A beam gazing into the heart of the Sun: The weather on our local star affects our own. A satellite is probing the patterns, say Heather Couper and Nigel Henbest

WEATHER affects us all - and not just earthly weather. We are almost totally dependent on the Sun, our local star, for heat, light and energy, so changes in its weather can seriously influence us. Dr Keith Strong, a solar physicist, says: 'From radio communication disruptions to satellite malfunctions to power outages to weather and climate changes, small variations on the Sun can affect us all in profound ways.'

The Sun has a well-documented long-term weather cycle. Roughly every 11 years, the number of sunspots - darker, cooler regions on the Sun's gaseous surface - builds to a maximum and then dies away again.

The underlying cause is thought to be due to the periodic build-up and decay of the magnetic field in the Sun's surface layers. Spots are places where loops of magnetic flux push up through the surface and into the Sun's atmosphere, forming what is known as an 'active region'. If loops of different active regions ever touch, the result is the solar equivalent of a short-circuit - a flare.

Solar flares pack the energy of millions of nuclear bombs, and blast clouds of fast-moving electrically charged particles into the solar system. When these particles hit the Earth, they affect its atmosphere and magnetic field - sometimes leading to the effects described by Dr Strong.

Although the Sun's weather cycle has been carefully observed, it is not well understood. But now solar physicists have a tool to help them comprehend our Sun's ever-changing moods: the Japanese satellite Yohkoh.

The satellite (its name means 'sunbeam') was launched on 30 August 1991 into an elliptical Earth orbit, containing a battery of equipment from Japanese, US and British researchers. Rather than look at the Sun's surface, the instruments are concentrating on its pearly corona - the glowing atmosphere that ordinary mortals only get to see during a total eclipse.

The corona is the starting point for solar wind, a stream of charged particles that boils off the Sun ceaselessly, buffeting the planets as it goes. It is also the site of flares, and 'coronal holes', which appear to be responsible for generating high-speed streams in the solar wind.

One of the most mysterious features about the corona is its very high temperature: one to three million degrees Celsius, and as high as 100 million degrees Celsius during solar flares. Dr Strong, whose company, Lockheed, built the soft X-ray telescope (SXT) on board Yohkoh, explains: 'We're trying to understand why there is a corona on the Sun at all. This star has a surface temperature of a few thousand degrees and yet its outer atmosphere has a temperature of between two and three million degrees. That's very strange.

'You can't heat an atmosphere to several million degrees with something that is only a few thousand degrees. Some other form of heating must be going on.'

The SXT takes pictures of the Sun's corona that record changes taking place over seconds and minutes - a vast improvement on old observations, such as Nasa's Skylab mission, which could only see changes over periods of hours.

During the next few years, Yohkoh will add to our understanding of the Sun's weather, and how it affects the planets. Given our record on Earth, however, to be able to predict the Sun's weather will be another matter altogether.

The planets

IF you didn't spot Mercury last month, that's it until June. This month, it moves into line with the Sun ('interior conjunction'), and is drowned in its glare. By the end of the month, Venus, too, will have moved into line with the Sun and disappeared from our skies.

Mars is also on the way out, fading by a half this month as the Earth draws away. Because the 'overtaking manoeuvre' between Earth and Mars is over, the Red Planet has resumed its 'direct motion', and is heading towards Castor and Pollux.

Jupiter is the 'star' of the month. On 30 March, it stands at 'opposition': opposite the Sun in its orbit, and closest to Earth at 660 million kilometres (390 million miles). It is also, at magnitude -2.4, more than twice the intensity of the brightest star, Sirius. On 10- 11 March the Moon will lie eight moonwidths south of Jupiter.

Saturn is just visible in the dawn twilight low in the south-east as a bright 'star' in Capricornus.

The stars

THE sight of Leo the lion riding high in the sky is a sure sign that spring is just around the corner. In the northern spring, the stars of Cancer, Leo and Virgo lie away from the Sun, so we get a good view of them in our night skies.

Leo is one of those rare constellations that looks like its namesake: a crouching lion. The 'head' is a sickle-shaped pattern of six stars, while the rest of its body stretches out to the left. In legend, Leo was the lion killed by Hercules as the first of his 12 labours, and it is one of the most ancient of the constellations.

The bright star Regulus, at the base of the sickle, marks Leo's heart. This blue-white star, 85 light years away, lies close to the Moon's path in the sky, and was often used by sailors as a navigational guide.

To see Leo's chief glory, however, you need a small telescope. Focus it on Algeiba, the star that marks the band in Leo's neck. Instead of one star, you will see two: a pair of golden-yellow orbs, one slightly fainter than the other, waltzing in a cosmic dance lasting almost seven centuries.

Diary

(After 28 March, times are BST)

1 3.46pm Moon at first quarter

8 9.46am Full Moon

9 Mercury at inferior conjunction

15 4.17am Moon at last quarter

20 2.41pm Spring Equinox

23 7.15am New Moon

28 2am British Summer Time begins (clocks forward)

30 Jupiter at opposition

31 5.10am Moon at first quarter