The forecasters in mid-October of 2003 were worried. For more than a week, they had watched plumes of material arcing out over our star's southeastern limb. Something on the far side of the Sun was venting vast plumes of plasma into space. Soon, the Sun's rotation spun the culprit into view: It was a region of sunspots more than 13 times the diameter of the Earth, bubbling with volatile magnetic fields.

Sunspots are the main sources for solar flares — brief pulses of intense radiation created when the Sun's magnetic loops spontaneously snap and rearrange themselves. Sometimes, a spate of solar flares will spur an even more violent phenomenon, a billion-ton belch of magnetized plasma that explodes out from our star at millions of miles per hour, plowing into anything in its path. Scientists call these solar belches "coronal mass ejections," or CMEs.

By October 28, the Sun's rotation had brought the sunspot region into direct alignment with Earth. And then it happened. Around 7 am Eastern time, the region released a pulse of high-energy photons in one of the strongest solar flares ever recorded. Eight minutes later, satellites detected the photons arriving at Earth, followed some minutes later by a shower of slower-moving, high-energy subatomic particles. The particles accumulated in the Earth's upper atmosphere, where they dramatically interfered with high-frequency radio communications and slightly increased radiation exposures for airplane crews and passengers. At a fuel cost of several tens of thousands of dollars per flight, commercial airlines began rerouting many of their planes on longer, safer routes that did not take them near the Earth's polar regions, where our planet's magnetic field caused most of the particles to linger. The flurry of particles also degraded GPS satellite signals, causing ground-based receivers to temporarily lose service or receive flawed navigation data.

As disruptive as the particle shower was, it was only the beginning. At 7:30 am, just after sunrise on the east coast of the United States, a satellite stationed directly between the Sun and the Earth observed our star gain an ominous glowing halo, the telltale sign of a CME aimed directly at our planet. All along the eastern seaboard, millions of people awoke to a seemingly normal, sunny day, unaware that they and our entire planet lay directly in the path of a vicious solar storm.

Shortly after 2 am Eastern time on October 29, the CME arrived at Earth, and the storm's major effects began. A magnetized plasma front slammed into our planet's magnetic field, pumping it full of energy to create a "geomagnetic storm" that sent powerful electric currents reverberating in and around the Earth. Vivid displays of auroral lights, normally restricted to higher latitudes, painted the night sky red and green in Florida and Australia.

A geomagnetic storm produces dangerous electrical currents in a manner analogous to a moving bar magnet raising currents in a coil of wire. When a CME hits the Earth's magnetic field and sends it oscillating, those undulating magnetic fields raise currents in conductive material within and on the Earth itself. The currents that ripple through our planet can easily enter transformers that serve as nodes in regional, national, and global power grids. They can also seep into and corrode the steel in lengthy stretches of oil and gas pipeline.

On October 29, power grids around the world felt the strain from the geomagnetic currents. In North America, utility companies scaled back electricity generation to protect the grid. In Sweden, a fraction of a CME-induced electric current overloaded a high-voltage transformer, and blacked out the city of Malmo for almost an hour. The CME dumped an even larger mass of energetic particles into Earth's upper atmosphere and orbital environment, where satellites began to fail because of cascading electronics glitches and anomalies. Most were recovered, but not all. Astronauts in low-Earth orbit inside the International Space Station retreated to the Station's shielded core to wait out the space-weather storm. Even there, the astronauts received elevated doses of radiation, and occasionally saw brief flashes of brilliant white and blue—bursts of secondary radiation caused when a stray particle passed directly through the vitreous humor of the astronauts' eyes at nearly light-speed.

Flares and CMEs from the Sun continued to bombard the Earth until early November of that year, when at last our star's most active surface regions rotated out of alignment with our planet. No lives were lost, but many hundreds of millions of dollars in damages had been sustained.

The event, now known as the Halloween Storm of 2003, deeply worried John Kappenman, an engineer and expert in geomagnetic storm effects. The Sun had fired a clear warning shot. Its activity roughly follows an 11-year cycle, and severe space weather tends to cluster around each cycle's peak. The Sun's next activity peak is expected to occur this year or next, and the chance of more disruptive geomagnetic storms will consequently increase.

Kappenman was particularly frightened by the blackout in Malmo. Subsequent investigations of the CME revealed that it had only struck a glancing blow — its magnetic field was aligned so that much of the potential impact was dampened, rather than enhanced, by the Earth's own. If, by chance, the alignment had been different, and our planet had absorbed the full brunt of the CME, who knew how large the blackout would have been, or how long it would have lasted?

