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Turbulence
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Turbulence is a universal phenomenon observed in fluids and plasmas. It is observed in a wide range of scales, from the vortices behind a bridge pier, to astrophysical scales. In plasmas, turbulence is characterized by disorganized behavior, swirling, chaotic, observed on all the characteristic quantities of the medium, the velocity but also the electromagnetic field.
LPP team
G. Belmont, R. Grappin, S. Galtier, L. Rezeau, F. Sahraoui
Selected publications
- Hadid L. Z., Sahraoui F., Kiyani K. H., Retinò A., Modolo R., Canu P., Masters J., Dougherty M. K., Nature of the MHD and kinetic scale turbulence in the magnetosheath of Saturn : CASSINI observations, The Astrophysical Journal Letters 813 (2), 2015.
- Meyrand R., Kiyani K. & Galtier S., Weak Magnetohydrodynamic Turbulence and Intermittency, J. Fluid Mech. 770, R1, (2015)
- Dong Y., Verdini A., Grappin R., Evolution of Turbulence in the Expanding Solar Wind, a Numerical Study, The Astrophysical Journal 793 118 (2014)
- Sahraoui F., M. L. Goldstein, P. Robert, Y. Khotyaintsev, Evidence of a Cascade and Dissipation of Solar-Wind Turbulence at the Electron Gyroscale, Phys. Rev. Lett. 102, 231102 (2009)
Turbulence observations
To study turbulence, LPP space plasma team mainly use in situ magnetic field measurements made onboard spacecraft. The main regions of turbulence observation are the Solar Wind and magnetosheath (compressed solar wind region between the
bow shock) and the magnetopause. In the Earth vicinity, the measurements are made with CLUSTER and MMS. Measurements have also been performed around Saturn with Cassini. Fluxgate magnetometer give access to low frequencies (large scales) ; they are complemented at higher frequency by search coil measurements, generally designed and built at LPP.
- Spectre des fluctuations magnétiques mesuré à bord de Cluster : : En rouge la partie mesurée avec le magnétomètre continu FGM, en bleu la partie mesurée avec le magnétomètre alternatif STAFF. La courbe en pointillé indique le niveau de bruit de l’instrument.
One observes wave spectra which present power laws (figure). This regular decreasing power behaviour is a characteristic of turbulent cascade : the energy is injected at large scale, then transmitted to smaller and smaller scales by means of vortices interaction. Signal processing work is being done at LPP for the precise analysis of turbulence : spectral analysis, filtering wave number (k-filtering) thanks to multi-satellite measurements, structure functions,....
Remote observations allow realizing that turbulence also exists in the stars or in the solar corona.
Theoretical models
The interest is to understand the mechanisms of generation, transfer and dissipation of turbulence, from the large MHD scales toward scales smaller than those of the dynamics of ions, at which kinetic processes come into play. One of the theoretical difficulties is to take into account the collisionless aspect of the studied space plasma. In the case of the solar wind, the evolution of the turbulence depends on the characteristics of the medium, expanding from the Sun and highly magnetized. This magnetic field also introduces anisotropy which must be taken into account in the models.
There are two levels of turbulence : the strong turbulence and weak turbulence. In the first case, fluctuations in the plasma (e.g. magnetic field) are at least of the same order of magnitude as the mean magnetic field. In the second case, these variations are significantly smaller, which makes it possible to undertake perturbation expansions. Analytical and numerical studies are conducted at LPP on these two regimes (see Figure below).
- Turbulence MHD forte et faible. . Simulation numérique directe de MHD en champ magnétique fort à la résolution 3072x3072x256. Cette image montre, dans une fraction du plan transverse au champ moyen, l’intensité des fluctuations du champ magnétique. La coexistence des régimes de turbulence forte et faible se manifeste par la présence de structures cohérentes (sous forme de gros tourbillons) et de structures incohérentes à plus petite échelle (Meyrand et al., 2015).
Numerical models
The equations of plasma physics are complex since they must combine description of the plasma itself with the equations of the electromagnetic fields. Turbulence is inherently non-linear, the numerical simulation is a valuable tool, extensively developed at LPP.
Expansion d’un volume de plasma de 0.2 à 1 UA : dynamique du champ de magnétique (à gauche) et du champ de vitesse fluctuant (après soustraction du champ radial moyen (à droite). Initialement (en bas), les lignes de vitesse et de champ sont isotropes. A la fin (en haut), après étirement transverse du volume de plasma par le vent radial, les lignes de champ magnétique se retrouvent principalement perpendiculaire à la radiale (la direction radiale moyenne est verticale), et les lignes de vitesse se retrouvent principalement radiales, formant naturellement des microjets. Ces deux évolutions combinées sont observées et ont des conséquences importantes sur la cascade turbulente (Dong et al, 2014).
Regarding the turbulence several types of codes are used : Hall MHD, electron MHD, MHD 3D code or hybrid 2D with coordinates comoving with the solar wind expansion, MHD 2D code overall axisymmetric, Shell-model codes, Landau fluid.
Dans la même rubrique :
- Modélisation théorique des plasmas sans collision
- Turbulence
- La reconnexion magnétique
- Accélération, rayonnement et turbulence dans les régions aurorales terrestres
- Les ondes de choc sans collision
- Génération du vent solaire
- Magnétopause de la Terre
- Magnétosphères planétaires
- Modélisation des environnements plasmas des petites planètes
- Météorologie de l’espace
- Sous-orages magnétosphériques
- Activité Solaire
