1,737 research outputs found
Collisionless Kelvin-Helmholtz instability and vortex-induced reconnection in the external region of the Earth magnetotail
No full textAbstract. In a magnetized plasma streaming with a non uniform velocity, the Kelvin-Helmholtz instability plays a major role in mixing different plasma regions and in stretching the magnetic field lines leading to the formation of layers with a sheared magnetic field where magnetic field line reconnection can take place. A relevant example is provided by the formation of a mixing layer between the Earth's magnetosphere and the solar wind at low latitudes during northward periods. In the considered configuration, in the presence of a magnetic field nearly perpendicular to the plane defined by the velocity field and its inhomogeneity direction, velocity shear drives a Kelvin-Helmholtz instability which advects and distorts the magnetic field configuration. If the Alfvén velocity associated to the in-plane magnetic field is sufficiently weak with respect to the variation of the fluid velocity in the plasma, the Kelvin-Helmholtz instability generates fully rolled-up vortices which advect..
Competing mechanisms of plasma transport in inhomogeneous configurations with velocity shear: The solar-wind interaction with earth's magnetosphere
No full textTwo-dimensional simulations of the Kelvin-Helmholtz instability in an inhomogeneous compressible plasma with a density gradient show that, in a transverse magnetic field configuration, the vortex pairing process and the Rayleigh-Taylor secondary instability compete during the nonlinear evolution of the vortices. Two different regimes exist depending on the value of the density jump across the velocity shear layer. These regimes have different physical signatures that can be crucial for the interpretation of satellite data of the interaction of the solar wind with the magnetospheric plasma
Collisionless magnetic reconnection
No full textMagnetic field line reconnection in collisionless fluid plasma regimes is discussed with the aim of understanding the role of conserved topological quantities in the nonlinear development of the reconnection instability, the role of the guide field in the transition from the Alfvenic regime to the whistler regime and the onset of secondary instabilities
Numerical evidence of undriven, fast reconnection in the solar-wind interaction with Earth's magnetosphere: Formation of electromagnetic coherent structures
No full textWe give evidence for the first time of the onset of undriven fast, collisionless magnetic reconnection during the evolution of an initially homogeneous magnetic field advected in a sheared velocity field. We consider the interaction of the solar wind with the magnetospheric plasma at low latitude and show that reconnection takes place in the layer between adjacent vortices generated by the Kelvin-Helmholtz instability. This process generates coherent magnetic structures with a size comparable to the ion inertial scale, much smaller than the system dimensions but much larger than the electron inertial scale. These magnetic structures are further advected in the plasma in a complex pattern but remain stable over a time interval much longer than their formation time. These results can be crucial for the interpretation of satellite data showing coherent magnetic structures in the Earth's magnetosheath or the magnetotail
Being on time in magnetic reconnection
No full textThe role of magnetic reconnection on the evolution of the Kelvin-Helmholtz instability is investigated in a plasma configuration with a velocity shear field. It is shown that the rate at which the large-scale dynamics drives the formation of steep current sheets, leading to the onset of secondary magnetic reconnection instabilities, and the rate at which magnetic reconnection occurs compete in shaping the final state of the plasma configuration. These conclusions are reached within a two-fluid plasma description on the basis of a series of two-dimensional numerical simulations. Special attention is given to the role of the Hall term. In these simulations, the boundary conditions, the symmetry of the initial configuration and the simulation box size have been optimized in order not to affect the evolution of the system artificially
Double mid-latitude dynamical reconnection at the magnetopause: An efficient mechanism allowing solar wind to enter the Earth’s magnetosphere
No full textThree-dimensional simulations of the Kelvin-Helmholtz (KH) instability in a magnetic configuration reproducing typical conditions at the flank Earth’s magnetosphere during northward periods show the system’s ability to generate favorable conditions for magnetic reconnection to occur at mid-latitude. Once these conditions are established, magnetic reconnection proceeds spontaneously in both hemispheres generating field lines that close on Earth but are connected to the solar wind at low latitude, allowing direct entrance of solar wind plasma into the magnetosphere. These results are consistent with recent observations of KH vortices showing the signature of reconnection events occurring well outside the equatorial plane (Bavassano M.B. et al., Ann. Geophys., 28 (2010) 893)
Kelvin-Helmholtz vortices and secondary instabilities in super-magnetosonic regimes
No full textThe nonlinear behaviour of the Kelvin-Helmholtz instability is investigated
with a two-fluid simulation code in both sub-magnetosonic and
super-magnetosonic regimes in a two-dimensional configuration chosen so as to
