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The Cluster Space Mission (ESA/NASA) discovers how the solar wind is heated

Cluster Space Mission

The sun, like the other stars in the universe, constantly gives out a flow of charged particles (electrons and protons) called the solar wind. Once emitted, this solar wind crosses space at considerable speed, varying between 400 and 800 km/s, and interacts with the various objects of the solar system: planets, comets, etc. For magnetized planets such as Earth or Jupiter, these interactions are the cause of dynamic phenomena as violent as the magnetic storms that produce the aurora borealis and australis at the North and South poles. Therefore, understanding the physical properties “upwind” of the Earth is crucial, in order better to apprehend the different physical processes produced by these long-distance interactions with the Sun.

The Solar wind is also the object of significant physical processes such as turbulence and particle acceleration. These universal phenomena also occur in so-called magnetic fusion plasmas or in far-off galaxies. Thus, the solar wind represents a huge laboratory, accessible for in situ measurements. It has been the object of several space exploration missions such as Voyager 1 & 2, Ulysses and Wind, with the future project Solar Orbiter in development.

Using the data from the Cluster probes, a team of research scientists from the Plasma Physics laboratoryNouvelle fenêtre (CNRS - UPMC, ParisTech, Paris-Sud), NASANouvelle fenêtre and the Swedish Institute of Space PhysicsNouvelle fenêtre has just made a major breakthrough in the understanding of the phenomenon of turbulence. Thanks in particular to very high resolution data from the alternative magnetometer designed and manufactured at the Plasma Physics laboratory and embarked on Cluster, the scientists were able for the first time to “follow” the transfer of energy from large scales (105 km) to small scales (10 km). This questions the belief that a very large part of the energy from turbulence dissipates at the scale of protons (~100 km). They have shown that energy continues its cascade to smaller scales, protons gaining only a fraction of this energy and therefore heating only partially. They were also able, for the first time, to localize clearly the real scale of dissipation: the electronic scale.

This new phenomenon, which enables the transfer of large quantities of energy from large to small electronic scales where it dissipates, explains several observations of electron acceleration in astrophysics. The study may have consequences for the classic models of coronal heating and for the theoretical modeling of magnetic reconnection.


These results were published in the June 10 edition of Physical Review LettersNouvelle fenêtre (102, 231102 (2009), “Evidence of a Cascade and Dissipation of Solar-Wind Turbulence at the Electron Gyroscale” by F. SahraouiNouvelle fenêtre, M. L. GoldsteinNouvelle fenêtre, P. RobertNouvelle fenêtre, Yu. V. KhotyaintsevNouvelle fenêtre.