They found that an "S"-shaped structure appearing on the surface of the Sun invariably precedes a solar explosion, which can hurl 11 billion tons of electrically charged particles at the Earth.
Coronal mass ejections, as the explosions are called, can travel at up to a million miles an hour and take about four days to reach Earth where they can cause havoc to electrical installations, power lines and communications networks.
One of the worst geomagnetic storms was in 1989. It knocked out the entire electrical supply of the Canadian province of Quebec, causing a cascade effect in neighbouring regions due to a dangerously low voltage.
A Nasa-sponsored study of the Sun's activity using a Japanese satellite found that the S-shaped pattern, called a sigmoid, began to form days before a coronal mass ejection.
Each sigmoid is like a "loaded gun" that scientists know has a high probability of going off, according to Richard Canfield, professor of physics at Montana State University. "We've found that the S-shaped regions are the dangerous ones. As soon as we can recognise that a region is S-shaped, we know that it is more likely to erupt," he said.
Coronal mass ejections are violent discharges of hot, electrically charged gas from the Sun's corona, or outer halo. The outbursts occur several times a day but only those shot towards Earth are potentially dangerous.
On reaching Earth they are known to cause errors in navigation equipment, knock out satellites, disrupt flow meters in oil pipelines, interfere with equipment for geological exploration and be a general hazard to astronauts who may be in space.
The scientists hope to refine the prediction technique to enable them to forecast the most likely ejections that may impinge on Earth, enabling companies and government agencies to take evasive action, such as turning away satellite arrays or protecting electrical equipment on the ground.
Professor Canfield said the sigmoid shape is likely to be caused by the development of a twisted solar magnetic field.Reuse content