represent typical conditions observed at the Earth's magnetopause flanks. It
is shown that in super-magnetosonic regimes the plasma density inside the
vortices produced by the development of the Kelvin-Helmholtz instability is
approximately uniform, making the plasma inside the vortices effectively
stable against the onset of secondary instabilities. However, the relative
motion of the vortices relative to the plasma flow can cause the formation of
shock structures. It is shown that in the region where the shocks are
attached to the vortex boundaries the plasma conditions change rapidly and
develop large gradients that allow for the onset of secondary instabilities
not observed in sub-magnetosonic regimes
Magnetic reconnection and Kelvin–Helmholtz instabilities at the Earth’s magnetopause
No full textKelvin–Helmholtz instability (KHI), driven by the velocity inhomogeneity at Earth’s magnetopause, has been shown to play a major role in mixing the magnetospheric and the solar wind plasma during northward periods. In fact, when the magneto-spheric and interplanetary magnetic fields are mostly perpendicular to the equatorial plane, KHI can develop at a low latitude without being significantly inhibited by the magnetic tension. In contrast, at a high latitude, the more complex magnetic configuration is believed to totally stabilize the instability. This intrinsic 3D dynamics is investigated in a simplified geometry showing that KHI is able to kink the magnetic field lines at a mid-latitude and to create current layers where magnetic reconnection spontaneously develops. It is shown that a mid-latitude reconnection is able to change the global topology of the magnetic field and to connect interplanetary field lines to the Earth’s cups, allowing the solar wind to directly enter the magnetosphere.Kelvin–Helmholtz instability (KHI), driven by the velocity inhomogeneity at Earth’s magnetopause, has been shown to play a major role in mixing the magnetospheric and the solar wind plasma during northward periods. In fact, when the magnetospheric and interplanetary magnetic fields are mostly perpendicular to the equatorial plane, KHI can develop at a low latitude without being significantly inhibited by the magnetic tension. In contrast, at a high latitude, the more complex magnetic configuration is believed to totally stabilize the instability. This intrinsic 3D dynamics is investigated in a simplified geometry showing that KHI is able to kink the magnetic field lines at a mid-latitude and to create current layers where magnetic reconnection spontaneously develops. It is shown that a mid-latitude reconnection is able to change the global topology of the magnetic field and to connect interplanetary field lines to the Earth’s cups, allowing the solar wind to directly enter the magnetosphere
Nonlinear vortex dynamics in an inhomogeneous magnetized plasma with a sheared velocity field
No full textThe long term evolution of a vortex chain generated by the nonlinear development of the Kelvin-Helmholtz instability in a magnetized two-fluid plasma with a sheared velocity field is investigated in a two-dimensional configuration. The different roles played by the in-plane magnetic field component and by the density inhomogeneity are elucidated. This investigation is of interest in understanding the plasma dynamics that arises from the nonlinear competition of different plasma instabilities involving the interplay of large and small spatial scales and, in particular, for understanding the mixing mechanism between the solar wind and the Earth's magnetospheric plasmas that occurs at the Earth's magnetospheric flanks
Magnetised Kelvin-Helmholtz instability in the intermediate regime between subsonic and supersonic regimes
No full textThe understanding of the dynamics at play at the Earth's Magnetopause, the boundary separating the Earth's magnetosphere and the solar wind plasmas, is of primary importance for space plasma modeling. We focus our attention on the low latitude flank of the Magnetosphere where the velocity shear between the Magnetosheath and the Magnetospheric plasmas is the energetic source of Kelvin-Helmholtz instability. On the shoulder of the resulting vortex chain, different secondary instabilities are at play depending on the local plasma parameters and compete with the vortex pairing process. Most important, secondary instabilities, among other magnetic reconnection, control the plasma mixing as well as the entry of solar wind plasma in the Magnetosphere.
We make use of a two-fluid model, including the Hall term and the electron mass in the generalized Ohm's law, to study the 2D non-linear evolution of the Kelvin-Helmholtz instability at the Magnetosheath -- Magnetosphere interface, in the intermediate regime between subsonic and supersonic regimes. We study the saturation mechanisms, depending on the density jump across the shear layer and the magnetic field strength in the plane.
In the presence of a weak in-plane magnetic field, the dynamics of the Kelvin-Helmholtz rolled-up vortices self-consistently generates thin current sheets where reconnection instability eventually enables fast reconnection to develop. Such a system enables to study guide field multiple-island collisionless magnetic reconnection as embedded in a large-scale dynamic system, unlike the classical static, "ad hoc" reconnection setups. In this regime, reconnection is shown to inhibit the vortex pairing process. This study provides a clear example of nonlinear, cross-scale, collisionless plasma dynamics
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