Brewing up a magnetic storm on the sun


At first sight, the solar disc is uninteresting and bland - a smooth ball of gas, marred occasionally by the dark blemish of a group of sunspots. But it is in reality a seething cauldron of activity, and it is around the sunspots themselves that much of the action can be found.

Sunspots are caused by the protrusion of a strong concentration of magnetic activity through the sun's surface; they are slightly cooler than their surroundings, which makes the regions appear black when viewed through the filters that reduce the light to bearable intensities for cameras and human eyes. Above the dark areas, the magnetic forces support the filaments of glowing gases that appear at the sun's edge as beautiful arched streamers of multicoloured bunting.

The tour de force of these magnetic regions is a solar flare. When two opposing magnetic "loops" come together within a complex sunspot region, they annihilate each other in a massive explosion which shoots out energy to interstellar space at unimaginable speeds.

The energy from a solar flare reaches Earth in stages. Electromagnetic waves in the form of X-rays, radio waves and, of course, the visible light which sometimes allows us to see the larger flares, take about eight minutes and give due warning of events to come. 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 - 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. In effect, these metal objects act like dynamos and can generate their own powerful surge of electricity.

Magnetic storms are most likely to occur, and to be at their most disruptive, near the maximum of the sunspot cycle, an event which occurs every 11 years or so, when there are more sunspots on the solar disc than usual. At the last solar sunspot maximum in 1989, a surge from a magnetic storm overloaded part of an electrical power grid and caused a blackout to cascade through much of the north-eastern United States and eastern Canada.

We must wait and see if the maximum of the present "cycle 23", expected by the experts to occur later on this year, presents us with any nasty surprises.