Magnetized Kelvin–Helmholtz instability: theory and simulations in the Earth’s magnetosphere context

Faganello, Matteo; Califano, Francesco

doi:10.1017/s0022377817000770

Cited by 67 publications

(69 citation statements)

References 226 publications

(896 reference statements)

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“…The signature of Kelvin-Helmoltz instabilities and intermittency may well persist in the statistics of such flows [44]. At even smaller scales, Hall-MHD, as well as electron dynamics are also observed [45][46][47][48]. Two-dimensional two-fluid Hall-MHD simulations have shown recently that there is a sizable proportion of the turbulent transfer, and hence of the dissipation, that is localized in coherent structures such as current sheets which are thin but have transverse dimensions of the order of the integral scale [49].…”

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confidence: 97%

Coupling Large Eddies and Waves in Turbulence: Case Study of Magnetic Helicity at the Ion Inertial Scale

2020

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In turbulence, for neutral or conducting fluids, a large ratio of scales is excited because of the possible occurrence of inverse cascades to large, global scales together with direct cascades to small, dissipative scales, as observed in the atmosphere and oceans, or in the solar environment. In this context, using direct numerical simulations with forcing, we analyze scale dynamics in the presence of magnetic fields with a generalized Ohm's law including a Hall current. The ion inertial length H serves as the control parameter at fixed Reynolds number. Both the magnetic and generalized helicity -invariants in the ideal case -grow linearly with time, as expected from classical arguments. The cross-correlation between the velocity and magnetic field grows as well, more so in relative terms for a stronger Hall current. We find that the helical growth rates vary exponentially with H , provided the ion inertial scale resides within the inverse cascade range. These exponential variations are recovered phenomenologically using simple scaling arguments. They are directly linked to the wavenumber power-law dependence of generalized and magnetic helicity, ∼ k −2 , in their inverse ranges. This illustrates and confirms the important role of the interplay between large and small scales in the dynamics of turbulent flows. arXiv:2001.11625v1 [physics.flu-dyn] 31 Jan 2020 and large scales play an essential role in estimating the efficiency of mixing in such flows [28][29][30], and it is found to vary linearly with the control parameter, namely the Froude number [31,32]. B. The case of space plasmasSimilar phenomena are observed as well for turbulent flows in the presence of magnetic fields. Such fields, together with charged particles, are abundant in the cosmos. At large scales, the magnetohydrodynamic (MHD) approximation, in which the displacement current is neglected in Maxwell's equations, is adequate, and observations of the Solar Wind, dating back to the Voyager spacecraft, confirmed the physical description of a medium governed by the interactions of turbulent, nonlinear eddies and Alfvén waves (see, e.g., for recent reviews, [33-36] and references therein). Turbulence is also found to play a central role in shaping these media [37][38][39].However, as the direct turbulent cascade of energy approaches smaller scales, plasma effects and dispersive waves come into effect, appearing for example through a generalized Ohm's law whose expression depends on the degree of ionization of the medium, which itself can differ greatly from the solar wind to the interstellar gas. Current spacecraft technologies allow for the resolution of much smaller temporal and spatial scales than what was available previously, and one can now reach the ion inertial length, H , and perhaps the electron inertial length (see for definitions the next section, and e.g. [40]). Other types of waves, kinetic Alfvén waves or whistler waves for example, come into play between the ion and electron scales, and the distribution of energy among modes is altered...

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confidence: 97%

Coupling Large Eddies and Waves in Turbulence: Case Study of Magnetic Helicity at the Ion Inertial Scale

2020

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“…where we have used the relation k FGM L u ∼ 0.4 derived in the compressible MHD limit (see Faganello & Califano 2017, and references therein). Therefore, the effects of such readjustment on the KHI growth are of order…”

Section: Readjustment Timescale Of Unbalanced Equilibriamentioning

confidence: 99%

Finite-Larmor-radius equilibrium and currents of the Earth’s flank magnetopause

Cerri

2018

J. Plasma Phys.

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We consider the one-dimensional equilibrium problem of a shear-flow boundary layer within an "extended fluid model" of plasma that includes the Hall and the electron pressure terms in the Ohm's law, as well as dynamic equations for anisotropic pressure for each species and first-order finite Larmor radius (FLR) corrections to the ion dynamics. We provide a generalized version of the analytic expressions for the equilibrium configuration given in Cerri et al. (2013), highlighting their intrinsic asymmetry due to the relative orientation of the magnetic field b = B/|B| and the fluid vorticity ω = ∇ × u ("ωb asymmetry"). Finally, we show that FLR effects can modify the Chapman-Ferraro current layer at the flank magnetopause in a way that is consistent with the observed structure reported by Haaland et al. (2014). In particular, we are able to qualitatively reproduce the following key features: (i) the dusk-dawn asymmetry of the current layer, (ii) a double-peak feature in the current profiles, and (iii) adjacent current sheets having thicknesses of several ion Larmor radii and with different current directions.

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“…Three-dimensional studies of local KHI evolutions at the Earth's magnetosphere have gradually realized the importance of the higher-latitude double reconnection process that is triggered by the KHI equatorial development. In this double midlatitude reconnection (DMLR) process (Borgogno et al, 2015;Faganello & Califano, 2017;Faganello et al, , 2014, the KHI self-consistently creates configurations liable to magnetic reconnection and subsequent particle acceleration. In this double midlatitude reconnection (DMLR) process (Borgogno et al, 2015;Faganello & Califano, 2017;Faganello et al, , 2014, the KHI self-consistently creates configurations liable to magnetic reconnection and subsequent particle acceleration.…”

Section: Introductionmentioning

confidence: 99%

Particle Orbits at the Magnetopause: Kelvin‐Helmholtz Induced Trapping

Leroy

Ripperda²,

Keppens

2019

JGR Space Physics

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The Kelvin‐Helmholtz instability is a known mechanism for penetration of solar wind matter into the magnetosphere. Using three‐dimensional, resistive magnetohydrodynamic simulations, the double midlatitude reconnection (DMLR) process was shown to efficiently exchange solar wind matter into the magnetosphere, through mixing and reconnection. Here we compute test particle orbits through DMLR configurations. In the instantaneous electromagnetic fields, charged particle trajectories are integrated using the guiding center approximation. The mechanisms involved in the electron particle orbits and their kinetic energy evolutions are studied in detail, to identify specific signatures of the DMLR through particle characteristics. The charged particle orbits are influenced mainly by magnetic curvature drifts. We identify complex, temporarily trapped trajectories where the combined electric field and (reconnected) magnetic field variations realize local cavities where particles gain energy before escaping. By comparing the orbits in strongly deformed fields due to the Kelvin‐Helmholtz instability development, with the textbook mirror‐drift orbits resulting from our initial configuration, we identify effects due to current sheets formed in the DMLR process. We do this in various representative stages during the DMLR development.

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Magnetized Kelvin–Helmholtz instability: theory and simulations in the Earth’s magnetosphere context

Cited by 67 publications

References 226 publications

Coupling Large Eddies and Waves in Turbulence: Case Study of Magnetic Helicity at the Ion Inertial Scale

Coupling Large Eddies and Waves in Turbulence: Case Study of Magnetic Helicity at the Ion Inertial Scale

Finite-Larmor-radius equilibrium and currents of the Earth’s flank magnetopause

Particle Orbits at the Magnetopause: Kelvin‐Helmholtz Induced Trapping

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