A very severe magnetic storm on Earth was feared last Saturday or Sunday, following a huge solar flare that had erupted some days before.
Telecommunications might be disrupted; orbiting satellites might shudder in the skies; and electrical black-outs here and there might be a very real possibility.
It had to do with sunspots. These are dark blemishes on the surface of the sun that increase and decrease in number over a regular cycle lasting about 11 years.
Sunspots are areas of intense magnetic activity, and this activity sometimes culminates in a solar flare, a kind of celestial Armageddon which shoots out great bursts of energy to interstellar space at unimaginable speeds.
Now, the more sunspots there are, the more intense these bursts of energy, and the sunspot cycle at present has only just passed maximum.
So NASA scientists were worried last Thursday when they spotted the largest sunspot cluster for a decade on the upper right-hand quarter of the sun. They feared the very worst.
The energy from a solar flare arrives on Earth in stages. Electromagnetic waves in the form of X-rays, radio waves and, of course, the visible light which sometimes lets us see the larger flares, take about eight minutes to reach Earth, and give due warning of anything to come: hence the NASA premonition.
Cosmic ray particles are the next to arrive; they take about an hour. But it takes about two days for the real magnetic storm to reach our planet.
Magnetic storm particles consist of ions and electrons, millions of tiny charged particles herded together into a coherent stream by powerful electric fields.
As they interact with the magnetic forces associated with the Earth, strong electrical currents are created in the upper atmosphere, and these induce sympathetic bursts of electrical activity in long metal objects on the ground below, such as railway lines, pipelines and electrical power lines. They also affect the density of the upper atmosphere, and thereby sometimes interfere with spacecraft.
But the havoc caused by a magnetic storm depends very much on the electrical and magnetic characteristics of the particular event, and this is almost impossible to predict beforehand. This was why nothing happened at the weekend. The aurora in northern latitudes were more spectacular than usual, but there was little else.
On the other hand, this same sunspot cluster released another solar flare on Sunday, whose effects might have been expected yesterday. So by the time you read these words a magnetic storm and the disastrous litany above may have occurred already - or it may not.
