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."