Considering the possibility of a long blackout, stretching over weeks, months, even years, Kappenman suddenly saw a foreboding societal reliance on electricity everywhere he looked. Perishable foods and medicines would spoil or freeze in warehouses suddenly stripped of climate control. Municipal stores of fuel and potable water relying on electric pumps would be rendered all but inaccessible. Telecommunications would crash, preventing the general dissemination of information and large-scale coordination between emergency responders. The twin specters of social collapse and mass starvation would stalk entire continents.

"If you lose electricity, within a matter of days you essentially lose almost everything else," Kappenman says. "After the initial blackout, we wouldn't really understand the seriousness of the situation until several days went by without having things restored. We'd rapidly lose the ability to provide the necessities for modern society."

All this may seem like doomsaying, but the historic record suggests otherwise: The Halloween Storm, in fact, pales in comparison to several earlier events. In 1989, ground currents from a less intense geomagnetic storm knocked out a high-voltage transformer at a hydroelectric power plant Quebec, plunging the Canadian province into a prolonged 9-hour blackout on an icy winter night. A far more extreme geomagnetic storm washed over the Earth in May of 1921, its magnitude illustrated in world-girdling aurorae and in fires that broke out in telegraph offices, telephone stations, and railroad routing terminals — sites that sucked up geomagnetic currents traveling through nascent power grids. An even more extreme storm in September 1859 caused geomagnetic currents so strong that for days telegraph operators could disconnect their equipment from battery power and send messages solely via the "auroral current" induced in their transmission lines. The 1859 storm is known as the "Carrington Event," after a British astronomer who witnessed an associated solar flare and connected it with the subsequent earthbound disturbances.

"The physics of the Sun and of Earth's magnetic field have not fundamentally changed, but we have," Kappenman says. "We decided to build the power grids, and we've progressively made them more vulnerable as we've connected them to every aspect of our lives. Another Carrington Event is going to occur someday." But unlike in 1859, when the telegraph network was the sole technology endangered by space weather, or in 1921, when electrification was in its infancy, today's vulnerable systems are legion.

Over the past 50 years, global power-grid infrastructure has grown by about a factor of ten. That growth has been accompanied by a shift to higher operating voltages, which increase the efficiency of electricity transmission but make the grid less resistant to exterior impinging currents. As the grid has grown, so too has the practice of importing and exporting electricity between regions, across interstate and international lines. The electricity to power a street light in upstate New York may sometimes come from a hydroelectric plant in Quebec; a neon sign outside a nightclub in Tijuana sometimes gets its juice from a natural-gas power plant in Southern California. This interdependency of nodes in the grid means a power outage in one region can more easily cascade into others, increasing the risk of widespread collapse. We have created a continent-sized antennae—one exquisitely tuned to soak up ground currents caused by space weather, yet poorly equipped to counter their negative influence.

Kappenman has made a career of understanding how a geomagnetic storm as powerful as 1859's Carrington Event could affect modern infrastructures, and has undertaken a series of studies on the topic underwritten by various branches of the U.S. federal government. He has consistently found that in a worst-case scenario where a great geomagnetic storm strikes with little forewarning, the excess current in the U.S. power grid could overheat hundreds or thousands of high-voltage transformers, melting crucial components and effectively crippling much of the nation's generation capacity. Based on current production rates, building replacement transformers would take as long as 4 to 10 years, during which more than a hundred million people would be without centrally provided power, causing an estimated economic impact in the U.S. of $1 to $2 trillion in the first year alone.

In direct response to Kappenman's work, last year the Department of Homeland Security asked an independent group of elite scientists, the JASON Defense Advisory Panel, to investigate his claims. In their report, issued in November 2011, the JASONs expressed skepticism that Kappenman's worst-case scenario could occur, pointing out that his analyses used proprietary techniques that prevented their full vetting and replication by other researchers. Nonetheless, they sided with Kappenman in stating that in its current form, the U.S. power grid was vulnerable to severe damage from space weather. Like Kappenman, the JASONs called for more space-weather safeguards, recommending that the U.S. grid be hardened against geomagnetic currents and that the nation's aging network of sun-observing satellites be bolstered.

