Science Briefs

Observing Sun-Earth Connections (by Dr. Daniel N. Baker)

Humankind has been fascinated since the dawn of recorded history with the Sun and its relationship to Earth. Virtually all civilizations have speculated about the place of our planet in the realm of the solar system and the stars of our galaxy. Only within the past few hundred years - since Copernicus - has there been acceptance of Earth revolving around the Sun and only since the time of Galileo has the changing face of the Sun been known. The role of the sun in driving disturbances in Earth's

magnetic field has come to be appreciated within the last century or so.

Thus, human understanding of solar-terrestrial relationships, though having a history of perhaps five thousand years or more, has proceeded at a painfully slow pace. Just since the beginning of the space age have we come to a relatively clear picture of the nature of solar variability and how this variability affects Earth. Astronomical observations make clear that our Sun is like many variable stars and so our Sun-Earth system is the physical prototype for stellar systems throughout the cosmos.

Rather remarkably, human technology on Earth has developed in close step with our evolving appreciation of the sun's influences. Mankind is now using a web of electrical and communication links that literally gird Earth. We also employ a vast array of spacecraft around Earth that give us nearly instantaneous communication, exact position information, knowledge of approaching weather systems, and military intelligence that makes the world safer for all of us. Every one of these human technologies can be adversely affected by disturbances in the solar-terrestrial


We now know in general terms that our sun reaches a maximum of activity every 11 years. As it reaches this state of coronal disturbance, the Sun is capable of expelling huge "plasmoids" of material (called coronal mass ejections - CMEs) which can move outward from the sun at more than 1000 km/s. The shock waves preceding such plasma structures can accelerate particles to huge energies sometimes over one hundred million electron volts. If the shock waves and CMEs strike Earth's magnetosphere, they can initiate huge geomagnetic storms that can thoroughly disrupt power systems, communication links, and the constellations of operational spacecraft on which our societies increasingly rely. The appreciation of CMEs as the agents of such profound solar disturbances of Earth and its environs has only come about in the past few years: This "paradigm" shift has had a far-reaching impact on how we think about solar-terrestrial relationships.

Given a many thousand-year wait to have the tools to study the Sun, Earth, and our place in between, we now have a most remarkable situation presented to us. The International Solar-Terrestrial Physics (ISTP) program has put into place the most astounding array of spacecraft and ground facilities ever conceived of for studying the space environment. There are exquisitely sensitive telescopes in space examining the Sun's many layers. There are spacecraft measuring the hot, high-speed plasmas flowing over Earth from the expanding solar corona. There is an unprecedented armada of spacecraft within Earth's magnetosphere examining continuously all facets of the plasmas which ebb and flow as the sun buffets our geospace environment. There is even an international web of ground stations that are recording quite exactingly the magnetospheric and ionospheric signatures of

the ever-changing interaction of the variable sun with our terrestrial environment. Humankind has never before had a "Great Observatory" of such power, precision, and completeness to study our most important star - the Sun - and our most important planet - Earth.



Perhaps ironically - certainly fortuitously - the tools offered to us by ISTP have fallen into place just as we are beginning a new solar cycle - Number 23 in the international parlance. As measured by sunspot number, this next solar maximum - probably to be reached in the year 2001 - will most likely be a large one. It may be the equal of the strongest solar maximum that has been experienced in the modern era. This would mean that solar disturbances of great power and destructive potential may be on their way to the geospace domain. It is an historic confluence of immense

importance that the ISTP armada is in place now and is operating flawlessly as the "Solar Maximum 2001" approaches. We have a possibility - perhaps never to be repeated - to study all aspects of the solar maximum and its consequent effects on the near-Earth environment. It is an epochal occurrence that for modest costs, the extended operation of ISTP and affiliated spacecraft can give us the scientific view and the practical knowledge that we need to finally understand the disturbed sun and the consequently-disrupted geospace environment. This proposal describes the

opportunity afforded by extended ISTP operation to revolutionize our understanding of solar-terrestrial physical processes.

