Cluster Mission Indicates Turbulent Eddies May Warm the Solar Wind |
The sun ejects a continuous flow of electrically charged particles and magnetic fields in the form of the solar wind — and this wind is hotter than it should be. A new study of data obtained by European Space Agency’s Cluster spacecraft may help explain the mystery.
The solar wind is made of an electrically-charged gas called plasma. One theory about the wind’s puzzling high temperatures is that irregularities in the flow of charged particles and magnetic fields in the plasma create turbulence, which, in turn, dissipates and adds heat to its surroundings. Using two separate sets of data sent back by Cluster, an international team of scientists has probed the spatial characteristics of this turbulence in more detail and at smaller scales than ever before. They saw evidence that the turbulence evolved to form very small “current sheets” — thin sheets of electrical current that separate regions of rotated magnetic field.
"For the first time, we were able to obtain direct evidence for the existence of current sheets at these very small scales, where dissipation of magnetic energy into heat is thought to occur," said Melvyn Goldstein, project scientist for Cluster at NASA’s Goddard Space Flight Center in Greenbelt, Md. Goldstein is a co-author of a paper on these results that appeared in the Nov. 9, 2012, issue of Physical Review Letters.
This solar wind is a non-stop gale of plasma, mainly protons and electrons, which originates in the sun’s searingly hot lower atmosphere. It blasts outward in all directions at an average speed of about 250 miles per second. The outflow is so energetic that it pulls along the sun’s magnetic field. The solar wind travels across the entire solar system, until it reaches the boundary with interstellar space. The plasma cools as it expands during its outward journey. However, the amount of cooling is much less than would be expected in a constant, smooth flow of solar particles since the density is so low that the particles cannot be receiving extra heat from the most common method on Earth: collisions. continue reading