Space Weather

Storms and hurricanes don't leave off where the atmosphere ends.

By Eric J. Lerner|Tuesday, August 01, 1995
RELATED TAGS: SOLAR SYSTEM
On March 13, 1989, in the early morning hours, technicians at the Hydro-Quebec power company in Montreal were in their control room as usual, watching the maplike mimic board that allows them to monitor the condition of their power grid. That grid supplies electricity not only to Montreal but to all of Quebec Province, to a total of 6 million people. At 2:44 A.M. a light started flashing on the mimic board: there was trouble up north. A voltage regulator had shut down on one of the main lines that run from the La Grande hydroelectric complex in northern Quebec to Montreal and other cities in the south. As the next 60 seconds ticked away, six more lights started flashing. Voltage levels in the grid became dangerously erratic. Then a circuit breaker tripped on one of the five lines coming out of La Grande; within a second the other four had gone as well. La Grande and its 9,500 megawatts were now completely isolated from the grid. As the startled technicians looked on helplessly, a cascade of broken circuits rippled around the province, cutting off the rest of Hydro-Quebec’s generators. In all, it had taken less than 90 seconds for power to collapse in the entire grid. The mimic board was now blinking like a Christmas tree. But all over Quebec the lights were out.

The blackout cost Hydro-Quebec more than $10 million, and it cost the power company’s customers tens if not hundreds of millions. Although power was restored to most of the province within nine hours, some places remained dark for days. In the postmortem analysis, Hydro-Quebec engineers had little trouble figuring out what had happened. Their conclusion was reassuring in one sense, scary in another. The blackout had not been caused by a design flaw, nor by operator negligence, nor indeed by any human error at all. The source of the problem had been the sun.

Every now and then the sun ejects a huge blob of hot, electrically charged gas, or plasma. The blobs travel hun-dreds of millions of miles out into the solar system, and they carry part of the sun’s magnetic field with them. Several times a year, one of the blobs happens to hit Earth. The result is called a geomagnetic storm. As the plasma plows into Earth’s magnetic field, it squashes the field violently, setting compass needles aflutter; charged particles pour into the space around Earth and zing down the magnetic field lines toward the poles; the northern and southern auroras spread their crazy glowing curtains across the sky. People who operate satellites go crazy, too. Geomagnetic storms often disable expensive satellites by triggering phantom commands in their fragile electronics; in 1994, for instance, a storm knocked out a quarter- billion-dollar Canadian communications satellite for nearly six months before engineers got it back under control.

But the most worrisome threat is to power grids. The fluctuations in Earth’s magnetic field during a geomagnetic storm induce rogue electric currents in Earth’s surface. In regions where the ground doesn’t happen to be a good conductor--such as Quebec, which sits on a shield of igneous rock--the currents gladly surge through power lines instead. The longer the lines--La Grande is 600 miles from Montreal--the greater the voltage difference induced by the magnetic fluctuations and the stronger the currents. Even so, the currents are never very powerful; the one that raced through Hydro-Quebec’s lines added only a few hundred kilowatts to a 21,000-megawatt grid. But it was a direct current rather than the usual 60- cycle alternating current, and power-line transformers aren’t equipped to handle that type of extra excitement. Their steel cores start to vibrate and whine like jet engines; parts of them get as hot as kitchen stoves. They start draining power out of the grid, which upsets voltage regulators, and they spit strange current spikes onto the grid, which trips circuit breakers and cuts off generators. When there’s no longer enough power in the grid to meet demand, it collapses. That’s what happened to Hydro- Quebec.

It could happen elsewhere. As local grids have been linked into large regional ones, and as utilities have come to rely more heavily on long-distance transmission lines, the grids have become more vulnerable to magnetic storms. The storm that hit Hydro-Quebec also knocked out power lines and transformers in New Jersey and Pennsylvania. By some accounts it came close to triggering an even more extensive cascade of power failures than occurred in the notorious blackout of 1965. The Northeast got lucky in 1989: had the magnetic storm happened on a hot summer day, when air conditioners were already sucking tremendous amounts of power out of the grid, the whole region might have been blacked out again. Having to rely on luck is not a happy situation for engineers; they aim for foresight. If Hydro-Quebec’s engineers had been given a forecast of the space weather that was bearing down on them, they too might have been able to avert disaster. They might have shut down a few parts of the grid to prevent a collapse of the whole thing.