Not everyone is optimistic that our modern society will successfully address the problem—including physicist Avi Schnurr, who is also the president of the Electric Infrastructure Security Council, a non-governmental organization advocating space-weather resilience. "If a Carrington Event happened right now it probably wouldn't be a wake-up alarm—it would be a goodnight call," he says. "This is a case where we have to do something that is not often successfully achieved by governments, and certainly not by democracies: We have to take concerted action against a predicted threatening event without having actually experienced the event itself in modern times."

Protecting the power grid on Earth is, in principle, relatively straightforward. (Countries such as Finland and Canada have already begun to take action, with promising results.) Most high-voltage transformers are directly connected to the ground to neutralize power surges from lightning strikes and other transient phenomena. They're vulnerable to space weather because geomagnetic currents flow upward through these ground connections.

By placing arrays of electrical resistors or capacitors as intermediaries between the ground and critical transformers, like those serving nuclear power plants and major metropolitan areas, that connection would be severed—and the space-weather threat greatly reduced if not entirely eliminated. Experts estimate this could be accomplished within a few years, at a cost of hundreds of thousands of dollars per transformer. In practice, however, it's not so easy. So far, U.S. power companies have balked at voluntary installation of such devices, and current government regulations don't require such protections.

In 2010, the U.S. House of Representatives unanimously passed the GRID Act, which would grant the federal government authority to take action to protect the national power grid in the event of an emergency, but the bill floundered in the Senate. Undaunted, in February of 2011 Congressional proponents introduced a new, nearly identical bill, the SHIELD Act, which as of this writing has still not come to a floor vote in the House or the Senate. The North American Electric Reliability Corporation, a self-regulatory body for North American electric utilities, formed a Geomagnetic Disturbance Task Force in 2010 to craft new standards and regulations to protect the grid from cataclysmic space-weather-induced failures, but the Task Force's reports are still forthcoming.

"The real danger here isn't astrophysical, it's institutional. The threat to everyone belongs to no one," says Peter Pry, a former official in the Central Intelligence Agency and the U.S. House Armed Services Committee who has tried to spur legislative action on the threat of space weather. After watching year after year in frustration as bills mandating protection of the grid repeatedly floundered in Congress, Pry helped form EMPACT America, a non-profit group chartered to raise public and governmental awareness of electromagnetic threats to the nation's infrastructure. Pry currently serves as EMPACT's president, and says the group is devoted to "ramrodding" the necessary legislation through Congress.

NASA

Power outages wouldn't be the only cause of cascading failures in the event of extreme space weather. According to Jane Lubchenco, the head of the National Oceanic and Atmospheric Administration (NOAA), society's reliance on GPS satellite networks has dramatically increased during the past decade, making this relatively new technology particularly vulnerable to space-weather disruption. GPS satellites broadcast precise timing signals to allow receivers to calculate geospatial position and to measure time to accuracies of billionths of a second. Those signals do more than provide directions for road trips; they synchronize cellphone conversations, orchestrate air traffic, and guide fleets of emergency vehicles.

Space weather probably couldn't take down the GPS network permanently, but it could cause hours or days of service disruption, exacerbating the crisis of a failing power grid. "Today, most financial transactions are date-stamped with GPS, and GPS guides the dynamic positioning of most deep-ocean oil and gas operations," Lubchenco says. "Can you imagine the financial disruption that a GPS outage would cause? Can you imagine the Deepwater Horizons that would occur if drilling platforms received erroneous GPS information?"

Until new layers of protection are put in place, the only way to ensure that power grids and satellite networks withstand a repeat of the Carrington Event would be to preemptively shut them down when a big storm is likely to occur. "That's really not a good solution," Kappenman says. For one thing, each self-enforced outage would cost billions of dollars in lost productivity. And for another, "forecast systems probably aren't ever going to be precise enough to avoid false alarms."

Thomas Bogdan, director of NOAA's Space Weather Prediction Center (SWPC) in Boulder, Colo., acknowledges "our ability to forecast is actually fairly poor." CMEs and solar flares will be particularly difficult to predict without better theoretical models of the circulation of plasma in the Sun, but CMEs reliably occur about four times per day during our star's activity peak, and approximately once a week during solar quiescence. "The only reason we really get by is that the sun has a regular activity cycle," Bodgan says.

SWPC relies on constant space-based surveillance of the Sun, which allows relatively rapid identification of potentially threatening events. Initial notifications come from a network of ground-based solar observatories operated by the U.S. Air Force, as well as a NOAA satellite network that watches for the telltale x-ray pulses that signal solar flares. But these early warnings cannot reveal whether a radiation storm or a CME is actually headed toward Earth. For that, forecasters rely on only a few satellites.

