The most powerful geomagnetic storm since December 2006 struck the Earth on Monday, a day earlier than expected.
On 3 April, the SOHO spacecraft spotted a cloud of charged particles called a coronal mass ejection (CME) shooting from the sun at 500 kilometres per second. This velocity suggested the front would reach Earth in roughly three days.
“It hit earlier and harder than forecast,” says Doug Biesecker of the US National Oceanic and Atmospheric Administration’s Space Weather Prediction Center in Boulder, Colorado.
Fortunately, the storm was not intense enough to interfere strongly with power grids or satellite navigation, but it did trigger dazzling auroras in places like Iceland (pictured).
Such storms highlight the uncertainty in the arrival times of CMEs, which can easily be 15 hours off predictions, Biesecker says. Better modelling of the solar wind, which can accelerate CMEs en route to Earth, could reduce the uncertainty.