Wonders of the Solar System Text Only. Andrew Cohen

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СКАЧАТЬ perfect and unchanging, but gradually we’ve come to realise that the Sun is far more dynamic then just a perfect beautiful orb in the sky. Even tiny fluctuations in its brightness can have huge effects here on Earth.

      SUNSPOTS: THE SEASONS OF THE SUN

      As long ago as 28 BC, Chinese astronomers in the Central Asian deserts had observed dark spots on the surface of the Sun. When the wind blew enough sand into the air to filter the Sun’s glare they could see these strange spots and recorded them in the Chinese history book, The Book of Han. Over the next 1,500 years many other people recorded these strange dark spots on the surface of the Sun, but it wasn’t until the invention of the telescope that Galileo was able to correctly explain the phenomena of sunspots.

      The picture on the opposite page was taken by the SOHO probe – the Solar and Helical Observatory that was launched in December 1995. SOHO is giving us unprecedented detail on the life of our Sun and delivering the most beautiful and intricate images of our star that we’ve ever seen. In the picture above (also taken by the SOHO probe) you can see a beautiful example of the birth, life and death of a sunspot. It may look small compared to the size of the Sun, but the sunspot you are looking at is in fact bigger than the Earth. Sunspots are transient events on the surface of the Sun that are caused by intense magnetic activity that inhibits the flow of heat from deep within the Sun up to the surface. These spots appear dark because they are dramatically cooler than the surrounding area – often 2,000 degrees Celsius cooler. In the eighteenth century it was thought they might even be cool enough to allow humans to land on the surface of a sunspot, but at a toasty 3,000–4,500 degrees Celsius even these cool spots on the Sun would melt a spaceship instantaneously.

      Sunspots expand and contract as they move across the surface of the Sun, and they can be as large as 80,000 kilometres (50,000 miles) in diameter, making larger ones visible from Earth without a telescope. Advanced technology and space observation now allows us to track their numbers as they ebb and flow across the face of the Sun.

      Since sunspots are cooler areas than on the rest of the Sun, we might have expected to find that the power of the Sun dimmed when the sunspot activity was at its height. In fact, we’ve found the opposite to be true: the greater the number of sunspots, the more powerful our star becomes. This variation is not just random, as we have studied the Sun in greater and greater detail we have begun to observe patterns emerging; patterns that seem to have direct links to our climate back here on Earth. We’ve discovered that the Sun has seasons.

      THE SUN AND EARTH: SHARING A RHYTHM?

      For decades scientists have sought to understand how these subtle changes in the Sun’s power might be affecting the Earth. It’s a puzzle that led one man to look away from the Sun and focus instead on the rivers around the Iguaçu Falls. Argentinean astrophysicist Pablo Mauas has spent the last decade analysing data that details every aspect of this river system – from water levels to flow rates – from 1904 all the way through the twentieth century. Unlike many of the world’s great rivers, the Parana is so large that it can be navigated by very big ships – and where there are ships there are records. These records enabled Pablo to uncover an extraordinary history and to reveal that, just like sunspots, the river too has a rhythm.

      Pablo and his team found that the stream flow of the river fluctuated dramatically three times during the last century, but the records gave no indication as to what was behind these fluctuations. The amount of water in the river Parana seems to follow a pattern, and Pablo had a hunch that this rhythm might be connected to the rhythms of the Sun. To try to link events on the Sun, 150 million kilometres away, to the flow of the great Parana, Pablo looked first at the most obvious rhythm of our star.

      We’ve known for over 150 years (since the German astronomer Heinrich Schwabe collated the data that stemmed back to Galileo’s earliest observations of sunspots) that the Sun follows a cycle that is repeated approximately every eleven years. This cycle reflects a rhythmic variation in the number of sunspots, which gives us a very clear indication of the amount of radiation given off by the Sun: the greater the number of sunspots, the greater the energy that is reaching the Earth. But when Pablo Mauas looked for a link between the Paranas rhythm and the eleven-year cycle, at first he found nothing. So instead he turned to calculations that described the Sun’s underlying brightness during the last century. We already know that climate change and events such as El Niño can boost the flow of the river, but when Pablo removed both of these effects from the data there appeared to be a strong relationship between the solar data and the stream flow. Superimpose the solar data on the water levels in the river (see below) and you see that when solar activity rises, the volume of water in the river goes up. There is a beautiful correlation between the flow in these rivers and the solar output. Pablo has revealed an amazing link across 150 million kilometres of space that may one day help us to not only better understand the impact of the Sun on our climate, but also to predict the likelihood of floods in the heavily populated waterways of one of South America’s greatest rivers.

      Changes in the Sun seem to move weather systems elsewhere, too. In India, the monsoon appears to follow a similar pattern to the river Parana, boosting precipitation when solar activity is at its greatest, whereas in the Sahara desert the opposite seems to occur: more solar activity means less rain. The exact mechanisms by which our star may affect Earth’s weather remain, for now, a mystery. We know that the energy production rate of the Sun – the power released in the fusion reactions at the core – is very constant, indeed. It doesn’t change, as far as we can tell, and so the changes that we see must be to do with the way in which the energy exits the Sun. And while the amount of radiation that falls onto the surface of the Earth is only at the tenths of a per cent level, it really does reveal the intimacy and delicacy of the connection between the Sun and the Earth.

      HOW TO CATCH A SUNBEAM

      We are tied to our star in the most intimate of ways. All the planets in our solar system are bathed in varying levels of sunlight, but only on one do we know of a phenomenon that does more than just passively receive the warmth of the Sun. Here on Earth, we actually feed on starlight. The Sun is the source of energy for almost all life on Earth; every plant, algae and many species of bacteria rely on the process of photosynthesis to create their own food using the power of the Sun. This in turn creates the foundations for the complex web of life here on Earth; not only does the process of photosynthesis maintain the normal level of oxygen in the atmosphere, but it is also the basis on which almost all life depends for its source of energy.

      We are only just beginning to understand the complex mechanisms by which plants capture sunlight; some of this explanation may take us off into the quantum world, but at its most basic chemical level photosynthesis is a simple process. Inside every leaf are millions of organelles called chloroplasts, and it’s these little units that do something magical when they capture a photon that has taken the eight-minute, 150-million-kilometre (93,000-million-mile) journey from the Sun. The chloroplasts take in carbon dioxide and water, and by capturing the energy from a sunbeam they convert this into oxygen and complex sugars. It’s these complex sugars, or carbohydrates, that are the basis of all the food we eat – whether directly through the consumption of photosynthetic plants or indirectly through the consumption of animals that feed on them. The amount of energy trapped by photosynthesis is immense – around 100 terawatt-years, which is six times larger than the power consumption of human civilisation.

      This intimate link between our planet and the Sun is all around us. Yet although we are surrounded by vast swathes of wonderful green machines that are all feeding on the Sun, the leaves and plants that cover so much of the planet do not just rely on any sunlight. In fact, plants are fussy eaters and have evolved to use just a fraction of the sunlight that makes its way through Earth’s atmosphere.

      On the surface of Earth sunlight may appear white, but when you pass it through a prism, you can see that it is made up of all the colours of the rainbow. Different wavelengths of light have different colours – from the blues with the shortest wavelength to СКАЧАТЬ