gamma rays and causes superheated plasma to explode into the portion of the corona where the flare began.

Scientists who have studied these flares have found spectral signatures of iron atoms in the coronal magnetic field with 25 of their 26 electrons stripped away. Applying the laws of physics, astronomers have concluded that these gases must be heated to 50 million K - three times hotter than the center of the sun itself. Yet the magnetic fields governing the creation of the flare are strong enough to keep the hot gases from expanding.

Although they occur more often and cause less damage on earth than flares, the related solar outcroppings known as prominences can be every bit as sensational. The smallest solar prominences have widths roughly equal to that of the earth; the largest may approach half the diameter of the sun itself. These arches of glowing gas float tens of thousands of miles above the solar surfaces, suspended by the sun's looping magnetic fields and sculpted into such shapes as hedgerows, funnels, and arcades. Some dance 3,000 miles high, while others loop to a spot 50,000 miles away.

Any prominence that rises more than 30,000 miles above the sun's surface is likely to burst within 48 hours. Most of these eruptive prominences fling their gases outward, tearing apart the overlying corona and injecting enormous quantities of plasma into space; others however, simply fall as a kind of rain into the chromosphere. Interestingly, two-thirds of the eruptive prominences re-form in the same shape and place several times, suggesting that the explosive releases are a normal part of their life-cycle and that special conditions favorable to the formation of a prominence prevail at particular sites.
A number of astronomers have proposed that loop prominences reveal the sun in the process of healing the wounds inflicted by a flare. In this scenario, a loop prominence is a sort of gigantic suture, showing exactly where the magnetic field lines pulled apart by a flare are slowly being stitched together again. Once the magnetic reconnection is complete, the prominence fades and disappears.

Sun Conclusion

I have written a fair amount of material for your digestion. The Solar Maximum peaked between November 1999 and February 2000, and a rare second peak concluded during the spring of 2002. Solar physicists predicted excess flare and auroral activity, stock traders were fearful of Y2K, astrologers were nervous of the May, 2000 planetary alignment, and doomsayers dreaded the end of the millennium. All of that passed without any disaster. The year of 2008 was a Solar Minimum, and had the lowest number of sunspot activity on record. We are reminded to be wary of excessive fear for natural phenomenon or pseudoscience predictions. You are strongly encouraged at this time to look at the SolarMax page now.

Please keep in mind that the sun produces incredible amounts of energy, but only a small potion hits the earth because we are so far away and space is so vast. Of the amount which does strike the Earth, much is reflected by the clouds, ice, or water, much is absorbed by the ground and oceans, and only .2% is actually available for life-giving photosynthesis. If the sun were hotter or cooler, farther or closer to Earth, or more variable, life as we know it would cease.

During the final two weeks of October, 2003 the Earth was blasted by two extremely powerful solar outbursts. The Sun held two Jupiter-sized sunspot groups, one of which let loose two of the most potent coronal mass ejections ever observed ... on successive days of October 28 and 29. These fast-moving energy fields struck the Earth the following evenings and produced spectacular auroras visible as far south as California, Oklahoma, and Florida. To see a special page devoted to the Solar Storm of the Century, click on Solar Storm.

I have included this very cool picture of the Sun's surface as photographed by the Transition Region and Coronal Explorer (TRACE)satellite. This may be my favorite website for images of the Sun, and you are strongly encouraged to go there for the pleasure the images bring, and then sense of awe that such photographs can give you too. Another wonderful site is that of the Solar and Heliospheric Observatory (SOHO). This site has pretty pictures as well as links to almost anything you would ever want to learn about the Sun.

 

 

You are now ready for the quiz and lab activity that accompanies this Unit on the Sun. You can go to the Sun Quiz, Sun Lab, or return to the Introduction to the Sun, or to the Syllabus.


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