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Accueil > A propos du LPP > Communication > Actualités archivées > 2009 > The ESA/NASA Cluster Mission Discovers how the Solar Wind is Heated

The ESA/NASA Cluster Mission Discovers how the Solar Wind is Heated

The solar wind is a permanent flow of charged particles (mostly electrons and protons) emitted by the Sun. The wind travels in the solar system at speeds varying from 400 km/s to 800 km/s and interacts with different solar system objects, such as planets and comets. In the case of magnetized planets (e.g., Earth or Jupiter), this interaction leads to highly energetic phenomena with dramatic effects on the environment of Earth [see, Figure 1] and other planets. The most widely experienced on Earth is the magnetic substorm with the beautiful displays of boreal and austral aurora.

Figure 1 : Sketch of the Sun-Magnetized planets interaction via the emitted solar wind particles.

Measurements of the temperature of the solar wind from 0.3 Astronomical Units (AU) to more than 10 AU (1 AU=150,000,000 km is the average distance between the Sun and Earth) have shown clearly that heat is continuously being added to the wind. It has also been known for decades that the solar wind is turbulent. In turbulent fluids, as large regions with one velocity encounter other regions with different velocities, the large regions produce smaller regions, which in turn produce yet smaller regions (this process is called a cascade)— think of waves in the ocean, water falls or cream being stirred in a coffee cup — until, at the smallest scales the motions finally dissipate and the energy goes into heat.

In “normal” fluids such as water and coffee, the smallest scales are determined by how viscous the fluid is — the less viscous, the smaller the scales. The amount of heat that can be produced by dissipating turbulence in the solar wind is about what is needed to explain the slow decrease of temperature with distance from the Sun. Even with turbulent heating, the temperature will decrease because of the spherical expansion of the solar wind. The solar wind, however, unlike water or coffee, consists of a fully ionized gas (of protons and electron) governed by electric fields and magnetic fields that originate at the Sun. The density is so low that normal fluid viscosity plays no role in dissipating turbulence. The magnetic and electric fields fluctuate in time and space and originate in the lower solar atmosphere or as a result of fast streams of gas slamming into slower speed streams in a way that mixes up the turbulence and produces a cascade of fluctuations from large scales to smaller and smaller scales. The combination of the fluctuating magnetic field and the charged particle solar wind gas leads to a complex mix that can interact in a multiplicity of ways. Although there are many theories as to how the waves are dissipated, until these observations it had not been possible to observe the dissipation directly.
In fact, even from a theoretical point of view, how such magnetic turbulence dissipates is unknown. Solving the problem is of fundamental importance to our understanding of how, for example, the solar corona is heated. Now observations made by experiments on the joint ESA/NASA Cluster mission have discovered how such turbulence is dissipated.
The Cluster mission consists of four identical spacecraft that were launched on two Soyouz rockets in July and August of 2000. The spacecraft contain a complete set of instruments designed to measure charged particles and waves. One instrument, STAFF (Spatio-Temporal Analysis of Field Fluctuation), is designed to measure high frequency magnetic waves. Using data from STAFF, the low frequency (fluxgate) magnetometer (FGM), and the electric fields and wave experiment (EFW), Dr. Fouad Sahraoui, a NASA Fellow (and a visitor from CNRS/LPP, France) and his collaborators, Dr. Melvyn Goldstein (NASA), P. Robert (Laboratoire de Physique des Plasmas, CNRS/LPP, France) and Dr. Yu. Khotyaintsev (Institute of Space Physics, Sweden) have discovered that solar wind turbulence extends down to scale sizes at which the electrons stop gyrating around the magnetic field. By “following” the energy cascade from large scales (105 km) down to the small scales ( 2 km), the team has been able to clearly refute the long-held belief that most of the energy is absorbed by protons (at 100 km), and to show that, at least for the conditions observed, electrons damp these fluctuations very efficiently. Solution of the equations that describe such fluctuations indicated that they are a type of plasma wave known as a Kinetic Alfvén Wave, consistent with recent theories on plasma turbulence.

This is the first direct observation of the dissipation of solar wind turbulence at these small (electron) scales. The fact that the turbulence consists of Kinetic Alfvén Waves rather than, say, ion cyclotron waves that predominately interact with the magnetic energy in the solar wind, may have profound implications for theories of how the solar corona itself is heated. Also, this new observation consisting of large energy deposit onto electrons may explain various observations of huge accelerations of electrons in astrophysics. Further research is planned to try to determine how common this process is. It will indeed take considerably more research before one can know if the process found by the present investigation is the most common mechanism, or whether there are other mechanisms that might dominate when different conditions are present in the solar wind.

For further information see : Sahraoui et al., Evidence of cascade and dissipation of solar wind turbulence at electrons scales, Physical Review Letters 102, 231102 (2009)

Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Exploitant du site : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Pascal Chabert (Directeur)