Space physicists, the people who study geomagnetic storms, are on the verge of being able to provide such forecasts. The collision between a cloud of billions of solar particles and Earth’s magnetic field is a complicated interaction--but not, researchers are coming to realize, an inherently unpredictable one. In some ways it’s even easier to predict than ordinary atmospheric weather. What space weather forecasters lack most these days are data: unlike meteorologists, they have no satellites that can give them a look at the weather that’s on its way.

That will soon change, at least temporarily. In 1997 two NASA satellites are scheduled to take up position a million miles from Earth, between us and the sun. From that vantage point the probes will be able to detect clouds of solar plasma before they reach us. Computer models now being polished in university and government labs may then be able to predict the onset of major storms as much as an hour in advance--enough time to protect power grids and shut down delicate satellites. Obviously more warning time would be better, says space physicist Ernest Hildner of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado. But an hour would be great compared with the present situation. An hour warning before a hurricane can allow you to save yourself, even if it’s too little time to board up all your windows.

Although the sun sends us a hurricane equivalent only a few times a year, there is always weather going on in the space around Earth. The sun’s outer atmosphere, the corona, continuously emits a weaker stream of electrons and protons called the solar wind. Like the denser, storm- inducing blobs, the solar wind carries the sun’s magnetic field lines with it, and it permanently deforms Earth’s magnetic field. Without it, the field would be shaped like that of a bar magnet: the field lines would run from South Pole to North, like an onion in cross section, and they would look the same on all sides of the planet. Instead the solar wind compresses the sun-facing dayside of the magnetic field while stretching the nightside field behind Earth into a long, cylindrical magnetotail. On a calm day, the magnetosphere--the region of Earth’s magnetic influence--extends about 40,000 miles toward the sun, about 100,000 miles to each side, and some 4 million miles along the magnetotail, far beyond the orbit of the moon.

The magnetosphere tends to keep out the solar wind, deflecting the charged particles in a direction perpendicular to both the magnetic field lines and the direction of the particles’ motion. Negative electrons are deflected to the right side of the planet, as seen from the sun--to the side where dusk is falling. Protons and other positive ions are deflected to the left--the dawn side. As they zing around the planet in opposite directions, some of the particles manage to cross the field lines and leak into the magnetosphere. Collecting in the magnetotail, they create poles of opposite charge. In other words, they create a generator. The result is spectacular.

The generator drives an electric current, which is mostly carried by the lightweight and mobile electrons. Part of the current simply flows across the magnetotail and returns along its outer surface. But another part follows an alternate route: the electrons spiral back along the stretched-out magnetic field lines toward Earth, crash into the atmosphere near the poles, zip around the magnetic poles through an electrically conducting layer called the ionosphere, and then ride other field lines back to the magnetotail. As they penetrate the upper atmosphere, the highly energetic electrons slam into oxygen and nitrogen atoms and cause them to glow like phosphor in a television screen. The oxygen glows whitish green; the nitrogen glows pink. Those shimmering curtains of light are the auroras, which are visible from the ground.

The auroras are a good gauge of the intensity of the solar wind. Only faintly visible during quiet periods, they increase in size and brightness when the wind gusts. When the wind storms--when the sun has emitted one of its huge, dense blobs of plasma--the auroras swell dramatically. During the Hydro-Quebec blackout, the aurora borealis, or northern lights, was so bright and extensive it was seen as far south as Georgia.

Space physicists don’t use the term blobs, of course. They call the sun’s plasma ejections coronal mass ejections, or CMEs. A CME weighs tens of billions of tons and travels as fast as 620 miles per second, twice as fast as the ordinary solar wind. It thus carries a mammoth energy--10 trillion trillion joules, enough to boil away the Mediterranean Sea. Only a fraction of that reaches Earth; by then the CME has expanded to a great size and its energy has been diluted. But a large part of the energy that does reach the planet is converted by the auroral generator into electricity, and lots of it. On an ordinary day the generator pumps out roughly 500,000 megawatts of power. During a geomagnetic storm the output may be a hundred times that.

