Much like a temperamental teenager, the sun has been acting up of late. As it approaches the peak of the 11-year solar activity cycle, predicted to occur next May, it has been displaying an increasing number of angry outbursts. These solar storms are technically called solar flares and are giant eruptions of radiation from the sun's atmosphere that cause significant brightening of the area where they occur. Solar flares are sometimes followed by coronal mass ejections (CMEs), which spew charged and magnetized particles into space.  Depending on the direction of their release, these particles sometimes reach Earth where they occasionally damage satellites and disrupt terrestrial power grids. In 1989 a solar storm knocked out electricity across Quebec for nine hours. In 2003 a solar storm crippled South Africa's power supply by damaging 15 large transformers, according to John Kappenman, an expert on how solar storms affect power grids.

On July 12, as a huge CME headed toward Earth, forecasters warned of possible power outages, spurring flight controllers to reroute aircraft on polar routes to lower latitudes, away from the shower of energetic particles. That storm produced spectacular auroral displays but caused no outages. Other CMEs followed on July 19 and 23 but, again, neither caused power failures.

Why is it that some CMEs cause disruptions whereas others of a similar magnitude or even larger do not? Experts, aided by new models, point to a couple of factors.

"For the first time, space weather forecasters now have models and tools for predicting how a CME is released from the sun, accelerated out into the solar wind, and ultimately ends up colliding with Earth's magnetosphere creating the geomagnetic storms that impact so many technologies and systems," says Rodney Viereck of the National Oceanic and Atmospheric Administration's (NOAA) Space Environment Center. Viereck's team is responsible for forecasts of geomagnetic storms caused by solar outbursts.

The first factor that influences whether a CME will be disruptive is the direction in which the charged particles are emitted. "Solar storms propagate like a bullet," says Tamas Gombosi, director of the Center of Space Environment Modeling at the University of Michigan. "Sometimes the bullets miss the Earth. When they originate far from the [sun's] central meridian that is facing the Earth, they miss the Earth."

The other factor is the orientation of the magnetic field of the charged particles streaming toward Earth. How the magnetic field of the CME interacts with Earth's magnetosphere, the magnetic shell covering and protecting the planet, determines how severe any terrestrial effects will be, notes Gombosi, who has built models of the interaction.

In general, if the charged particles from a CME hit Earth's magnetosphere head on and the ejection has a strong magnetic field pointing south, then the disruptive effects are greater, Gombosi says.

According to him, some storms are most troublesome because of a process called magnetic reconnection, in which the magnetic field of the CME interacts directly with the Earth's magnetic field. During the interaction, the magnetic field lines that normally connect the planet's north and south poles may get reconfigured and essentially plug into the CME's field lines for a short time, then disconnect and regroup again into a north-south configuration.

"When the CME's magnetic field has a big southward component, there is a high probability of reconnection," Gombosi explains. "On the other hand, if there is a high northward component, there is a low probability of reconnection."

The way reconnection disturbs terrestrial power grids is complex but, in essence, the process mimics what happens in electric generators, where a fluctuating magnetic field (usually a moving magnet) produces a current in a coil of wire, says Adam Szabo, director of NASA's Heliophysics Laboratory. "Just as in regular electric generators, when moving magnetic fields cross long electrical conductors, electric currents will be generated. Power lines are such long conductors. The generated excess current can overload transformers and substations causing a domino effect of outages."

Power-grid expert Kappenman warns that, under the right conditions, a CME could have planetary scale consequences that would take more than a couple of days to recover from, in part because damaged transformers would be difficult to replace quickly.

"It's hard to manufacture these 200-ton devices," he says. "In the best of times, they take a year to a year and a half to make. There was nearly a three-year backlog a few years ago."

Better space weather forecasts
New understanding of the physics of reconnection helps to explain why today's forecasts of Earth-bound CMEs are better than in the past, when "forecasters had to rely on estimates and 'rules of thumb' in developing forecasts," Viereck says.

Until a few years ago, the commonly held scientific view was that two kinds of solar events affect us—short-duration events caused by solar flares and long-duration events from CMEs. Scientists incorrectly used different mechanisms to explain how the two occurred. "It appears that there is only one acceleration mechanism," says Szabo, who notes that the scale of the magnetic reconnection produced is the only difference.

Although a geomagnetic storm forecast today is more accurate than in the past, it can only give an accurate warning up to a few hours, at most. To improve those lead times and also give more advance notice of the damage potential of an incoming CME, researchers say, they need more complete data about how the storms are produced on the sun and how they change over time. With such insights, they might be able to model more precisely how CMEs will behave when they reach Earth.

This modeling would be aided by observations that somehow capture the direction of the magnetic field within the CME as it leaves the sun and by "a better understanding of how the magnetic field will change and evolve as the CME makes its multiday transit from the sun to Earth," Viereck says.

But observations of CMEs on and near the sun—whose surface temperature is about 5,500 degrees Celsius—are extremely difficult to accomplish. Because intense heat disables electronics, it is very difficult to station a data-gathering spacecraft close to the sun. Although NASA's MESSENGER spacecraft currently orbits Mercury, the solar system's innermost planet, "even at the orbit of Mercury, which is 60 solar radii, it is probably not adequate," Gombosi says. "We need to go to three to five solar radii."

Other obstacles confront space weather forecasters as well. Even though satellites give very reliable information of when a CME started out from the sun, no one knows how to predict the north-south component of a geomagnetic storm, Gombosi says, and there are no tools able to measure that. Viereck says modelers also need a more detailed picture of the exact conditions in Earth's magnetosphere at the moment a CME hits.

Don't expect better forecasts as the sun nears solar maximum next year. "We are still at least a decade away from developing a detailed enough understanding that can be converted in any sort of operational forecasting," Szabo notes.