Space Weather: the Sun-Earth connection

From 93 million miles away, the Sun looks serene. But in fact, our closest star is a seething cauldron that churns, boils and frequently explodes. Parts of the Sunís surface and atmosphere are constantly being blown into space, where it becomes the solar wind. Made up of hot, charged particles, this wind streams out from the Sun and buffets the solar system. Moving at a million miles per hour, the solar wind takes three or four days to get to Earth, and like earthly winds, it can be gusty and turbulent. When it arrives at Earth, it interacts with our magnetic field, generating millions of amps of electric current. The solar wind blows the Earthís magnetic field into a comet-shaped region called the magnetosphere. Collectively, the eruptions from the Sun, the disturbances in the solar wind, and the twisting and stretching of Earthís magnetosphere are called space weather. And, just like weather here on Earth, it can be both mildóand wild.

During the month of March in 1989, a staff scientist strolling outside Arizonaís Kitt Peak National Observatory observed a red glow in the night sky that he thought was caused by forest fires. Then, seeing a greenish fringe and vertical streamers stretching like ribbons above the horizon, he realized what it was ó the aurora borealis, or Northern Lights. It was very unusual to see this mysterious and awe-inspiring phenomenon so far south of the Arctic region where they are a common sight. Their appearance on this night was made possible by a series of events that began several days before on the Sunís surface-some 93 million miles away.

In early March, an immense area of sunspots (darker, cooler regions of the Sunís surface) large enough to contain 70 earth-size planets had come into view around the eastern rim of the Sun. Created by intense magnetic fields, the giant sunspot group suddenly brightened and expanded to cover hundreds of thousands of square miles. This solar eruption, called a flare, was accompanied by a huge burst of electromagnetic radiation and billions of tons of matter that were flung far into space that scientists refer to as a coronal mass ejection. The radiation, mostly in the form of X-rays, traveled at the speed of light and was detected on Earth about eight minutes after the flare erupted. Carried along by the solar wind that blows continuously away from the Sun at speeds of up to several million m.p.h., the energetic particles from this large solar flare happened to intercept Earth in its orbit around the Sun.

Most people are familiar with the three states of matter common on Earth: solid, liquid, and gas. The most common form of matter in the universe, however, is "plasma," the fourth state of matter. In a plasma, the atoms and molecules are broken up into positively and negatively charged particles. Plasmas are electrical conductors so that they can interact strongly with electric and magnetic fields. The Sun is so hot that it is entirely composed of plasma including the solar wind which blows ever outward from the Sun.

Earth has a strong magnetic field that extends far into space. Like a rock in a stream, the solar wind mostly flows around this magnetic cavity called the magnetosphere. During the solar eruptions in March 1989, the solar wind buffeted Earthís magnetosphere resulting in large "magnetic storms." These caused huge auroral displays that extended much farther to the South than usual. They also caused radio interference, increased drag on satellites orbiting near Earth, and were responsible for shutting down the Hydro-Quebec power system that blacked out parts of Montreal and the province of Quebec for as long as 9 hours causing millions of dollars in damages.

The Sun is now approaching another maximum - are we prepared?

Our Changing Sun

Although the average distance from Earth to the Sun is a whopping 149,600,000 kilometers (93,000,000 miles), careful observation from Earth reveals a surprisingly large number of different visible features. The most obvious and best known feature is the sunspot. Typically moving in groups, these dark (in visible light), planet-sized features have been known to humankind for centuries. As sunspots form and disappear over periods of days or weeks, they also appear to move across the Sunís surface. Composed of strong magnetic fields, sunspots are shaped much like a horseshoe magnet that rises from below the Sunís surface. The rising hot gas is trapped by the sunspotsí intense magnetic field which cools the sunspots from 6000 ˚C to about 4200 ˚C. The cool area appears dark compared to the area around it. Thus, from Earth, we see spots on the Sun. In some photographs, we can also see light colored areas around groups of sunspots that resemble tufts of cotton candy. We call these fluffy looking fringes, plages.

Sunspots are the source of massive releases of energy called solar flares, the most violent events in the solar system. In a matter of minutes to several hours, a solar flare releases about 10,000 times the annual energy consumption of the U.S. Solar flares give off radiation that includes X-rays and ultraviolet rays, and charged particles called protons and electrons. This sudden surge in radiation can damage spacecraft and even give a dose or radiation to travelers flying in airplanes over the polar regions.

