Stormy Weather

When the sun's fury maxes out, Earth may take a hit

Part one in a two-part series on the Earth-sun connection.

False-color ultraviolet image shows brightening of Earth’s auroral oval last July 15, minutes after a magnetic storm struck our planet. The storm had erupted on the sun the previous day. The ultraviolet emissions, recorded by the IMAGE satellite, stem from energetic electrons bombarding molecular nitrogen in Earth’s atmosphere. Stephen Mende, Harald Frey/UC Berkeley
Close-ups of a flare that erupted on the sun July 14th. Left: The relatively cool, dense loops of electrically charged gas in this magnetically active region of the sun’s corona are held down by a lid of hotter, more tenuous loops that aren’t visible in this picture. Center: The lid blows off, the overlying coronal loops fly outward, and the pent-up energy is released as a flare. Right: After the flare erupts, gases flow from the sun’s surface and fill the newly formed, less-stressed coronal loops. NASA/Lockheed Martin Solar & Astrophys.Lab./TRACE
Illustration of a shock wave (blue arc) from of a coronal mass ejection (yellow) about to hit Earth’s magnetosphere. NASA

On July 14, 1789, an angry crowd stormed the infamous Bastille prison. This act of defiance ignited a revolution that turned the streets of Paris crimson, as mobs carried aloft the guillotined heads of aristocrats.

This past July 14 also marked a time of horrific violence, but on a much larger scale.

At 5:03 a.m. eastern time, a region on the sun large enough to swallow Earth suddenly became 10 times brighter, firing a torrent of high-energy radiation into space. Then, the sun’s outer atmosphere belched a billion-ton cloud of magnetized, charged particles. Traveling more than 6 million kilometers an hour, the magnetic cloud headed straight for Earth.

Twenty-five hours later, the cloud hurtled past the Solar and Heliospheric Observatory (SOHO), a European Space Agency-NASA satellite that continuously monitors the sun. One of the craft’s solar panels—its power source—suffered in seconds as much damage as it normally accrues in a year’s exposure to the harsh environment of space. Another craft, NASA’s flagship X-ray observatory, Chandra, was forced into hibernation.

The cloud also temporarily knocked out the sun sensor on a satellite that measures the solar wind, the breeze of charged particles that blows out from the sun. Operating blind, the craft didn’t know where to point for several days.

ASCA, a Japanese X-ray satellite, was even less lucky. Accelerated by the cloud, charged particles fried the craft’s flight computer, spinning ASCA out of control. The craft never regained power. Reduced to a frozen piece of space junk, ASCA will later this year crash into Earth’s atmosphere.

About 26 hours after it shot out from the sun, the cloud reached Earth. Ramming into the magnetosphere, the invisible magnetic shield that surrounds our planet, the storm revved up charged particles and dumped the equivalent of 1,500 gigawatts of power into the atmosphere. That’s four times the power generated by the U.S. power grid. The disturbance severely damaged two large power transformers and disturbed electric power systems, shutting down voltage-regulating devices all along the East Coast.

Scientists forecast storms as severe, or even worse, over the next 2 years.

Welcome to solar maximum.

Pole reversals

Like Earth’s magnetic field, the magnetic field of the sun has a north pole and a south pole. But every 11 years, those poles reverse direction—with great commotion. The peak of that activity is called the solar maximum, which scientists determine by counting the dark blotches on the sun, where bundles of magnetic field lines concentrate. The number of these sunspots appears to have peaked last summer, more than 6 months later than researchers had predicted. The peak, however, is a broad one, and the solar maximum is expected to last for another 18 months.

Until the Bastille Day event, this solar maximum—the 23rd on record—seemed relatively puny. Even now, after solar storms pummeled Earth twice last November, it isn’t likely to stand out as one of the strongest, says forecaster Ernest Hildner, director of the National Oceanic and Atmospheric Administration’s Space Environment Center in Boulder, Colo. But in two other respects, he notes, this solar cycle is like no other.

With 2,000 communications satellites launched since the last solar maximum, astronauts taking longer trips in space, and a society ever more dependent on computers, cell phones, automated banking systems, and other electronic equipment, Earth has never been as vulnerable to the havoc that an electric disturbance from the sun can wreak.

In May 1998, for instance, a communications satellite called Galaxy IV abruptly failed. Although the cause of the failure is not known definitively, it occurred just after an unusually intense period of solar activity. When the satellite went belly-up, 45 million pagers suddenly went dead.

Characteristics of the power grids that transmit electricity make matters worse, says power-systems engineer John Kappenman of Metatech Corp in Duluth, Minn., a firm that monitors solar disturbances for several businesses. Today, power is being transmitted over greater and greater distances to more and more customers, yet utilities haven’t proportionately increased the number of devices they employ to regulate voltage, he notes. Because they help minimize any sudden surge in current, such devices play a critical role when a solar storm hits Earth.

