August 19, 2012
Unexplained Solar Wind Heating Possibly Caused By Magnetic Turbulence
redOrbit Staff & Wire Reports - Your Universe Online
Magnetic turbulence is most likely the reason that solar wind moving away from our sun and our solar system is hotter than it theoretically should be, according to new research from scientists at the University of Warwick.
As solar wind leaves the sun and expands beyond the solar system, it should begin to cool off due to the lack of particle collisions to dissipate energy, the university explained in a Friday press release. That isn't actually the case, though, as the solar wind is actually hotter than experts believe it should be, and that phenomenon has stumped researchers for years.
In a pair of papers recently published in the journal Physical Review Letters, physicist and lead author Dr. Kareem Osman believes that he has uncovered the answer to the conundrum: magnetic turbulence, which is found in stars, stellar winds, galaxies, and other cosmic entities throughout the universe. Understanding this turbulence is vital to interpreting astrophysical observations, the institute explained, and the solar wind and near-Earth environment offered them an opportunity to examine the relationship between them.
"The solar wind is much hotter than would be expected if it were just expanding outward from the Sun. Turbulence is the likely source of this heating," the university said. "For neutral fluids such as fast flowing water, energy dissipation occurs through many microscopic collisions. As is the case for many astrophysical plasmas, the near-Earth solar wind is thin and spread out, which means collisions between particles are rare to the point that the plasma is considered collisionless. A major outstanding problem is how, in the absence of those collisions, does plasma turbulence move energy to small scales to heat the solar wind."
"Turbulence stretches and bends magnetic field lines, and often two oppositely directed field lines can come together to form a current sheet," Dr. Osman added. "These current sheets, which are distributed randomly in space, could be sites where the magnetic field snaps and reconnects transferring energy to particle heating. There are also many more ways that current sheets can heat and accelerate the plasma."
In order to determine how proton temperature correlated with the current sheet strength, Osman and colleagues set thresholds in the strength of those current sheets. Their research, which was funded by the Science and Technology Facilities Council (STFC), "convincingly" demonstrates that there is a relationship between the sheet strength and the temperature, and that the strongest bonds are also the hottest.
"While each current sheet does not provide a lot of heating, collectively the current sheets account for 50% of the solar wind internal energy despite only representing 19% of all the solar wind data," the university revealed. "Even more striking, the strongest current sheets which only make up 2% of the solar wind were found to be responsible for 11% of the internal energy of the system."
"The researchers also found that current sheets heat the solar wind in a very interesting manner; the heating is not equal in all directions," they added. "This temperature anisotropy can drive plasma instabilities and the strongest current sheets where preferentially found in plasma that is unstable to particular types of these instabilities called 'firehose' and 'mirror'."