Space Weather: Storms from the Sun
Once upon a time, back in the twentieth century, the weather was straightforward: it rained or snowed, skies were sunny or cloudy. However, in the twenty-first century—the era of globalization and digitalization—a whole new kind weather is critical to consider: space weather.
Space weather is direct product of our local star, the Sun. The Sun continuously sheds its skin, blowing a fierce wind of charged particles in all directions, including Earth's. From time to time, storms on the Sun's surface—solar flares, coronal mass ejections—toss off added masses of energy and ions. When that turbulence slams into Earth, it produces space weather. The consequences can be spectacular, from colorful auroras to satellite, power and communications failures.
Space weather isn't new: the Sun has buffeted Earth with solar particles since the planet first formed. What has changed is society. This feature reveals how our increasing use of satellite technology has made us vulnerable to solar storms, and how solar scientists—“space weathermen”—are learning how to predict and forecast the Sun’s activity.
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Like all stars, the Sun is an immense and glowing mass of neutrons, protons and other subatomic particles, all bound by gravity and a roiling magnetic field. As a cosmic neighbor, it is immensely influential: every second, the Sun produces an amount of energy equivalent to the detonation of about 10 trillion Hiroshima-scale atomic bombs. The light it generates burns with a brightness equivalent to 100 trillion trillion 100-watt light bulbs shining simultaneously.
As stars go, though, the Sun is merely typical: about 850,000 miles wide, or roughly 200 times the width of Earth; with a mass roughly 300,000 times that of Earth and a density equal to Jupiter's, or about one-fourth Earth's. That mediocrity serves science well. Through close study, solar researchers have learned a great deal not only about the Sun, but also about the workings of similar, more distant stars. "There is no other star we can observe in such detail," says Harrison Jones, a solar physicist with NASA's Goddard Space Flight Center.
Advances in technology have revealed that the Sun is comprised of several layers, rather like a giant flaming onion. Its power derives entirely from nuclear reactions at the core. There, under tremendous pressure, hydrogen atoms fuse to form helium atoms, releasing vast amounts of energy; the temperature at the core is about 15 million degrees Celsius. This energy travels outward through the various layers of the Sun and onward into space. It is a slow process: a photon of light takes more than 10,000 years to travel the quarter-million miles from the core to the Sun's surface.
In the core and the surrounding layer, called the radiative zone, the gas particles are so densely packed that the overall gas, or plasma, barely moves. In these innermost regions, solar energy travels by radiation: photons of light bounce from one gas particle to the next and slowly leak outward. In the convective zone, the outermost of the Sun's interior layer, the gas particles have a little more elbow room. They collide less frequently and less violently, so the temperature falls, and the plasma begins to move, boiling upward on vast thermal plumes, like soup in a kettle (onion soup, presumably). The plumes, or convection currents, carry energy to the Sun's surface and cool as they rise. The temperature at the surface, or photosphere, is "only" 6,000 degrees Celsius.
The photosphere is not a hard surface, but a thin, roiling layer of magnetically charged plasma. Only about 60 miles thick, the photosphere is the source of much of the visible radiation, or sunlight, that leaves the Sun. The Sun that the casual viewer sees--that white-hot sphere suspended in the sky--is really only the photosphere, the thinnest layer of the Sun's true self.
The photosphere is a frenzy of magnetic activity. Some regions are centers of positive polarity, others are regions of negative polarity, like positive and negative ends of bar magnets. These regions are linked by magnetic field lines that may loop high into the Sun's atmosphere. The regions of positive and negative polarity constantly shift, however, and as they do, the field lines break and reform. This process can produce violent storms in the Sun's outer regions.
The average temperature of the photosphere is 6,000 degrees Celsius. In regions where magnetic activity is particularly intense, temperatures are slightly cooler. Against the bright, hot photosphere, these regions look comparatively dark and are known as sunspots. Sunspots are a useful gauge of solar activity. During the quieter phase of a solar cycle, scientists may detect only a handful of sunspots at any given time. During solar maximum, as many as 250 might exist.
