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Entanglement between magnetic reconnection and turbulence in astrophysical plasmas
Toutes les versions de cet article : [English] [français]
Decades of observational research have shown that astrophysical plasmas are generally in a turbulent state (Fig. 1) ; the most popular signature being the ubiquitous power-law scaling of the magnetic field energy spectrum. In the classical theory of turbulence, legacy of A. Kolmogorov, kinetic energy, assumed to be injected at the largest scales, progressively cascades to smaller and smaller scales until it is eventually dissipated into thermal heating of the fluid at smallest scales of the system. The concept of this scale-by-scale cascade can be well illustrated in hydrodynamics : eddies of similar size interact, break down and give birth to smaller eddies (Fig. 2). This process stops when the eddy size becomes comparable to the dissipation scale, controlled by molecular collisions.
This simple qualitative picture, while very helpful, fails to describe most of the properties of collisionless, magnetized plasma turbulence. For instance, turbulence in plasmas can lead to the formation of thin layers of electric current. In such structures magnetic reconnection can occur, leading to a change in the magnetic field topology, intense local energy dissipation and particle heating or acceleration. This fundamental plasma physics process, and the importance of its interplay with plasma turbulence, is testified by the wealth of literature on the subject. However, a quantitative measure of the associated local turbulent cascade is, to date, missing because of the lack of appropriate theoretical tools. This gap in now filled by the work published recently by Manzini, Sahraoui & Califano in Phys. Rev. Lett. [1]
Thanks to a novel technique named “coarse-graining” and based on a scale by scale space filtering [2], it was indeed possible to compute the turbulent energy transfer across a given scale and at each position in space, thus relaxing the constraint of the classical “Kolmogorov-like” theories that require averages over large portions of plasma.
Applying this tool to data taken in the terrestrial magnetosheath by the NASA Magnetospheric Multiscale (MMS) Mission, it is shown that locations of magnetic reconnection are associated to intense turbulent energy transfer, effective at sub-ion scales (Fig. 3((e)).
The results are consistently confirmed in numerical simulations : a one dimensional cut across the simulation domain shows that turbulent transfer is enhanced at reconnecting locations (denoted 1-4 in Fig.4(e)) and at scales smaller than the ion gyroradius.
This work elucidates how magnetic reconnection can be the main driver of turbulence at scales below the ion gyroradius. This new driving mechanism is of particular importance as it can be faster than the classical scale-by-scale cascade, meaning that it can “shortcut” the latter and drive sub-ion scale fluctuations. This “shortcut” to small scales can potentially solve a pending paradox raised by spacecraft observations in various planetary magnetosheaths, namely the ubiquity of turbulent fluctuations at sub-ion scales even when no energy cascade emerges from the larger (MHD) scales [3]. This work also opens new pathways to investigate the interplay between turbulence, reconnection and energy dissipation in collisionless magnetized plasmas.
References :
[1] Manzini, D., F. Sahraoui, F. Califano, Sub-ion scale turbulence driven by magnetic reconnection, Phys. Rev. Lett., 130, 205201 (2023)
[2] Manzini, D., F. Sahraoui, F. Califano, Local energy transfer and dissipation in incompressible Hall magnetohydrodynamic turbulence : The coarse-graining approach, Phys. Rev. E, 106, 035202 (2022)
[3] Huang, S. Y., L. Z. Hadid, F. Sahraoui, Z. G. Yuan, X. H. Deng, On the existence of the Kolmogorov inertial range in the terrestrial magnetosheath turbulence, ApJL, 836 (1), L10, 2017
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