Also visible for only minutes, are granulations in the Sunís photosphere. Granulations are rising and falling columns of hot gases that look like fluffy marshmallows arranged in a honeycomb pattern. The tops of these granules form the Sunís "surface". Although we refer to the Sunís "surface" as the photosphere, you probably know, that the Sun has no solid surface, unlike Earth. It is an uneven sphere of glowing, hot gas!

Just as the Sun disappears behind the Moon during a total solar eclipse, a flash of bright red light appears. This colorful layer of the Sun, called the chromosphere, becomes visible for a brief instant. Although we know little about the chromosphere, there are curious, permanent features of the chromosphere, called spicules, that we can study in more detail. There are so many of these fine, bright, hairlike features, that they are always visible near the Sunís edge, even though an individual spicule lasts only minutes. Like sunspots, spicules rise and fall vertically above the Sunís surface.

One of the most spectacular features of the Sun are solar prominences. They appear to stream, loop and arch away from the Sun. The most recognizable prominences appear as huge arching columns of gas above the limb (edge) of the Sun. However, when prominences are photographed on the surface of the Sun, they appear as long, dark, threadlike objects and are called filaments. Like sunspots, prominences are cooler (about 10,000 ˚C) in relation to the much hotter background of the Sunís outer atmosphere (about 1,500,000 ˚C). Prominences can also erupt from the Sun with a tremendous burst of energy.

If you have seen photographs of a solar eclipse, then you have probably noticed a bright halo around the Sun, called the corona. Sometimes parts of the corona appear to be missing. Logically, we call this area a coronal hole. Scientists believe that the solar wind, a million mile per hour gale that blows away from the Sun, originates in coronal holes. Unlike wind on Earth, the solar wind is a stream of ionized (electrically charged) particles speeding away from the Sun.

The Sunís corona changes with sunspot activity. When there are more sunspots, the corona appears to be held closely to the Sun; when there are fewer sunspots, the corona streams out into space in a shape that resembles the spike on a warlike, peaked helmet called helmet streamers. While helmet streamers are long-lived, their demise often occurs abruptly through a massive and powerful eruption called a coronal mass ejection (CMEs).

These huge clouds of hot solar gas and magnetic field are often associated with solar flares. They can cause magnetic storms when they hit Earth's magnetic field and damage human technological systems in space and on the ground. For example, in 1989, the Quebec province in Canada suffered an electrical blackout because many transformers were destroyed by a large magnetic storm. That one storm caused many millions of dollars worth of damage. A powerful solar flare erupted from the Sun about three days before the start of the storm at Earth. Even when the Sun is not too active, solar storms can cause problems. A magnetic storm on January 11, 1997 was blamed for the loss of a $270 million dollar AT&T communications satellite. This moderate storm was caused by a coronal mass ejection that erupted from the Sun even though there were no noticeable sunspots.

Planet Earth: a great magnet

Without the almost constant supply of light and heat from the Sun, life on Earth would be impossible. But the Sun does vary, and it is in constant motion. For example, the Sun emits powerful streams of particles which bathe our solar system in a hot wind of ionized gases. Such gases are hot enough to conduct electricity and are called "plasmas." The solar wind plasma buffets the magnetic field surrounding Earth and causes electric power "blackouts," communication problems and the mysterious and beautiful aurora.

As we study the Sunís profound effect on Earthís environment, we learn about processes that can also help us decipher the secrets of distant stars, and perhaps even galaxies.

A few hundred miles above our heads, Earthís atmosphere gradually ends and space begins. Yet, contrary to popular opinion, space is not empty! It contains gases hot enough to conduct electricity, with radiation at sometimes dangerous levels, and with invisible magnetic fields. The tenuous conducting gases in space are called plasmas and they occur throughout nature. By exploring this environment, we gain knowledge about important processes that happen throughout the universe. This natural space laboratory is our window to the stars.