What causes the sun to spit out CMEs several times a month (of which only a fraction head our way) is still a matter of controversy. The most popular theory holds that the sun’s corona, at a temperature of more than 2 million degrees Fahrenheit, is restrained by broad arches of magnetic field lines, which charged particles tend not to cross (just as they are later restrained from entering Earth’s magnetosphere). Occasionally these lines weaken, though, and the coronal plasma escapes, rushing out into space. A CME is what happens when the magnetic field loses its grip, says Art Hundhausen, an astronomical physicist at the University of Colorado. The plasma just blows out. Observations made by the Ulysses spacecraft during the summer of 1994 tend to support this theory. As it flew by the sun, Ulysses found that plasma was blowing off the corona at a much faster rate at the poles, where the magnetic field lines are almost perpendicular to the solar surface and thus less able to contain the corona, than it was at the equator.

Yet even after Ulysses it’s still possible to hold pretty much the opposite view about CMEs. James Chen, an astronomer at the Naval Research Laboratory in Washington, D.C., thinks the sun’s magnetic field is what launches the blobs, not what holds them back. Moreover, he thinks the blobs are really loops--vast loops of electric current spiraling around magnetic field lines that are anchored in the corona. From time to time, Chen says, a surge of magnetic energy from deep inside the sun causes one of these 500,000-mile-long loops to billow out into space. With its magnetic feet still rooted in the sun, it can billow right past Earth. What looks like a blob to us, says Chen, is actually a piece of an arch.

Whatever a CME is, it takes only two or three days to travel from the sun to Earth. So if space physicists are ever to forecast geomagnetic storms more than two or three days in advance, they are going to have to decide what causes CMEs and learn to forecast them. That goal seems a long way off. Space weather forecasters have their sights set at the moment on predicting just the next few hours of space weather rather than the next few days. That’s hard enough.

For one thing, just because a CME is headed for Earth doesn’t mean a storm is coming. The orientation of the solar magnetic field lines that are frozen into the blob is a crucial variable. If they happen to be aligned with Earth’s field lines--if the north pole of the plasma coincides with Earth’s magnetic north--then nothing much happens. Since like magnets repel, the magnetosphere deflects the attacking blob as it would the tamer solar wind. The magnetosphere may get compressed slightly more than usual, but the weather inside it remains calm.

It’s when the CME’s magnetic field lines are oriented opposite Earth’s--that is, pointed southward--that all hell breaks loose. The scientific word for it is reconnection of the magnetic field lines, but you can think of it as two bar magnets attracting each other with a violent slap. The Earth’s magnetic field lines suddenly merge with those of the plasma blob, opening a yawning breach in Earth’s defenses. Solar plasma rushes into the magnetosphere in great gusts.

That pushes the auroral generator into overdrive. The current of electrons rushing back toward Earth from the magnetotail surges dramatically; the aurora surges down to lower latitudes, where more of us can see it. But some of the electrons land on satellites, triggering electric currents that mimic commands from Earth and may send the spacecraft spinning out of control. And some of the electrons get deflected onto a different path, intensifying a ring of current that flows around Earth near the equator. As the plasma blob is compressing the magnetosphere from the outside, this ring current’s own magnetic field is undermining Earth’s field from within. The field collapses to half its normal size and oscillates violently; compass needles swerve by several degrees. The collapsing magnetic field is what can collapse a power grid like Hydro- Quebec’s, by inducing rogue currents in the power lines.

Between the disaster of a south-oriented CME and the non-event of a north-oriented one lies a continuous range of possibilities; a CME can arrive with any magnetic orientation. And there would seem to be any number of other variables that might determine how a speeding blob of billions of charged particles would interact with the magnetic field and electric currents around Earth--so many possible influences on the collision, in fact, that the effects should be essentially random and therefore unpredictable. That’s what space physicists thought for a long time, and it was a discouraging thought. A CME might black out New York or it might steal harmlessly past Earth into the night; the real outcome would be unforecastable.

But last year that idea was laid to rest--and the nascent field of space weather forecasting was given a tremendous theoretical boost--by Surja Sharma, a physicist at the University of Maryland. Sharma analyzed long series of measurements made both by satellites and by magnetometers on Earth, measurements that reflected the interaction of the solar wind and the magnetosphere. He found irregularity in the data sets; he found complexity. But he also found pattern, which you don’t get in random data. The magnetosphere, Sharma concluded, is not so much random as chaotic. That was very good news.

