Tripping the light fantastic to the surface of the Sun
Monday 08 September 1997
There is a reason for their apparently obscure aim. After a year of careful observation, using the orbiting Soho (Solar and Heliospheric Observatory) spacecraft, they have discovered that the Sun's surface is a churning mass with its own "rivers" of superheated electrically charged gases, or plasma, flowing at a temperature of 5,800C.
The swirling movement of these streams causes sunspots - cooler areas (just 3,800C) on the surface, which show up as dark patches - and solar storms. And these affect the weather, and even communications, on Earth.
Although the Sun's surface temperature seems unimaginable, the temperature is hundreds of times greater at its core, where the energy that powers the star (and in turn, warms the Earth) is produced. There, the immense gravitational forces generated by the mass of the Sun crush together single protons, each one originally the nucleus of an interstellar hydrogen atom, to form a helium nucleus.
Surplus energy is thrown off and eventually reaches us as sunlight. Millions of tons of hydrogen are consumed every second in this process - although the Sun is expected to burn for another 5 billion years or so, being about halfway through its life.
But the light particles (or photons) generated in the fusion process do not stream directly from the heart of the Sun to its surface and then out into space. The core is so dense that the photons must take an atomic Dodgem ride to the surface, bouncing off the atoms in their way as they rush outwards.
Scientists have calculated that it can take a single photon several years to reach the surface of the Sun. From there, however, it enjoys an uninterrupted journey outwards. The tiny proportion which reaches us takes just eight minutes to travel from the Sun's surface to the Earth, 93 million miles (149 kilometres) away.
Long-term variations in the Earth's temperature may be linked to sunspots, while solar storms, which can throw out flares of plasma millions of kilometres into space, can cause radio interference, damage telecommunications satellites and even knock out power stations.
The new data emerged from observations carried out jointly by the European Space Agency (ESA) and Nasa, the United States space agency.
"We have detected motion similar to the weather patterns in the Earth's atmosphere," said Jesper Schou, of Stanford University, California.
The joint European and US team also discovered that the surface of the Sun is slowly moving: the outer layer, to a depth of about 15,000 miles, is flowing at about 50 miles per hour from the Sun's equator to its poles. On that basis, it would take almost two years for any area of plasma to journey from the equator to the poles.
Studying the patterns might make it possible to predict them - giving valuable warnings about looming solar changes.
The observations were carried out by Soho, which is studying the Sun from a spacecraft about 1 million miles from Earth. On board Soho is an instrument which can effectively measure sound waves inside the Sun. "These techniques allow us to peer inside it, much as a doctor can look inside a pregnant woman using ultrasound," Professor Schou said.
The team was astonished to find a complex pattern of streams and currents under the surface. "What we have here is an inroad into understanding the solar cycle, the 11-year cycle of sunspots that has been puzzling us for centuries," said Craig DeForest, of Stanford University.
One stream circles the poles, while a series of others migrates towards the solar equator. By terrestrial standards they are huge: "You can fit almost 100 Earths inside this jetstream," Professor Schou said.
The belts also rub against slower-moving plasma inside the Sun. "That's where the sunspots form," said Professor Douglas Gough, of Cambridge University. The same processes could also underlie solar flares and storms. Last week a flare shot out of the Sun - although scientists say there is no risk to the Earth.
Professor Gough said the streams, which generate huge magnetic and electrical fields, create opposing forces which eventually must find an escape. "Think of them like elastic bands," he said. The bands are twisted and pulled by the motion: "then it slips. It either snaps, or it contracts and shoots out material like a slingshot".
The scientists compared the solar "rivers" to atmospheric currents on Earth. "We are just beginning to understand how the Earth's atmosphere operates," Professor Gough said. "Now we are getting tremendous and interesting details from the Sun."
Professor DeForest said that knowing this could help scientists to predict sunspot activity. "We can predict where on the Sun these things arise."
He compared it to meteorology - which allows prediction of general weather patterns, but not of localised events. In the same way, the Soho findings will make it easier to predict trends, but it will not be possible to predict where an individual sunspot will arise - "just like it's not easy to predict where a thunderstorm will break out".
However, knowing the right area might add a few days to the warnings that are now given when a solar storm is coming. And that, in turn, could mean an easier time back on Earth - even if it is raining.
Heat and dust: A solar flare pushes out from the surface of the Sun, as recorded by Soho, the European/United States Solar and Heliospheric Observatory, 1 million miles from Earth
Bottom left, solar rotation rate with depth: The (false) colours represent speed; red material is rotating the fastest and dark blue the slowest. The left side of the figure shows rotation speed at the surface of the Sun. Red material at the equator is moving approximately 3,000 miles per hour faster than the blue material at the poles. The cutaway reveals rotation speed inside the Sun. The large dark-red bank is a massive fast flow of hot, electrically charged gas called plasma, beneath the solar equator
Bottom centre, variations in solar motion: This image represents the difference in speeds of areas on the surface and in the interior of the Sun. Red and yellow are faster than average, and blue is slower. The cutaway reveals speed variations in the interior. The red ovals embedded in the green areas at the poles are the newly discovered polar plasma "jet streams", each large enough to engulf two Earths
Bottom right, polar flows: The flow lines, showing the surface flow from the equator to the poles, are set over an image of the rotation speed at the surface. The cutaway represents the observed polar flow 15,000 miles beneath the surface and a hypothetical return flow 120,000 miles under the surface Photographs: Nasa
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