Earthís invisible magnetic field extends far out into space to form a region we call the magnetosphere. The force of the solar wind pushes on the magnetosphere, squeezing in the sunward side and stretching the night side into a long tail (called the magetotail) that extends hundreds of thousands of miles into space. As the solar wind flows past the magnetosphere, it acts like a cosmic generator, producing millions of amps of electric current. Some of this electric current flows into Earthís upper atmosphere which can light up like a neon tube to create the mysterious Northern and Southern Lights.

The First Major Discovery of the Space Age

The Van Allen Radiation Belts, one of the many regions within the magnetosphere, were the first major scientific discovery of the Space Age. In 1958, Geiger counters aboard the U.S. satellites Explorers 1 and 3, detected energetic charged particles trapped in magnetic field regions fairly close to Earth.

Over 98% of the charged particles from the Sun and from galactic cosmic rays that strike Earth's magnetosphere are deflected by it. The Van Allen Radiation Belts -- two doughnut shaped belts that surround the Earth -- trap the rest of the harmful particles, which bounce back and forth along magnetic field lines between Earth's north and south magnetic poles like beads on a wire.

The Earthís magnetic field acts like a shield to the supersonic solar wind. A shock wave (called the bow shock), like that produced by supersonic aircraft, slows and heats the solar wind. During the Sun's most active periods, solar wind disturbances, created by coronal mass ejections, buffet Earth's magnetosphere and they can produce large magnetic storms lasting one or more days.

If a very large amount of energy from the solar wind enters the magnetosphere a disturbance called a magnetic storm develops. Magnetic storms have been known to seriously damage electric power networks, affect communications, and damage satellites orbiting near Earth.

Auroras: billboards for electric space

Spectacular auroras are a phenomena whose beauty almost defies description. Triggered by disturbances in Earthís magnetic field, these curtains of light occur when high energy electrons swirling around Earth are pulled towards the polar regions by Earthís magnetic field. Here, they plunge into Earthís upper atmosphere, where they slam into gas molecules, making them glow.

Auroras act as a gauge of whatís happening tens of thousands of miles away in Earthís magnetic field. On days that are calm, auroras are only slightly visible, even in the northern latitudes. But during magnetic storms, auroras expand and can be seen as far south as Colorado or even Florida.

Auroras are created when energized electrons move down Earthís magnetic field lines until they collide with air molecules about 70 miles above Earthís surface. The collisions make the air glow: oxygen molecules glow whitish-green, nitrogen molecules glow pink.

NASAís Discovery astronauts got a great look at the auroraís "curtains" in 1991. This photo of the aurora was taken from the orbiting space shuttle several hundred miles above Earthís South Pole.

Auroral Forms:


The Sunspot Cycle

Besides its dazzling brightness, the Sunís next most obvious features are the appearance of dark, cool areas called sunspots that can bee seen when the Sun's image is projected onto a piece of white cardboard through a pin-hole (never look at the Sun directly!!). Sunspots are huge magnetic field bundles that are shaped somewhat like a horseshoe magnet. These magnetic fields result in cooler, darker regions on the surface of the Sun that we see as sunspots. Sunspots are sources of a tremendous amount of energy including solar flares, the most violent events in the solar system. In a matter of minutes, a large flare releases a million times more energy than the largest earthquake.

Sunspots and the resulting solar flares affect us, here on Earth. In fact, the more we learn about sunspots and solar flares, the greater their influence on Earth appears to be. Solar flares emit radiation that includes X-rays and ultraviolet rays, and charged particles called protons and electrons. This radiation surge may damage electrical power systems, interfere with telecommunications, disrupt high-tech ship navigation systems, harm an astronaut in space, or create the spectacular aurora (Northern and Southern lights).

Through years of study we now know that the chances of a sudden surge in radiation on Earth caused by a solar flare is related to an increase in the number and complexity of sunspots. Therefore, researchers plot average sunspot numbers over a period of time so that we know when to expect the next series of disruptive, potentially dangerous solar flares. The sunspot number (also called the Wolf number after Rudolph Wolf, who devised the method) is not actually the number of sunspots on the Sun. It is calculated with a formula that does depend on the number of visible sunspots recorded at selected observatories around the world.

