Two satellites, operated by Nasa and the US Air Force, have been quietly put into space more than a million miles from the Earth to try to give advance notice of the storms, which could black out electrical grids, disrupt radio and navigation signals, stop credit card transactions, and even blind nuclear early warning satellites.
But there is good news too. The storms threaten to silence mobile phones and pagers - and could bring the Northern Lights far enough south to be seen from the Equator.
The storms, which give out vast bursts of energy, reach a maximum every 11 years as part of a natural solar cycle. But the US government is warning that this "solar max" could cause far more disruption than usual because the Earth has more power grids and satellites vulnerable to the flares.
At the last peak, early in 1989, a surge of magnetic activity shut down a power grid in Quebec, Canada, leaving 3 million people without electricity for nine hours. And as the present solar max built up last year, flares knocked out a satellite over the US. Over the subsequent three days 40 million pagers and mobile phones were out of action, television broadcasts were disrupted, and credit card transactions were blocked.
More alarmingly, the British American Security Information Council reports that flares have several times blinded one or two of the three US nuclear early warning sensors, severely weakening its defence against attack.