“In general, power grids are more vulnerable than they were 10 years ago,” Kappenman says.

At the same time, never before have planetary scientists had so large a flotilla of spacecraft to monitor the sun and its effects on Earth. SOHO, NASA’s TRACE (Transition Region and Coronal Explorer), and the Japanese satellite Yohkoh track aspects of the sun’s roiling physics. Also, the European Space Agency’s Ulysses craft is getting ready to make its second tour of the sun’s poles (SN: 9/23/00, p. 203).

But wait, there’s more. ACE (Advanced Composition Explorer), like SOHO, lies 1 million miles closer to the sun than Earth does. By monitoring changes in the space environment at that location, it provides warning of a solar storm 1 hour before it reaches our planet. There’s also WIND, which measures the solar wind that fills interplanetary space not far from Earth. Depending on its density and speed, this wind can boost or diminish the force of a solar eruption as it impinges on planets.

Another spacecraft, called IMAGE (Imager for Magnetopause-to-Auroral Global Exploration), is the first to observe charged particles and neutral atoms throughout the magnetosphere. Furthermore, both IMAGE and the Polar spacecraft examine auroras, the shimmering electric disturbances at Earth’s poles generated when charged particles from the sun crash into the atmosphere. Meanwhile, Japan’s Geotail spacecraft examines the back side of Earth’s magnetosphere.

“These spacecraft are a wonderful way to understand cause and effect,” says NOAA’s Hildner. “You can watch an event leave the sun and come all the way to Earth, and you can measure it with this constellation of spacecraft. This is how we develop our models.”

Solar eruptions

Solar eruptions generally fall into two classes: flares and coronal mass ejections (CMEs). Both are generated by the sudden release of energy stored in a magnetic field, although scientists are unsure exactly how.

Magnetic field lines emerging from deep within the sun form giant, arching loops in the sun’s corona. As the sun rotates, these loops become twisted, tangled, or stretched, storing vast amounts of energy. When stretched too far, the loops can suddenly snap or rearrange themselves, generating the largest explosions in the solar system. Some of the energy is released as a burst of radiation—a flare.

When the ultraviolet and visible light from a flare heads in our direction, it takes just 8 minutes to reach Earth. The greatest danger comes from the solar protons that may be accelerated by this explosive release of radiation, which arrive about 20 minutes after the flare does. If high-energy protons happen to strike astronauts outside the shelter of their spacecraft, they could be severely injured or even kill them. Because the ionosphere absorbs much of the protons’ energy, they don’t pose a threat to people or electrical systems on Earth.

There’s a second way that a twisted magnetic field may release its energy. Like a stretched rubber band shot across a room, the field may fling itself from the sun’s outer atmosphere, or corona, carrying the ionized gas surrounding it. These are CMEs: billion-ton parcels of ionized gas, or plasma, and the magnetic field holding them together. Sometimes referred to as magnetic clouds, these parcels can be bigger than planets and have much greater impact on Earth than flares.

Like a plane racing through the air at supersonic speed, a CME hurtles through the solar wind, creating a shock wave that accelerates the charged particles it meets. CMEs shoot radially outward from the sun, and only a few are directed toward Earth. When a CME and its charged-particle entourage strike Earth’s magnetosphere—usually 3 to 4 days after the CME erupts in the corona—the cloud can trigger potentially devastating electrical events.

So-called killer electrons, which are charged particles revved up to energies greater than 1 million electron-volts, punch through the skin of spacecraft. If enough charge builds up in delicate electronic components inside the craft, electric arcing occurs. Jumping from one piece of electronics to the next, these tiny bolts of electricity can destroy a satellite’s sensitive electronic equipment, including computers and communications devices.

By compressing and jostling Earth’s magnetosphere, the shock wave barreling in front of a CME delivers a punch that energizes charged particles within this magnetic shield. The magnetosphere may suddenly shrink, as it did in the aftermath of the Bastille Day storm. Normally extending 64,000 km from Earth’s surface, the magnetosphere constricted to nearly half that length, notes space scientist Nicola J. Fox of NASA’s Goddard Space Flight Center in Greenbelt, Md. She and other scientists described the Bastille Day eruptions late last month at a NASA press briefing.

Some CMEs are more dangerous than others. If one reaches Earth when its magnetic field happens to point southward, opposite to the direction of Earth’s magnetic field, then the cloud can directly connect with the terrestrial field.

A southward-pointing CME dramatically alters Earth’s magnetic field. In so doing, it creates a strong electric current, or ring current, which partially girdles the magnetosphere’s equator and is much greater on Earth’s nightside than dayside.

By linking to currents much closer to Earth, the ring current plays a pivotal role in disturbing power grids on the ground. The ring current on the nightside completes a closed circuit by connecting with currents that flow along Earth’s magnetic field into and out of the ionosphere, the electrically conducting layer of ionized gas in the upper atmosphere.

