François PETRELIS
CNRS, Laboratoire de Physique de l'Ecole Normale Supérieure (ENS Paris)

Magnetohydrodynamics and dynamo effect

The dynamo effect is an instability that, in an electrically conductive fluid, converts kinetic energy into magnetic energy. It is this process that generates the magnetic field of astrophysical objects (planets, stars, galaxies). In a liquid metal, dynamo instability occurs in highly turbulent flows. We are therefore dealing with an instability in which the unstable field (the magnetic field) is forced by a highly fluctuating field (the velocity field). Even though the idea of dynamo instability was presented by Larmor more than hundred years ago, no experimental observation was achieved until the early 2000s.

Selected results :
Experimental observation of the dynamo effect and of spontaneous reversals of the magnetic field in a turbulent flow of liquid sodium (VKS collaboration)

vksf1
Time series of the magnetic field measured in the VKS experiment. The magnetic field remains of same direction for hundreds of seconds and abruptly reverses in an apparently random manner.
  •    Generation of a magnetic field by dynamo action in a turbulent flow of liquid sodium
    Monchaux R., Berhanu M., Bourgoin M., Moulin M., Odier Ph ., Pinton J. -F., Volk R., Fauve S., Mordant N., Pétrélis F, Chiffaudel A. , Daviaud F., Dubrulle B., Gasquet C., Marie L., Ravelet F.
    Physical Review Letters 98 (4) 044502 (2007)

  •    Magnetic field reversals in an experimental turbulent dynamo
    Berhanu M, Monchaux R, Fauve S, Mordant N, Pétrélis F, Chiffaudel A, Daviaud F, Dubrulle B, Marié L, Ravelet F, Bourgoin M, Odier P, Pinton JF, Volk R
    Europhysics Letters 77, 59001 (2007)

    • Model for the reversals of the magnetic field.

    A low dimensional model based on the competition between two modes of magnetic field captures the variety of regimes observed in the VKS experiment as well as what is known of the Earth magnetic field and other astrophysical objets.
    QBO3
    Comparison between the time series of reversing magnetic field as observed in the VKS experiment, the low dimensional model and the Earth magnetic field.
  •    Chaotic dynamics of the magnetic field generated by dynamo action in a turbulent flow
    Pétrélis F, Fauve S,
    J. Phys. : Condens. Matter (20) 494203 (2008) pdf

  •    Simple Mechanism for Reversals of Earth’s Magnetic Field
    Pétrélis F, Fauve S, Dormy E, Valet J-P
    Physical Review Letters (102), 144503 (2009)

  •    From Reversing to Hemispherical Dynamos
    Gallet B, Pétrélis F
    Physical Review E (80) 035302 (2009)

  •    Mechanisms for magnetic field reversals
    Pétrélis F, Fauve S
    Philosophical Transactions of the Royal society A (368), 1595-1605 (2010)

  •    Experimental dynamos: from models to applications to the geodynamo
    F. Pétrélis,
    Comptes Rendus. Physique, 1-17 (2024)

    • Observation in numerical dynamos


    Relating Earth reversal frequency and plate tectonics

  •    Plate tectonics may control geomagnetic reversal frequency
    F. Pétrélis, J. Besse, J-P Valet
    Geophysical Research Letters (38), L19303 (2011)

    • Dynamo due to variations of electrical conductivity or magnetic permeability

  •    Dynamo action due to spatially dependent magnetic permeability
    B. Gallet, F. Pétrélis, S. Fauve
    Europhysics Letters (97), 69001 (2012)

  •    Spatial variations of magnetic permeability as a source of dynamo action
    B. Gallet, F. Pétrélis, S. Fauve
    Journal of Fluid Mechanics 727, 161-190 (2013)

  •    Optimum reduction of the dynamo threshold by a ferroma- gnetic layer located in the flow
    J. Herault, F. Pétrélis
    Physical Review E 90, 033015 (2014)

  •    Fluctuations of electrical conductivity: a new source for astrophysical magnetic fields
    Francois Petrelis, Alexandros Alexakis, Christophe Gissinger
    Phys. Rev. Lett. 116, 161102 (2016)