The next layer above the photosphere is the chromosphere; after that is the transition region. Magnetic activity near the surface generates violent storms in these outer regions: solar flares, coronal holes, coronal mass ejections and other phenomena that blast hot plasma outward into space. Due to all the activity, the temperature skyrockets from about 100,000 degrees Celsius at the inner edge of the transition layer to several million degrees at its outer perimeter.
Finally, the outermost region of the Sun, the corona, extends millions of miles into space. Temperatures in the corona constantly fluctuate, so the strength of the solar wind--a breeze of solar particles that blows outward in all directions--is always changing.
Space-weather scientists have benefited greatly from studies of the Sun's internal structure. Understanding its complexities helps forecasters better predict where and when solar storms are likely to develop.
The National Oceanographic and Atmospheric Administration's (NOAA) Space Environment Center, located in Boulder, Colorado, is the nation's space weather center. Every day, data and images of the Sun's latest activity stream into the Space Environment Center from satellites, space-based telescopes and ground-based instruments around the world. Staff scientists like Larry Combs then pore over this information for signs of brewing solar trouble. Much like traditional meteorologists, forecasters of space weather size up existing storms (solar ones, in this case), predict the eruption of new ones, determine what effect those events are likely to have on Earth's environment, and, when necessary, alert the people and industries telecommunication and power companies, the military, NASA that may be affected by inclement space weather.
"We're looking for a storm to arrive from the Sun," Combs says. "We're looking at the solar wind–its density, its temperature, and all that goes along with that."
The driving force behind solar activity is the Sun's magnetic field. Like Earth, the Sun is a giant rotating magnet. Unlike Earth (or a simple magnet), however, the Sun is not a solid body: it is a highly conducting fluid, so its rotation, and the magnetic field that arises as a consequence, is complex and constantly on the move. Material near the solar poles rotates faster than material near the solar equator. Meanwhile, new material is constantly rising to the surface by convection, stirring the fluid further. As a solid body, Earth has a tidy magnetic field, with straight north-south field lines that surround the planet like a neatly wrapped ball of yarn. In comparison, the Sun is a knitter's nightmare: a huge sphere of rubbery ropes, of ever-shifting field lines that twist, kink, tangle and snap as the liquid body spins.
The Sun's magnetic field defines the shape and motion of its gaseous atmosphere. "Much of the structure we see on the Sun is the gas aligning along the magnetic lines of force," Combs says. Loops and prominences, two of the more charismatic solar features, are yawning arches of solar gas; they may last for months and can extend 30,000 miles above the Sun's surface. As the Sun progresses through many rotations, its magnetic field becomes increasingly distorted; this tumult, known as solar maximum, reaches a peak every 11 years or so. During this period, solar eruptions are more frequent and severe and the solar wind blows with added ferocity. "The sun has a cycle," Combs says. "It has a maximum period and a minimum period. Right now we're in that maximum period."
Using different photographic filters, the space-weather forecaster watches the Sun's magnetic field as it changes over a period of months, days, hours, even minutes. "We look at areas on the Sun that have the potential to produce activity," says Harrison Jones, a solar physicist with NASA's Goddard Space Flight Center. A magnetogram image, for example, reveals the varying intensity of the magnetic field in shades of gray. "Areas that are bright are areas where the magnetic field is pointing towards us and is strong. Areas that are black are areas where the magnetic field is strong, but pointing away from us. An area of strong magnetic field is an area that is likely to evolve and release high-energy particles." Forecasters also look for sunspots, slightly cooler regions of the Sun that appear dark in visible-light and ultraviolet images. Sunspots arise where the magnetic field is in flux, and are often sites of developing solar storms.
