Solar Winds May Be Warmed By Turbulent Plasma
December 19, 2012

ESA Study Says Solar Winds May Be Warmed By Turbulent Plasma

Watch the video "Simulated Vision of Solar Wind Turbulence"

April Flowers for - Your Universe Online

Scientists have zoomed in on solar wind to reveal the finest detail yet using the European Space Agency's (ESA) Cluster quartet of satellites as a space plasma microscope. They found tiny turbulent swirls that could play a big role in heating the winds. The fact that these winds stay hotter than they current models predict has been a long-standing puzzle in solar physics — one that the new study may help to resolve.

Found all around us in water flowing from a tap, around an aircraft wing, in experimental fusion reactors on Earth and in space, turbulence is highly complex physical phenomenon. In the solar winds — streams of charged particles emitted by the Sun — turbulence is thought to play a key part in maintaining the wind's heat as it streams away, racing across the Solar System.

The wind originates in the Sun's searingly hot lower atmosphere, blasting outward in all directions at an average speed of 250 miles per second. This energetic burst of plasma is pulled along the Sun's magnetic field, traveling across the entire solar system before reaching the boundary with interstellar space. The solar wind cools down as it expands, but to a much smaller extent — especially inside the orbit of Jupiter — than would be expected in a smooth flow without turbulence.

Irregularities in the flow of particles and magnetic field lines create turbulence that stretches and bends those magnetic field lines. However, understanding how the energy is transferred from the large scales at its point of origin, to the small scales at the point of dissipation, is likened to attempting to trace energy as it is transferred from the smooth, laminar flow of a river down to the small eddies formed at the bottom of a waterfall.

Two of the four Cluster satellites (C2 and C4) made extremely detailed observations of plasma turbulence in the solar wind on January 10, 2004. The satellites were separated by just over 12 miles along the direction of plasma flow. The spacecraft operated in "burst" mode to take 450 measurements per second. These readings were taken using the Spatio Temporal Analysis Field Fluctuation (STAFF) instrument that is carried on each Cluster spacecraft. Capable of detecting rapid variations in the magnetic fields, STAFF can recognize very small spatial structures in the plasma. Additional data was obtained by Cluster 2 on March 19, 2006.

"During the 2004 observation, both spacecraft were so close that they observed almost simultaneously the same 'quasi-stationary', rotating structure in the solar wind as it passed them by," said Silvia Perri of the Università della Calabria, Italy.

"The magnetic field data showed the typical signature of a current sheet crossing. At that time, the solar wind was flowing at about 550 km/s [340 m/s]. Since the current sheet event lasted only 0.07 seconds for both satellites, this corresponded to a spatial size of about 38 km [23.5 m]."

"During the second event, the four Cluster spacecraft were also in the solar wind, but they were too far apart to make a two- or three-dimensional study of the plasma flow. However, the STAFF instrument on Cluster 2 obtained a one-dimensional snapshot of the fluctuations in the turbulent flow and found evidence of a discontinuity in the solar wind which was similar to the previous event."

Scientists confirmed the existence of sheets of electric current just over 12 miles across on the borders of turbulent swirls by comparing the results of the observations with computer simulations. Current sheets are more or less two-dimensional rather than spread throughout a large volume of space.

“This shows for the first time that the solar wind plasma is extremely structured at this high resolution,” says Perri. The findings of this study were recently published in Physical Review Letters.

The Cluster quartet previously detected current sheets on much larger scales of up to 62 miles. These larger sheets were observed in the magnetosheath, the region between Earth's magnetic bubble — the magnetosphere — and the bow shock that is created as the magnetosphere meets the solar wind.

The process of "magnetic reconnection" was detected at the borders of these turbulent eddies. In this process, oppositely directed field lines spontaneously break and reconnect with other field lines, releasing their energy.

Until this study, the precise mechanism and scale of the magnetic reconnection process has remained uncertain. However, there is considerable evidence for reconnection at relatively large scales in the solar wind.

“Although we haven´t yet detected reconnection occurring at these new, smaller scales, it is clear that we are seeing a cascade of energy which may contribute to the overall heating of the solar wind,” said Dr Perri.

Determining whether similar processes are also in play closer to the Sun will be the job of future missions such as ESA's Solar Orbiter and NASA's Solar Probe Plus. NASA´s Magnetospheric Multiscale mission will specifically probe the small-scale regions where reconnection can occur.

“This Cluster result demonstrates the mission´s unique capability to probe universal physical phenomena, in this case pushing the mission´s instrument measurement capabilities to their limit to unlock features at small scales,” explained Matt Taylor, ESA´s Cluster Project Scientist.

“Future multi-spacecraft missions will make very detailed studies of these small-scale plasma phenomena and provide further context to our Cluster measurements. “¦ Cluster is primarily designed to explore Earth's magnetosphere, but the instruments on board each spacecraft are also able to provide important insights into the nature of the solar wind."

"This study shows how data from multiple satellites flying in close formation can contribute to our understanding of spatial variations in the solar wind and elsewhere."