Cluster measures turbulence in Earth’s magnetic environment

Cluster data has found the key to address a major, long unsolved question in plasma physics.
Cluster © ESA - CC BY-SA 3.0 IGO

Scientists have estimated how much energy is transferred from large to small scales within the magnetosheath, the boundary region between the solar wind and the magnetic bubble that protects our planet.

Based on data collected by the European Space Agency’s (ESA) Cluster and NASA’s THEMIS missions over several years, the study revealed that turbulence is the key to the transfer of energy from large to small scales, making this process a hundred times more efficient than in the solar wind.

The planets in the Solar System, including Earth, are bathed in the solar wind, a supersonic flow of highly energetic charged particles released by the Sun. Our planet and a few others stand out in this stream of particles: these are the planets that have a magnetic field of their own, representing an obstacle to the solar winds.

The interaction between Earth’s magnetic field and the solar wind create an intricate structure; the magnetosphere. This is a protective bubble that shields the planet from the vast majority of solar winds.

Until now, scientist have achieved a fairly good understanding of the physical processes that take place in the solar wind plasma and in the magnetosphere. However, there are some aspects that are still missing regarding the relationship between these two environments.

Lina Zafer Hadid, from the Swedish Institute of Space Physics in Uppsala, Sweden said: “To learn how energy is transferred from the solar wind to the magnetosphere, we need to understand what goes on in the magnetosheath, the ‘grey area’ between them.”

Fouad Sahraoui from the Laboratory of Plasma Physics in France said: “In the solar wind, we know that turbulence contributes to the dissipation of energy from large scales of hundreds of thousands of kilometres to [the] smaller scale of a kilometre, where plasma particles are heated up and accelerated to higher energies.”

“We suspected that a similar mechanism must be at play in the magnetosheath too, but we could never test it until now,” he added.

The magnetosheath plasma is more turbulent, home to a greater extent of density fluctuations and can be compressed to a much higher degree than the solar wind. It is substantially more complex, and scientists have only in recent years developed the theoretical framework required to study the physical processes taking place in such an environment.

In the new study, the researchers looked through a vast volume of data collected between 2007 and 2011 by the four spacecraft of ESA’s Cluster and two of the five spacecraft of NASA’s THEMIS missions, which fly through Earth’s magnetic environment.

Lina explains: “We found that density and magnetic fluctuations caused by turbulence within the magnetosheath amplify the rate at which energy cascades from large to small scales by at least a hundred times with respect to what is observed in the solar wind,”

Philippe Escoubet, Cluster Project Scientist at ESA, said: “It is very exciting that a study based on several years of Cluster data has found the key to address a major, long unsolved question in plasma physics.”

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