Solar storms occupy much of a forecaster's attention. They come in a variety of forms, each promising a unique brand of potential trouble. Solar flares are massive explosions on the Sun's surface. They often arise near sunspots and release a wide spectrum of energy-charged particles, X-rays and gamma rays-outward into space. Coronal holes are more subtle phenomena; to the forecaster, they appear as large, dark regions of the corona. The magnetic field lines around coronal holes are "open": instead of looping downward and trapping plasma against the Sun's surface, they point outward, spewing solar particles into space as quickly as 500 miles per second, adding extra punch to the solar wind. The biggest solar storms are coronal mass ejections. A CME is an enormous bubble of plasma expelled by the Sun; it contains billions of tons of fast-moving solar particles as well as the magnetic field that binds them. "A coronal mass ejection is actually ripping a part of that magnetic field away from the Sun and sending it out into space," says Terry Onsager, a scientist at the Space Environment Center.
Most solar storms are directed away from Earth. But when a storm points our way, the forecasters at the Space Environment Center begin to calculate what sort of space weather it may generate on Earth, and how soon to expect the turbulence.
"The Sun doesn't give a hoot about us," says Joe Kunches, the lead forecaster at the Space Environment Center. "We just happen to be in the way."
Space weather begins on the Sun, but it isn't really space weather until it hits Earth. Fortunately, Earth's magnetosphere spares the planet's residents from the worst damage. Earth's rotation generates a large magnetic field around the planet, extending hundreds of miles into space. This field, known as the magnetosphere, deflects or safely absorbs much of the hazardous energy and particles the Sun sends in our direction. "We're in a cocoon provided by the Earth's magnetic field," says Terry Onsager, a solar scientist at NOAA's Space Environment Center. "It's a shield, a bubble that we live inside."
"We're looking for a storm to arrive from the Sun," Combs says. "We're looking at the solar wind — its density, its temperature, and all that goes along with that."
Technically, space weather is what happens when solar particles and energy interact with Earth's magnetosphere. In sizing up a space-weather event, then, scientists at NOAA's Space Environment Center in fact keep an eye on two environments simultaneously: the Sun's environment and the near-Earth environment. "The geomagnetic forecast and the solar forecasts are actually two different forecasts," says Larry Combs. "One is what we expect the Sun to do at different areas. The other is how we expect Earth's geomagnetic field to respond."
"There are three different things we look for," Combs continues. "The first are electromagnetic emissions, in the form of X-rays and ultraviolet rays. Those go right through the magnetosphere and enter our upper atmosphere and ionosphere." Solar flares are the primary sources of these high-energy events; their effect is felt on Earth within minutes. The X-rays, ultraviolet rays and gamma rays heat up Earth's upper atmosphere; this creates more drag, which slows orbiting spacecraft and satellites and disrupts ground communications and navigation devices that rely on satellites to work effectively. NASA controllers must constantly boost the orbit of the Hubble Space Telescope to counteract the effects of space weather. High-energy radio waves emitted by solar flares also can upset the ionosphere, a layer of charged gas 60 to several hundred miles above Earth's surface. We use the ionosphere to reflect radio signals back to Earth over long distances; sudden disruptions of the ionosphere can cause radio blackouts around the globe.
The second thing forecasters look out for is an influx of energetic particles, primarily protons and electrons, released by the Sun. "They travel more slowly," Combs says. "It may take anywhere from thirty minutes to several hours for them to reach the magnetosphere." Solar flares and coronal holes, which create extra-strong gusts of solar wind, are the main causes of particle events. A sudden influx of solar particles into Earth's upper atmosphere can damage the sensitive electronics of satellites, thereby disrupting communications and navigation on the ground. An excess of solar particles also raises radiation levels in the upper atmosphere to dangerous levels; astronauts are careful to schedule their spacewalks around times when solar activity is at a minimum.
The third thing forecasters watch for is fast-moving solar plasma, large bubbles of solar particles bound up in a magnetic field. Typically produced by coronal mass ejections, a slab of plasma may take three or four days to slam into Earth, but when it does, the results can be severe. Like a wet electric blanket, the plasma can cause Earth's magnetosphere to tremble, generating massive electrical currents that can black out power grids on Earth's surface. In 1989, a huge geomagnetic storm temporarily damaged the Hydro-Quebec Power company, plunging all of Quebec into darkness for several hours. These currents also heat Earth's atmosphere, slowing satellites in their orbits.
