Magnetic turbulence trumps collisions to heat solar wind

Aug 17, 2012

(Phys.org) -- New research, led by University of Warwick physicist Dr Kareem Osman, has provided significant insight into how the solar wind heats up when it should not. The solar wind rushes outwards from the raging inferno that is our Sun, but from then on the wind should only get cooler as it expands beyond our solar system since there are no particle collisions to dissipate energy. However, the solar wind is surprisingly hotter than it should be, which has puzzled scientists for decades. Two new research papers led by Dr Osman may have solved that puzzle.

Turbulence pervades the universe, being found in stars, , , galaxies, and even the material between galaxies. It also plays a critical role in the evolution of many laboratory plasmas, causing diminished confinement times in fusion devices. Therefore, understanding plasma turbulence is essential to the interpretation of a large body of laboratory, space, and astrophysical observations. The solar wind and near-Earth environment provide an excellent laboratory for the study of turbulence, and are the only in-situ accessible astrophysical plasmas.

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. For neutral fluids such as fast flowing water, 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.

The new research led by Dr. Kareem Osman at the University of Warwick's Centre for Fusion, Space and Astrophysics has revealed how turbulence heats the solar wind. He says:

"Turbulence stretches and bends , and often two oppositely directed field lines can come together to form a current sheet. 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."

The researchers set thresholds in the strength of these current sheets, to determine how proton temperature was related to current sheet strength. The results show convincingly that these current sheets are associated with temperature enhancements, and that the strongest 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. 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 in a very interesting manner; the heating is not equal in all directions.

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'.

Explore further: Orion on track at T MINUS 1 Week to first blastoff

More information: K.T. Osman, W.H. Matthaeus, M. Wan, and A.F. Rappazzo, Intermittency and Local Heating in the Solar Wind, Phys. Rev. Lett. 108, 261102 (2012).

K.T. Osman, W.H. Matthaeus, B. Hnat, and S.C. Chapman, Kinetic Signatures and Intermittent Turbulence in the Solar Wind Plasma, Phys. Rev. Lett. 108, 261103 (2012).

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User comments : 4

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cantdrive85
1 / 5 (7) Aug 17, 2012
"Magnetic reconnection is pseudo-science" Nobel Laureate Hannes Alfven.

There is no hope for any legitimate growth in our knowledge of the Cosmos when the scientists leading the way don't even understand basic plasma physics!
HannesAlfven
2.3 / 5 (6) Aug 17, 2012
Re: "There are also many more ways that current sheets can heat and accelerate the plasma"

If we weren't only proposing inferences for the solar wind's acceleration and heating which confirmed the dominance of gravity to the solar system, a legitimate question would be: Can an electric field centered at the Sun also heat the solar wind, in addition to accelerating it? After all, the way in which we accelerate charged particles in the laboratory is by applying an electric field. But, the existence of an electric field at the Sun would suggest a larger galactic electro-DYNAMIC circuit maintaining that charge imbalance (plasma physics is not the study of electrostatics).

Given that NASA is already at the point of asking if an electrical circuit involving the thermosphere and ionosphere might be responsible for lightning, shouldn't it also be fair game to wonder if the Sun is also part of a larger electrical circuit? An electron drift would be incredibly hard to see unless we looked.
GSwift7
5 / 5 (1) Aug 17, 2012
An electron drift would be incredibly hard to see unless we looked.


But wouldn't the potential required to generate a current over such a long distance be enormous? What does the math say in regard to constraining the voltage drop?

Doesn't that also imply that certain phenomenae we observe as being omnidirectional should be polarized in stead?
barakn
5 / 5 (1) Sep 08, 2012
More importantly, what is the energy source that replenishes this electric field, considering that the solar wind it drives and heats should deplete its strength? Surely it would weaken over time, and unless we accept that the original field strength had an impossibly high starting value, this would imply the Sun is not nearly as old as mainstream science suggests. Wouldn't it be funny if the Electric Universe loonies turned out to be closet Young Earth Creationists?

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