March 27, 2009
The four satellites of ESA's Cluster mission have revealed how turbulence develops in space just outside Earth's magnetic environment. This result improves the understanding of turbulence, a key physical process by which energy throughout the universe is transported from large scales at which it is input to small scales where it is dissipated.
Just as the turbulence experienced by aircraft is caused by high-speed winds, turbulence in space can be caused by the solar wind, an uninterrupted flow of high-speed solar particles. Thanks to Earth's magnetic field, most of this solar material is deflected around the Earth's magnetosphere (blue region in Figure 1), delimited by the magnetopause.
The solar wind, when it reaches a magnetized planet, is first decelerated from supersonic to subsonic speed by a shock wave (called the bow shock), located in front of the magnetopause. The region between the bow shock and the magnetopause is called the magnetosheath. This region is one of the most turbulent environments of near-Earth space, making it an excellent laboratory in which to study turbulence. Other environments such as the solar atmosphere, or accretion disks around young stars, are likely to exhibit similar behavior, but the terrestrial magnetosheath is by far the most accessible place to perform measurements. Characterizing the properties of the magnetic turbulence in this region is of prime importance to understand its role in fundamental processes such as energy dissipation or the acceleration of particles to high-energies.
In March 2007, Nature Physics publication reported for the first time that not only did turbulence exist in the magnetosheath in the form of many small magnetic islands - with a typical size of 60 miles (100 kilometers), but also that magnetic reconnection occurred within these islands. The observations showed that the turbulent plasma is accelerated and heated during the reconnection process. The discovery of reconnection in turbulent plasma has significant implications for the study of laboratory and astrophysical plasmas, where both turbulence and reconnection naturally develop and thus where turbulent reconnection is very likely to occur. Possible applications range from the dissipation of the magnetic energy in fusion devices on Earth, such as the International Thermonuclear Experimental Reactor (ITER) project to the acceleration of high-energy particles in solar flares. This discovery was based on data collected by the four Cluster spacecraft when they were flying together in formation with a very short separation distance - about 60 miles (100 kilometers).
A more recent study, published May 23, 2008, again made use of data collected by the four Cluster satellites, but this time it was with an inter-spacecraft separation of several thousands of miles. This configuration enabled the study of magnetosheath turbulence at large scales. Magnetic field data collected April 16, 2003, by the four satellites.
At this time the Cluster satellites were widely spread in space, from just behind the bow shock (C3 and C4) to more than 5,000 miles away (C1), while C2 was located in between. One property of the turbulence, the intermittency, was studied in particular and was accurately reproduced with a theoretical model. The intermittency appears in the transfer (or the cascade) of the energy from large scales to small scales as an uneven distribution of the energy between the scales, and this was used to examine the evolution of the turbulence in the magnetosheath.
"For the first time, we use multi-spacecraft observations to characterize the evolution of magnetosheath turbulence," said Emiliya Yordanova, lead author of this study. "The intermittency significantly changes over the distance, being increasingly stronger away from the bow shock."
"Magnetic reconnection, turbulence, and shocks are three fundamental ingredients of the plasma universe," said Matt Taylor, acting ESA Cluster project scientist. "The detailed understanding of these key processes and their associated multi-scale physics is a challenge for the future of space physics. One of the lessons learned from the Cluster mission is the need for new space missions equipped with instruments of higher sensitivity and better time resolution along with a larger number of satellites to sample different scale sizes simultaneously. Such a mission concept exists - a fleet of 12 satellites named Cross-Scale. This is a candidate mission within the ESA Cosmic Vision program."