A chaotic system, unlike a random one, is not unpredictable. It’s a system whose behavior can be predicted for only a limited time into the future, because immeasurably small changes in inputs to the system are capable, given enough time, of producing very large changes in output. A system doesn’t have to be buffeted by innumerable random forces to behave like that. What people have learned from studying chaotic systems is that you can get very complicated behavior from a very simple system, Sharma says. In principle, he found, the weather in space should be describable by equations containing just a few variables--and so it should be predictable by taking only a few key measurements. Sharma believes that it should be possible to predict space storms several hours in advance.

To actually make predictions, though, researchers need two things: they need a detailed physical model of what is going on in a geomagnetic storm, so that they know what key measurements to make. And then they need the measurements. A lot of models have been proposed. Daniel Baker, a physicist at the University of Colorado, has compared the magnetosphere to a classic chaotic system: the dripping faucet. Plasma flowing in from the sun, he says, collects in the magnetotail until it reaches a critical mass and energy. At that point the magnetosphere can no longer hold the plasmoid, which drips down the magnetotail like water dripping from a faucet. The magnetic field lines then snap back toward Earth, generating the intense electric currents that produce the aurora and other symptoms of space weather.

This process happens every two or three hours even on quiet days, producing what space physicists call substorms. But the drops come bigger and faster during a geomagnetic storm. Baker’s model is designed to predict one drop ahead. Using only three measurements--the strength of the solar wind and of the magnetic and electric fields in the tail--the model has done well in predicting past storms from historical data. If Baker had some contemporary data, he could take a shot at predicting the future.

It’s not that no data exist or that no one has tried predicting geomagnetic storms. For the past 30 years, in fact, the lab now led by Ernest Hildner at NOAA in Boulder has tried to predict storms by watching the formation of CMEs at the sun’s surface. The researchers monitor the sun with radio, optical, and X-ray telescopes, looking for clues that a CME is about to depart. We take all the straws in the wind that we can get, and we kind of integrate and average them to come up with our prediction, says Hildner. The researchers can often see a CME at the very moment of departure because it emits a characteristic burst of radio waves. From this, says Hildner, we can measure the speed of the ejection and calculate roughly how long it will take to reach Earth. If we think the ejection is big enough to cause a storm, we issue a warning for the day we think it will arrive.

Like picnickers desperate for any weather forecast, no matter how unreliable, satellite operators and utilities have a big appetite for Hildner’s warnings. But his group misses most geomagnetic storms, and conversely most of the warnings it issues are wrong. Since the researchers can’t know which way the magnetic field in a CME is pointing, their false- alarm rate is guaranteed to be at least 50 percent, because half of all CMEs will turn out to have harmless, north-pointing fields. The problem is, we can’t see the CME as it moves between the sun and us, Hildner says. By the time CMEs arrive, some are pussycats and others are roaring tigers, and we can’t tell which is which from how they look leaving the sun. It’s like trying to forecast weather for Washington, D.C., when the only upwind weather station is in San Francisco.

The two NASA satellites now in the works will provide additional space weather stations--temporarily. The first one, called Wind, was launched last year. In 1997 it is scheduled to settle into orbit around the L1 point, a point 1 million miles from Earth at which the planet’s gravity is precisely canceled by the sun’s. Tagging along with Earth around the sun, the satellite will always remain between the two. At that distance, it will intercept an oncoming CME about one hour before the blob reaches Earth’s magnetosphere. It will measure the CME’s density, velocity, and magnetic field.

Wind is only a research satellite, though, not a weather- forecasting satellite; it will function for only a year, and it will send data back to Earth for only two hours each day. Any CME that happens to blow by outside that time window will plow into Earth before Wind has had a chance to warn of its arrival. A second satellite, the Advanced Composition Explorer, will join Wind at the L1 point later in 1997, and it will provide 24-hour-a-day data on the solar wind. That will at last allow Hildner and other researchers to try their hand at making reliable forecasts of geomagnetic storms. But ACE will have only a three-year life. After that, says Hildner, we’re in the dark again.

There are no plans in the works for a satellite to replace ACE; there are no plans to establish the National Space Weather Service that Hildner and other space physicists dream of and say they are ready to make work. There are many more pressing demands on the nation and on the federal budget. On the other hand, that calculus could change. If a large blob of hot plasma were to blow in from the sun with a south-pointing magnetic field; if the aurora were to swell as far south as Georgia once again; and if the lights were to go out this time not just in Quebec but all over the northeastern United States as well, that calculus could change in a day.
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