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Accueil > A propos du LPP > Communication > Actualités archivées > 2018 > A new technique reveals how coherent structures drive energy transfer and dissipation in space plasma turbulence

A new technique reveals how coherent structures drive energy transfer and dissipation in space plasma turbulence

The Sun emits a supersonic stream of charged particles, the so-called solar wind, which interacts with planetary magnetospheres in the Solar System. Upon interaction with magnetospheres, planetary shocks form and the plasma downstream is decelerated and heated in the so-called magnetosheaths. The plasma in all these regions – solar wind, shock and magnetosheath - is highly turbulent.

In turbulent flows, large-scale motions (big whirls) affect small-scale motions (little whirls) and this corresponds to a net energy transfer between different scales. Such energy is eventually dissipated at kinetic scales, that is, scales comparable with particles’ gyroradia. Yet the way such transfer and dissipation occurs is still a key open question. Transfer and dissipation can be mediated at kinetic scales by different mechanisms such aslinear wave damping, magnetic reconnection and stochastic heating. It is now largely accepted that the presence of localized "coherent structure" enhances the energy transfer and dissipation channels and the kinetic features of the plasma. Such structures, observed in situ by spacecraft, naturally emerge in heliospheric turbulent plasmas in the form of elongated sheets where the magnetic field and the electrical currents are higher than elsewhere. The energy transfer is very inhomogeneous : most of the transfer happens in a small fraction of space. Despite of all this, quantitative methods to express the relationship between scale-to-scale energy transfer and the presence of spatial structures are still missing.

A recent study by E. Camporeale from the Dutch National Research Institute for Mathematics and Computer Science (Centrum Wiskunde & Informatica - CWI), co-signed by the LPP researcher A. Retinò, employs the technique of space filtering to quantify such transfer. Such technique is applied to two-dimensional high-resolution Hall magnetohydrodynamic simulations to derive the amount of energy transfer from large to small scales in different regions of the simulation and to quantify the importance of coherent structures foe dissipation.The figure below, which made the cover page of Physical Review Letters on 19 March 2018, shows the formation of coherent structures in the simulation domain, where spatial scales are normalized to the ion inertial length. The component of the magnetic field out of the simulation plane, Bz, is shown. The coherent structures are the small-scale structures where Bz is enhanced, corresponding to regions of strong electrical current.

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Structures cohérentes dans un plasma turbulent

In the study by Camporeale et al., the characterization of coherent structures is performed by means of a two-dimensional wavelet transformation. By studying the correlation between the energy flux and the wavelet amplitude, the study demonstrates the strong relationship between scale-to-scale transfer and coherent structures. Furthermore, by conditioning one quantity with respect to the other, the study quantifies for the first time the inhomogeneity of the turbulence cascade induced by topological structures in the magnetic field. Half of the energy transfer is localized in only 25% of space.

For more information :
https://journals.aps.org/prl/issues/120/12
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.125101


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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Directeur de la publication : Pascal Chabert (Directeur)