The Sun ejects a continuous flow of electrically charged particles and magnetic fields in the form of the solar wind. One of the long-standing puzzles of solar physics is that the solar wind is hotter than it should be. However, a new study of data obtained by ESA's Cluster spacecraft may help to explain the mystery.

Earth and all of the other planets of our Solar System are like aircraft travelling through a non-stop gale of plasma, mainly protons and electrons, which originates at the Sun. This so-called solar wind begins in the Sun's searingly hot lower atmosphere and is blasted outward in all directions at an average speed of about 400 km/s (almost 1.5 million km per hour).

The outflowing plasma 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, but the amount of cooling is much less than would be expected in a constant, smooth flow of solar particles. Instead, spacecraft measurements have shown that the solar wind is hotter than it should be, especially inside the orbit of Jupiter.

How is it heated on its odyssey across the Solar System, especially where space is largely empty and collisions between particles are rare? One process favoured by heliophysicists is the dissipation of turbulence in the plasma, which arises from irregularities in the flow of particles and magnetic fields. This turbulence stretches and bends magnetic field lines, often resulting in the formation of current sheets - thin sheets of electrical current which separate regions of oppositely directed magnetic field.

Such sheets are more or less two-dimensional, rather than being spread throughout a large volume of space. They are also sites where the magnetic field lines frequently reconnect and break, resulting in the transfer of energy and particle heating.

Until now, the precise mechanism and scale of this magnetic reconnection process have remained uncertain. However, in a previous study involving Cluster, plasma turbulence was observed in the magnetosheath, the region between Earth's bow shock, where the solar wind meets the magnetic field of the Earth, and the magnetosphere - the magnetic bubble which surrounds it. In addition, there is considerable evidence for reconnection at relatively large scales in the solar wind.

Furthermore, the process of magnetic reconnection in the magnetosheath was detected at the border of the turbulent eddies. Here, the reconnection occurred in electrical current sheets, no more than 100 km across, hinting at a possible mechanism that might also occur at smaller scales in the solar wind.

Now, using two separate sets of data sent back by the Cluster spacecraft, an international team of scientists has been able to probe this turbulence in more detail and at smaller scales than ever before observed.

Their study made use of the high time resolution of the Spatio Temporal Analysis Field Fluctuation (STAFF) magnetometer, which is carried on each of the four Cluster spacecraft. STAFF is capable of detecting rapid variations in magnetic fields, which means that very small spatial structures can be recognised within the plasma.

Writing in a recent paper in Physical Review Letters, the team examined two sets of STAFF observations. The first data were obtained on 10 January 2004, when two Cluster spacecraft (C2 and C4) were flying only 20 km apart in the Sun-Earth direction, while the two other spacecraft were much further away. At that time, STAFF was operating in rapid 'burst' mode, during which it recorded 450 measurements of the magnetic field per second. Additional data were obtained by a single spacecraft (Cluster 2) on 19 March 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 Universita della Calabria, Italy, lead author of the paper.

"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. Since the current sheet event lasted only 0.07 seconds for both satellites, this corresponded to a spatial size of about 38 km."

"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."

"The data show that the plasma turbulence is virtually two-dimensional and consists of thin current sheets that lie perpendicular to the plane of the average magnetic field of the solar wind," said Melvyn Goldstein of NASA's Goddard Space Flight Center, a co-author of the paper and the NASA Project Scientist for Cluster.

"For the first time, we were able to obtain direct evidence for the existence of current sheets at very small scales, where heat dissipation might occur as a result of magnetic reconnection."

"By feeding the data into a computer simulation, we were able to visualise the Cluster observations. The result was a new, striking, 2-D view of the solar wind turbulence, with thin current sheets located at the edges of turbulent swirls some 20 km across."

"This shows for the first time that the solar wind plasma is extremely structured at these very small scales," said Silvia Perri. "Although we haven't yet detected reconnection occurring at these sites, it is clear that we are seeing a release of energy approaching smaller and smaller scales, which may contribute to the overall heating of the solar wind."

"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," said Matt Taylor, ESA's Project Scientist for Cluster.

"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."

The study described in this article is based on "Detection of Small-Scale Structures in the Dissipation Regime of Solar-Wind Turbulence" by S. Perri, M. L. Goldstein, J. C. Dorelli, and F. Sahraoui, published in Phys. Rev. Lett. 109, 191101 (2012); doi:10.1103/PhysRevLett.109.191101