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THE sun is gearing up for a peak in activity at a time when technology makes our planet more vulnerable to solar outbursts than ever before. Monitoring has improved since the last solar maximum, so what are the big risks this time around?
About once every 11 years, the sun goes ballistic, throwing out more bursts of magnetic activity than normal. As a large but harmless solar flare signalled last week, the next solar maximum is due in 2013.
In the past, these storms have triggered extra currents in power lines, destroying transformers and leading to blackouts. This time around, blackouts could be more common. John Kappenman of Storm Analysis Consultants in Duluth, Minnesota, found that many transformers in the US are ageing and therefore extra fragile.
He also points out that while new transformers consume less power, that means relatively small currents from solar storms can overload and damage them. "If anything, we're making things on the grid more vulnerable," he says.
As well as causing blackouts, solar storms can fry satellite electronics, which we rely on more and more for communication, navigation and weather forecasts.
They calculated that there are 994 working satellites in orbit today compared with 629 during the sun's last peak. Better storm forecasting should make them less vulnerable (see "And now for the solar forecast"). Ground controllers can command satellites to switch off sensitive parts temporarily in response to a forecast.
However, there is another risk that barely existed 11 years ago. Many passenger flights between North America and Asia now take shortcuts over the North Pole. This saves flying time and cuts fuel consumption, but it leaves planes vulnerable to solar storms.
Earth's magnetic defences are weakest at the poles (see diagram), allowing electrons and protons to pour into the atmosphere during solar storms. This can interfere with planes' communication and navigation signals. Airlines reroute polar flights when solar storms are predicted, as they did last week, adding hours of flight time and costing tens of thousands of dollars in extra fuel per flight.
Astronauts on the International Space Station, meanwhile, get an extra dose of radiation during a storm: there are six there now compared with three 11 years ago.
We can take some comfort in the knowledge that the looming maximum is supposed to be relatively weak, but we shouldn't be complacent. In 1859, during an otherwise weak cycle, a solar storm made telegraph wires spark, starting fires. "You've got the opportunity for flares, and they can be big ones," warns David Hathaway of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
And now for the solar forecast
Predicting the weather is tricky and solar storms are no exception. We've improved in leaps and bounds since the last solar maximum but we still can't say whether an approaching flare will be a perfect storm or a just a damp squib.
In 2000, the best early warning tool was NASA's Solar and Heliospheric Observatory. Potentially damaging plasma clouds would show up on the spacecraft's images of the sun. However, the images were transmitted to Earth just once a day and since solar outbursts can travel all the way to Earth in less than a day, some clouds were missed.
In 2010, NASA launched the Solar Dynamics Observatory, which streams images of the sun to Earth in real time. That is one reason why the predicted arrival time for a plasma blast last week - the strongest since 2003 - was "exceptionally good", says David Hathaway of NASA's Goddard Space Flight Center in Greenbelt, Maryland, where SDO is managed. The forecast was accurate to within 13 minutes.
SDO has limitations too, though. The most violent storms tend to come from plasma clouds that have a strong magnetic field in opposite alignment to Earth's, but SDO's images cannot reveal a cloud's magnetic properties. So we don't really know what we're in for until an hour ahead of impact, when the cloud engulfs NASA'sAdvanced Composition Explorer. Positioned between the sun and Earth, it can measure the cloud's magnetic field, though even ACE is ageing and needs to be replaced.