Another mysterious connection between sunspots and life on Earth has to do with climate. During a period from 1645 to 1715 known as the Maunder minimum, almost no sunspots were seen. During that same time, Earthís Northern Hemisphere experienced the "Little Ice Age" as average temperatures dipped and rivers froze. What is the connection? Scientists are not certain, but sunspots must be telling us something important about how the Sun works and produces energy.

Thus, as our knowledge of the interaction between the Sun and Earth improves and our use of the space environment increases, space weather forecasting becomes more important. Although we may feel, see, or hear nothing unusual, our electrical power may fail, our telecommunications may falter, and satellites may no longer function. Some people may speak of mysterious, superstitious forces, but we know that none other than our closest star, the Sun, is the culprit. And in the long term, studying sunspots could help us solve the many mysteries of our nearest star - mysteries that could mean life or death for planet Earth.

Forecasting Space Weather

Space weather affects both people and equipment not only in airline and space travel, but also in Earth based jobs such as long-line telephone communication systems, pipeline operations, and electric power distribution.

Utilizing a fleet of spacecraft from NASA and other space agencies, scientists in dozens of countries observe the Sun, the solar wind, auroras and Earthís environment in space. Telescopes, radar and ground observatories that monitor Earthís magnetic field combine with the spacecraft of the International Solar Terrestrial Physics (ISTP) program to provide a picture of current and future space weather conditions.

Scientists at the Space Environment Center (SEC), in Boulder, Colorado, conduct research in space physics and develop new techniques for forecasting solar events like solar flares and CMEs. The SECís Space Environment Services Center is the national and world warning center for these disturbances. These efforts are being further advanced by the National Space Weather Program (NSWP), a government-wide effort that includes the National Science Foundation (NSF). As the Sun approaches the maximum of its activity cycle in the year 2001, scientists will be scrutinizing the stormy relationship between Earth and the Sun.

Plasma: the fourth state of matter

Matter can exist in one of four states: solid, liquid, gas, and plasma. Plasma, the least familiar state of matter, is actually the most common form of matter in the universe. The term has nothing to do with blood plasma. On Earth, the plasma state of matter is rare, except in lightning discharges and artificial devices like fluorescent lights. Beyond Earthís surface, plasmas are common -- from the aurora flickering in the uppermost reaches of Earthís atmosphere, to the "wind" that blows out from the Sun and deflects the tails of comets, to the Sun, itself, and all other stars and in the colossal jets that explode into space from distant galaxies. Indeed, plasmas make up more than 99% of the visible matter in the universe.

If you added enough heat to the block of steel (solid), you could melt it (liquid), boil it away (gas), then ionize the gas (plasma). By taking away heat, you could reverse the process. Many materials can go through these transformations between the four states of matter.

Plasma is mostly invisible and untouchable. Plasmas come in many shapes and sizes.

How are Plasmas Created?

Plasma, like all matter is made up of atoms. Atoms are so tiny that more than a million can fit across the head of a pin. They are composed of one or more negatively charged electrons that orbit a positively charged nucleus (made up of neutral particles, called neutrons, and positively charged particles, called protons). Atoms are electrically neutral; they have the same number of positive and negative electrical charges.

Plasma occurs when gases become ionized. When gases are exposed to heat or radiation, their electrically neutral atoms split into positively charged fragments called ions and negatively charged free electrons. Another term for plasma is "ionized gas." Because plasma contains electrically charged particles, it acts very differently from ordinary forms of gas.

Understanding how plasmas interact with electric and magnetic fields gives us a better idea of what is happening between the Sun and Earth and elsewhere in the universe, but many mysteries remain. The region between our star and our planet has become a large, working laboratory, where space scientists use the latest technology to learn more about what is happening out in the plasma environment of space.

Plasmas Can Be Noisy

Charged particles in a plasma are always in motion, generating a variety of electromagnetic waves, like radio waves and, at extreme temperatures, even X-rays. Some plasma waves can be converted into an audio signal that you can hear, just as a radio converts radio waves to audible sound waves.

Press the buttons to listen to three types of waves and learn where they were generated in Earth's magnetosphere.