The coronal heating problem has been established since the late 1930s, when the Swedish spectroscopist Bengt Edlén and the German astrophysicist Walter Grotrian first observed phenomena in the Sun’s corona that could only be present if its temperature was a few million degrees celsius. Our recent study has finally achieved this, validating Alfvén’s 80 year-old theory and taking us a step closer to harnessing this high-energy phenomenon here on Earth. The theory had been tentatively accepted – but we still needed proof, in the form of empirical observation, that these waves existed. He theorised that magnetised waves of plasma could carry huge amounts of energy along the Sun’s magnetic field from its interior to the corona, bypassing the photosphere before exploding with heat in the Sun’s upper atmosphere. In 1942, the Swedish scientist Hannes Alfvén proposed an explanation. This spike in temperature, despite the increased distance from the Sun’s main energy source, has been observed in most stars, and represents a fundamental puzzle that astrophysicists have mulled over for decades. But a few thousand kilometres above it – a small distance when we consider the size of the Sun – the solar atmosphere, also called the corona, is hundreds of times hotter, reaching a million degrees celsius or higher. With direct access to the sun's atmosphere, Solar Probe Plus will provide unprecedented information on how the solar corona is heated and revolutionize our knowledge of the origin and evolution of the solar wind.The visible surface of the Sun, or the photosphere, is around 6,000☌. "The mission will directly measure the density, velocity and magnetic field of the solar material there, allowing us to understand how motion and heat in the corona and solar wind are generated." "This research provides confidence that Solar Probe Plus, as designed, will be exploring the inner solar magnetic system," said Marco Velli, a Solar Probe Plus scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. Scientists knew the mission would be gathering information closer to the sun than ever before, but couldn't be sure it would travel through the corona proper. Realizing that the corona extends much further than previously thought has important consequences for NASA's Solar Probe Plus because the mission will travel to within 4 million miles of the sun. Beyond that boundary, however, solar material streams away in a steady flow called the solar wind - out there, the material has separated from the star and its movement can't affect the corona. That is to say that even out to 5 million miles from the sun, giant solar storms or coronal mass ejections can create ripple effects felt through the corona. Tracking magnetosonic waves showed DeForest and his team that the material throughout this extended space remained connected to the solar material much further in. Unlike sound waves on Earth, which oscillate several hundred times per second, these waves oscillate about once every four hours - and are about 10 times the length of Earth. The researchers studied waves known as magnetosonic waves, and they are a hybrid of sound waves and magnetic waves called Alfven waves. The results were published in The Astrophysical Journal on May 12, 2014. "We can't hear the sounds directly through the vacuum of space, but with careful analysis we can see them rippling through the corona." "We've tracked sound-like waves through the outer corona and used these to map the atmosphere," said Craig DeForest of the Southwest Research Institute in Boulder, Colorado. Combined with measurements from Voyager 1 of the outer boundary of the heliosphere, we have now defined the extent of this entire local bubble. These STEREO observations provide the first direct measurements of the inner boundary of the heliosphere - the giant bubble sparsely filled with solar particles that surrounds the sun and all the planets. This information has implications for NASA's upcoming Solar Probe Plus mission, due to launch in 2018 and go closer to the sun than any man-made technology ever has before. Now, using NASA's Solar Terrestrial Relations Observatory, scientists have found that this atmosphere, called the corona, is even larger than thought, extending out some 5 million miles above the sun's surface - the equivalent of 12 solar radii. Surrounding the sun is a vast atmosphere of solar particles, through which magnetic fields swarm, solar flares erupt, and gigantic columns of material rise, fall and jostle each other around.
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