Within the ionosphere, the current is known as the electrojet, which is associated with Earth’s auroras and normally circulates at polar latitudes. A surge in the ring current jolts the electrojet and may also push it to lower latitudes. In the northern hemisphere, this brings it—and the aurora—farther south, where the magnetic disturbances it creates on the Earth’s surface can induce currents in transmission lines over highly populated areas and disrupt many power systems.

For several hours, until the CME passes by, Earth is at its mercy. Many of the disruptive effects of the July 14 eruption stem from its southward-pointing CME.

Another southward-pointing CME, which struck Earth during the last solar cycle, had far more serious consequences. In the wee hours of March 13, 1989, the Hydro-Quebec power company was operating normally. Then, Earth’s magnetic field fluctuated violently in the region along the U.S.-Canadian border. Voltage sagged, and within 90 seconds, all of Quebec province went black. Six million people woke that chilly late-winter morning without heat or electric lights.

Earthling predictions

So, how are earthlings doing in predicting when a solar storm will occur and how severe it will be? “If the weather predictions were as bad as our ability to predict [the onset of a solar storm], we’d fire all the weathermen,” says solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo.

But once a storm erupts “we’re doing fairly well,” in determining when it will strike Earth, argues Kappenman. He and other scientists credit the ACE satellite, which experiences the brunt of a solar storm an hour before Earth does, with warning satellite and power companies about the severity of approaching storms. “For the first time, we can do continuous predictive forecasts of geomagnetic storms,” he says.

An hour’s lead time is enough to batten down the hatches of a satellite or alert a power company that may need to rely on reserves and minimize use of long-distance transmission lines.

Scientists still have a long way to go before they can make reliable predictions days to weeks in advance, but there’s been some progress. After reviewing more than 2 years of data on solar storms, researchers reported in 1999 that magnetically active regions on the sun often exhibit an S-shaped pattern in their X-ray emissions (SN: 3/13/99, p. 164).

S marks the spot because the twisted or stretched magnetic field lines create this sinuous pattern. But researchers don’t know how soon after an S appears an active region will erupt. And since the sun rotates once every 28 days, the timing is critical for determining if the region will be in line with Earth when it sends out a CME. “If you see an S shape, you know the gun is loaded, but you don’t know when the gun is going off, so you don’t know which way it will be pointed,” explains DeForest.

Once an eruption occurs, an instrument on SOHO can determine whether or not the storm is headed Earthward. That gives forecasters 2 to 3 days, warning that something may hit, although it provides little information on severity.

Two independent teams of astronomers recently found different ways to discern activity on the sun’s far side, its hidden half, days before it rotates into view. One technique relies on the detection of ultraviolet radiation emitted by hydrogen gas in the sun’s vicinity. When energy from a solar outburst hits the gas, it creates ultraviolet hot spots that SOHO can detect, even if they occur on the sun’s far side.

In the other method, researchers use an instrument on SOHO to listen in to the sun’s acoustic vibrations. A slight increase in the velocity of sound waves bouncing off the sun’s far side may alert scientists about a region likely to erupt before the far side rotates into view (SN: 3/18/00, p. 183).

In another diagnostic effort, solar physicists have developed what appears to be a reliable method for predicting how long it will take a given CME to reach Earth (SN: 6/24/00, p. 404: Model Tracks Storms from the Sun). They determine the arrival time of a CME by taking into account how much the solar wind can speed up or slow down the storm.

Another team, which includes participants from the Air Force, is going after the greatest challenge in the space weather business—attempting to determine if the magnetic field of a CME will point south. In trying to model this critical feature, researchers must take into account not only the initial characteristics of a CME but also the speed and density of the solar wind through which it travels. The wind can reorient the CME’s magnetic field into more or less dangerous directions, says Murray Dryer of the Space Environment Center.

Looking toward the future, two NASA craft set for launch in 2004 are intended to view the sun’s corona from different directions, providing a three-dimensional view of solar storms. The mission, dubbed Stereo, is expected to identify those storms heading earthward more quickly than the current array of satellites does.

In 2006, the space agency plans to launch the Solar Dynamics Observatory as the next-generation SOHO. The craft will provide nearly 24-hour coverage of the sun, says project scientist Barbara J. Thompson of Goddard.

Researchers still hope to revive the concept of Geostorm, a proposed mission that was not funded this fiscal year (SN: 8/21/99, p. 120: The craft would use ultrathin sails that take advantage of the push of sunlight to reach orbits closer to the sun than those of ACE. Geostorm could provide data on an impending storm 2 to 3 hours before it reaches Earth.

Such a mission probably wouldn’t be launched until the next solar cycle –just in time to greet the next great wave of violence from the sun.

Next week: Science News explores the effect of solar activity on Earth’s weather and climate in “Pinning Down the Sun-Climate Connection.” Available at Pinning Down the Sun-Climate Connection

More Stories from Science News on Planetary Science