Phew! What a scorcher!

Next month, the Sun's surface will flare up as part of an 11-year cycle. Here, radios will crackle and the best wines will be produced. What's going on?

Below the surface of the Sun, 140,000 miles below, to be precise, something is stirring. Vast rivers of gas, wider than the combined diameters of many Earths, are subtly beginning to change their motion. Gigantic filaments comprising millions of tons of superheated plasma - electrically charged gas - travelling at hundreds of miles an hour are beginning to writhe and coil in another direction. It is the beginning of the next solar cycle.

Below the surface of the Sun, 140,000 miles below, to be precise, something is stirring. Vast rivers of gas, wider than the combined diameters of many Earths, are subtly beginning to change their motion. Gigantic filaments comprising millions of tons of superheated plasma - electrically charged gas - travelling at hundreds of miles an hour are beginning to writhe and coil in another direction. It is the beginning of the next solar cycle.

The effect will not be felt for years. It happens every 16 months and astronomers do not yet understand how this solar heartbeat shows itself on the Sun's surface as an 11-year cycle of activity. Every 11 years, the whole system winds itself into a crescendo called the Solar maximum, and next month is it.

The Sun's powerhouse is at its core. There, the 15 million-degree temperatures and the denser-than-lead superheated gas are strong enough to force protons to fuse and become helium nuclei, a small amount of energy being released in the process. It is a very temperature-sensitive reaction, which is why it only happens in the heart of the Sun and rapidly tails off away from the solar core.

The energy liberated by the nuclear fusion reaction emerges as a photon of high-energy light that gets scattered back-and-forth, performing what scientists call a "random walk" away from the core. This occupies the photon for a million years. This means that if the Sun's power production region were to suddenly shut off, we would not notice it affecting the Sun's brightness for a million years.

When the photon has travelled a total distance of six million million million miles, easily enough to travel to the nearest galaxy - and back again - it has actually only traversed the 600,000 miles as the crow flies to the base of the interface layer. Here the photon must stop as the Sun suddenly becomes opaque.

Stopped in its tracks, the energy given up by the photon must go somewhere so the gas starts to move. Vast convection bubbles, thousands of miles across, swell and rise towards the solar surface where it cools and descends back into the interior, only to rise again.

With proper equipment you can see this convection on the surface. The whole of the Sun's disk seethes with bright cells, a thousand miles across and with dark fuzzy borders, that come and go every 30 minutes or so. Astronomers call it granulation. The movement is supersonic; if you were there, the sonic booms would be constant.

It is this convection zone that is the key to the Sun's 11-year cycle of activity. Because the Sun spins faster on the inside and at the equator, the interplay of rotation and convection on streams of electrically charged gas causes titanic magnetic fields to be generated.

These magnetic fields form into tubes, or flux ropes, and as they break surface they can be an awesome sight. They cool the surface slightly and the result is a sunspot. As they ascend into the solar atmosphere, they become filled and outlined by super-hot, thin gas. X-ray telescopes on satellites see them as beautiful loops and whirls. But the base of the flux tubes are moving and other tubes are pushing up from beneath, so it is an unstable situation and something has to give. The tubes become stretched and coiled until the magnetic tension is too great and the magnetic field suddenly and explosively collapses.

The result is a solar flare, the sudden and dramatic heating of vast quantities of gas and charged particles. Some of these particles rain down in energetic streams on the Sun's surface, heating it and causing it to glow. Observers call this a white-light flare. Electrons are sent spiralling along magnetic field lines, now flailing into space. The electrons radiate micro and radio waves. On the distant Earth, crackling and spluttering is heard in the radio spectrum.

Gas clouds are also ejected into space, 10 billion tons of gas heaved outward at a million miles an hour. Four days later it may collide with the Earth. If the magnetic field that threads its way through the gas cloud is in the right configuration, which it is 50 per cent of the time, then as it reaches the Earth's magnetic sheath a great interaction takes place and magnetic fields ripple around our planet, inducing electric currents on the surface. The result is a magnetic storm that can knock out power supplies and interfere with communications satellites.

In these geomagnetic storms, high-latitude power lines may be at risk. Power companies usually isolate them to limit the damage-inducing currents. But in 1989 in Canada this was not the case, and vast areas suffered a blackout. Satellites may be damaged; air navigation systems on aircraft, and even the migratory mechanism within the brain circuits of birds, may become confused.

The Earth responds in more ways to the solar cycle. The multicoloured curtains and rays of the aurorae light up the polar skies as a result of an interaction between the solar storms, the Earth's magnetic field and ionised gas in the upper atmosphere. For some reason, never satisfactorily explained, the best wines are produced at times of a solar maximum, and some people claim they can see 11- or 22-year cycles in the weather as well.

Sometimes the Sun's cycle falters and the sunspots fail to appear. The last time this happened was between 1645 and 1715, not long after the telescope was first turned towards the heavens and the Sun. Few sunspots were seen, although at that time the 11-year cycle had not been recognised, and the Earth chilled.

Climatologists and historians call this period the "little Ice Age". Frost fairs were common on the Thames and winters were harsh. During one particularly severe winter, Eskimos arrived on the north coast of Scotland looking for more hospitable climes. In these days of global warming, we have nothing like that, and the solar cycle seems to be regular. But we understand so little about the Sun's pulse that we can make no predictions about when it may turn off again. There are suggestions of other prolonged minima in the past (eg 1460 to 1550), but the conclusion is that it could happen any year.

Scientists can only tell if the Sun has reached its peak about seven months afterwards. That is because they rely on "smoothed" monthly averages of sunspot numbers. Some cycles, like the last one, are double peaked, separated by a year or more. But their gut feeling is that maximum occurs next month.

There is still so much that is unexplained about the Sun. Exactly how do the sub-surface currents produce the solar cycle; why are there fewer particles called neutrinos emerging from the nuclear reactions at its core; why is it surrounded by a thin, million-degree atmosphere, and exactly how do solar flares work? As this is the first solar maximum in the era of the internet, it is possible to watch online sunspots moving across the face of the Sun and keep an eye on solar flare monitors (real-time data from solar monitoring satellites are put on the Net). We can all watch, marvel, and lay in stocks of this year's vintage.

David Whitehouse is science editor of BBC News Online