The Solar and Heliospheric Observatory (SOHO), a spacecraft positioned nearly a million miles directly sunward of Earth, looks for CMEs directed toward our planet. The two spacecraft in the Solar Terrestrial Relations Observatory (STEREO) monitor the Sun from two widely separated points in space to provide a three-dimensional view of CME trajectories that, by chance, fall within their fields of view. The final watcher is the Advanced Composition Explorer (ACE), a spacecraft in the vicinity of SOHO that measures the intensity and magnetic orientation of any CMEs that pass by. Twenty to 50 minutes after a CME sweeps by ACE, it reaches Earth, and forecasters are reduced to little more than distributing observational updates for the unfolding geomagnetic storm.

Disturbingly, SOHO and ACE are both well past their nominal lifetimes, with no certain replacements. "Once SOHO ceases functioning, probably in the next year or so, we won't have its unique 'looking down the barrel of a gun' perspective on the Sun for forecasting Earth-directed CMEs," says Sten Odenwald, an astronomer affiliated with NASA's Goddard Space Flight Center. ACE has sufficient propellant to continue operations until roughly 2024, but there are no guarantees its instruments will last that long. Without ACE's measurements, Odenwald says, "we'll be able to see a CME coming toward us, but we won't know whether its interaction with Earth's magnetic field will cause major fireworks or be relatively harmless."

STEREO and another satellite, the Solar Dynamics Observatory, may be able to compensate for SOHO's eventual loss, but Lubchenco and other experts unanimously believe allowing ACE's unique observational capabilities to expire without a replacement would constitute a blind spot too large and risky to ignore. And, in fact, a spacecraft that could replace ACE currently sits in ignominious storage at NASA's Goddard Space Flight Center in Greenbelt, Md. The Deep Space Climate Observatory, or DSCOVR, is fully assembled and all but ready for launch. Its backers simply lacked the funding to buy a rocket ride into space. As part of the Obama Administration's budget request for 2012, NOAA would receive $47.3 million to refurbish and launch DSCOVR to act as ACE's replacement, but the initiative died in the House. DSCOVR may yet launch, but for now it sits in storage, and the Sun continues to seethe.

Just as SOHO and ACE are falling into twilight, a new generation of computational models can use the satellites' unique data to generate substantially improved forecasts of space-weather effects on our planet.

"In many ways, space-weather forecasting is where meteorology was in the late 1950s and early 1960s," Bogdan says. "That's when numerical weather-forecast models were first added to the forecaster's arsenal, and we're just now getting similar operational models that rely less on statistics based on prior events and more on a robust physical understanding of the fundamental processes at work." Similar to how modern hurricane forecasts feature cone-shaped spreads for a storm's future path, the SWPC has developed models that, within a few hours of a CME being observed, will compute its probable route through the rest of the solar system. When displayed, the result looks a bit like a phonograph horn, with the central region being the most likely CME trajectory. If the Earth lies in the center, trouble is most probably on the way.

"Right now," Bogdan says, "this is only telling us when something will bash into Earth, but we really need a more regional picture. We need a model that translates astrophysical measurements into terms of ground impacts on the grid, because operators can't react to just a solar-wind velocity. They want to know current, amps, when a disturbance will begin, and how long it will last where they are." The SWPC is now partnering with the U.S. Geological Survey to incorporate the Survey's nationwide monitoring of fluctuations in Earth's magnetic field into new rapid-response regional predictive models. Given a warning of days or hours, operators could then take a number of precautions to reduce the risk of ground currents in otherwise unprotected equipment, by shifting the distributions of the grid's electric loads or rapidly ramping down activity at select power stations.

Such a model requires an intimate understanding of all the things on our planet that affect the regional distribution of geomagnetic current intensities, such as the local geography and the varying electrical resistances of different materials within the Earth. In 2007, researchers at NASA's Goddard Space Flight Center initiated "Solar Shield," a collaboration with the Electric Power Research Institute to construct one-of-a-kind physics-based models incorporating those and other variables to predict geomagnetic storm impacts. The project's models began delivering forecasts to power grid customers in 2009, but no one will really know how it performs against another Carrington Event until the next one occurs.

"Another great geomagnetic storm probably won't happen tomorrow, but that doesn't mean we shouldn't worry," Bogdan says. "The good news is, we've got time to prepare for this, but the bad news is, if we don't hedge our bets and buy down some risk, one day, we're gonna get clobbered."

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