Society has grown increasingly vulnerable to space weather in recent decades, largely because it has put more at stake: Earth's upper atmosphere is busy with spacecraft, communication satellites and orbiting telescopes (including ones that study space weather) as never before. This vulnerability in turn has prompted a vigorous and fruitful effort to improve space-weather forecasting. Of course, no forecast can ever be perfect; weather, even the space kind, is fickle.
"I could hang up the phone, there could be a coronal mass ejection, and in two days, away we go," says Joe Kunches, the lead forecaster at the Space Environment Center. "So far, though, it looks like a normal day."
Every storm cloud has a silver lining; in the case of space weather, that lining is the aurora borealis, more commonly known as the Northern Lights. (Viewers in the southern hemisphere are treated to an equivalent version called the aurora australis, or Southern Lights.) The phenomenon is best observed on a clear, cold night around the spring or autumn equinox. Find an open patch of sky well away from the interfering lights of the city, and you may catch a glimpse of the spectacle: curtains of pale light-green and blue, sometimes red or violet-shimmering above the northern horizon for minutes or even hours at a time.
Auroras occur when electrons and protons from the Sun strike gas molecules in Earth's upper atmosphere. As the solar particles encounter Earth's magnetosphere, they are drawn along the magnetic field lines and funneled toward the North and South poles. There, high above Earth's surface, they collide with atmospheric molecules, energizing them and causing them to glow. The colors that result depend on the gas molecules involved. The brightest and most common auroral color, a brilliant yellow-green, is produced by the glow of oxygen molecules roughly 60 miles above Earth. Ionized nitrogen molecules emit blue light when hit by solar particles; neutral nitrogen molecules emit a purplish-red light. All-red auroras are rare; they are caused by the glow of oxygen atoms 200 miles above Earth. The size and intensity of the aurora varies from night to night, and moment to moment, depending on the strength of the solar wind. On April 6, 2001, a large geomagnetic storm produced an aurora that was seen as far south as Alabama. The scientific understanding of auroras has advanced enormously in recent years with the launch of satellites designed expressly to study them. Instruments aboard NASA's Polar spacecraft monitor ultraviolet radiation and chemical changes in the upper atmosphere, effectively offering an up-to-the-minute report on the shape and intensity of the aurora. The Imager for Magnetopause-to-Aurora Exploration (IMAGE) spacecraft, launched in 2000, studies Earth's magnetosphere in astounding detail. It can watch auroras evolve over a period of hours, and can even see auroras flickering in the far-ultraviolet wavelength. Recently and for the first time, scientists observed a phenomenon known as "black auroras." A black aurora isn't really an aurora at all: it's the dark, empty space within a colorful aurora where one would otherwise expect auroral activity to be visible. Nonetheless, black auroras exhibit distinct patterns, including curls, rings and writhing black patches. Nowadays, scientists often can forecast a spectacular aurora hours or days in advance, so it's worth checking space weather websites (See Related Links) with some regularity.
In the 1970s, with the aid of the Hubble Space Telescope, it became apparent that Earth is not the only planet with auroras. On both Jupiter and Saturn, auroras appear pink due to the large amounts of hydrogen in those planets' atmospheres. Jupiter's aurora has proved to be particularly intriguing. On Earth, the aurora is powered by a barrage of charged particles from the Sun. On Jupiter, auroras are generated instead by volcanic particles from the Jovian moon Io. These particles become ionized, expand and then are trapped in Jupiter's tremendous magnetic field. Rotating once every ten hours, Jupiter generates auroras many times more powerful than those on Earth. However, Earth's auroras remain unique in one respect: they are (at times, anyway) green. Indeed, Earth is the only known planet with green auroras, because it is the only known planet with an oxygen-rich atmosphere. As scientists look deeper into the universe for signs of other, potentially habitable worlds, auroras are one clue they examine. If a distant, unknown planet has shimmering green auroras, that's a strong indication that its atmosphere is rich in oxygen, perhaps enough to support life. Whether that life is capable of appreciating the auroras—well, that's another issue.