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M. Roth - One of the best experts on this subject based on the ideXlab platform.

  • Reconstruction of the magnetopause and low-latitude boundary layer topology using Cluster multi-point measurements
    Annales Geophysicae, 2004
    Co-Authors: John Keyser, Mark Dunlop, Göran Gustafsson, Henri Rème, Fabien Darrouzet, Andrew N. Fazakerley, P M E Decreau, M. Roth, Nicole Cornilleau-wehrlin
    Abstract:

    Multi-spacecraft data from Cluster allow for a more detailed magnetopause and boundary layer structure determination than ever before. Reconstruction methods, in which the time variability observed during a pass is interpreted as being due to boundary motion and/or spatial structure, are particularly well suited for this task. Such methods rely on the availability of plasma and field data and adopt the Tangential Discontinuity hypothesis to determine the motion, acceleration, boundary structure, boundary curvature and surface wave speed over an extended time interval. In this paper one- and two-dimensional reconstruction methods are applied to multi-spacecraft data for the first time.

  • equilibrium conditions and magnetic field rotation at the Tangential Discontinuity magnetopause
    Journal of Geophysical Research, 1998
    Co-Authors: J. De Keyser, M. Roth
    Abstract:

    De Keyser and Roth recently have developed a kinetic model of the Tangential Discontinuity magnetopause. This model predicts (1) that not all configurations of magnetic field vectors and magnetosheath velocity allow an equilibrium to exist and (2) that there is a preference for a particular magnetic field rotation sense across the magnetopause due to the different response of ions and electrons to the electric field in the current layer. In the present paper we extend the original model to allow for different magnetospheric and magnetosheath densities and temperatures, and we show that the conclusions remain essentially unchanged. Given the large-scale magnetosheath flow pattern around the magnetosphere, we also compute which regions of the dayside magnetopause may be in Tangential Discontinuity equilibrium for a given magnetosheath field orientation.

  • Equilibrium conditions for the Tangential Discontinuity magnetopause
    Journal of Geophysical Research, 1997
    Co-Authors: J. De Keyser, M. Roth
    Abstract:

    Early satellite observations of the dayside magnetopause have suggested that the magnetic field typically rotates clockwise above the solar-magnetospheric equatorial plane and counterclockwise below it, in agreement with the predictions of first-order orbit theory for magnetopause crossings of the rotational Discontinuity type. The present paper treats the Tangential Discontinuity (TD) case. The influence of magnetosheath magnetic field and plasma flow on the magnetopause equilibrium structure is analyzed by means of a Vlasov model. The nature of the current layer plays a major role; the analysis is carried out for ion-dominated, electron-dominated, and mixed layers. Necessary and sufficient conditions for the existence of an equilibrium magnetopause are derived. It is found that (1) the magnetopause is best modeled as a transition layer of mixed type; (2) for high magnetic shear the magnetic field at the dayside magnetopause preferentially rotates clockwise above the equatorial plane and counterclockwise below it, while it rotates counterclockwise above and clockwise below the equatorial plane at the tail flanks; this effect becomes more manifest as the magnetosheath flow is faster and as the difference in proton and electron transition lengths is more pronounced; (3) for low magnetic shear, TD equilibrium is expected to be lost more easily at the dawnside than at the duskside; and (4) the model provides a magnetopause thickness estimate; in particular, the low magnetic shear dawn magnetopause is predicted to be thinner than the dusk magnetopause.

  • Theoretical plasma distributions consistent with Ulysses magnetic field observations in a solar wind Tangential Discontinuity
    Solar Physics, 1996
    Co-Authors: J. De Keyser, M. Roth, Joseph Lemaire, Bruce T. Tsurutani, C. M. Hammond
    Abstract:

    The overall multi-layer structure of the magnetic field profile observed by Ulysses across a broad solar wind Tangential Discontinuity can be reproduced fairly well by means of a kinetic model. Such a simulation provides complementary information about the velocity distribution functions, which are not always available from the plasma experiment due to the low time resolution inherent in plasma measurements. The success of such a simulation proves that the kinetic model can be used as a realistic basis for further studies of the structure and stability of solar wind Tangential discontinuities.

C. M. Hammond - One of the best experts on this subject based on the ideXlab platform.

Nick Omidi - One of the best experts on this subject based on the ideXlab platform.

  • themis observations of Tangential Discontinuity driven foreshock bubbles
    Geophysical Research Letters, 2015
    Co-Authors: Z Liu, D L Turner, V Angelopoulos, Nick Omidi
    Abstract:

    Foreshock bubbles (FBs), transient ion foreshock phenomena formed from highly concentrated suprathermal foreshock ions upstream of solar wind discontinuities, produce intense disturbances in the magnetosphere-ionosphere system and can accelerate particles to even higher energies. Rotational discontinuities are known to drive FBs. Tangential discontinuities (TDs), however, have not previously been considered as drivers of FBs because they have no normal magnetic field component, preventing access of field-aligned particles upstream. However, given that suprathermal foreshock ions have gyroradii larger than the width of TDs, they may pass upstream of TDs and generate FBs. Using multipoint observations from Advanced Composition Explorer, WIND, and Time History of Events and Macroscale Interactions during Substorms (THEMIS), we report on two cases of TD-driven FBs. The FBs were identified using classical FB selection criteria, and the driving TDs were identified using ideal MHD criteria, to within the limits of observational error. Our results add another potential solar wind driver of FBs and imply that FBs may be even more common than previously thought.

  • THEMIS observations of Tangential Discontinuity‐driven foreshock bubbles
    Geophysical Research Letters, 2015
    Co-Authors: Z Liu, V Angelopoulos, Drew Turner, Nick Omidi
    Abstract:

    Foreshock bubbles (FBs), transient ion foreshock phenomena formed from highly concentrated suprathermal foreshock ions upstream of solar wind discontinuities, produce intense disturbances in the magnetosphere-ionosphere system and can accelerate particles to even higher energies. Rotational discontinuities are known to drive FBs. Tangential discontinuities (TDs), however, have not previously been considered as drivers of FBs because they have no normal magnetic field component, preventing access of field-aligned particles upstream. However, given that suprathermal foreshock ions have gyroradii larger than the width of TDs, they may pass upstream of TDs and generate FBs. Using multipoint observations from Advanced Composition Explorer, WIND, and Time History of Events and Macroscale Interactions during Substorms (THEMIS), we report on two cases of TD-driven FBs. The FBs were identified using classical FB selection criteria, and the driving TDs were identified using ideal MHD criteria, to within the limits of observational error. Our results add another potential solar wind driver of FBs and imply that FBs may be even more common than previously thought.

Michael S. Ruderman - One of the best experts on this subject based on the ideXlab platform.

  • Propagation of leaky surface waves on contact magnetohydrodynamic discontinuities in incompressible plasmas
    Physics of Plasmas, 2018
    Co-Authors: Michael S. Ruderman, E. Vickers, Istvan Ballai, Robert Erdélyi
    Abstract:

    We study the wave propagation on a magnetohydrodynamic contact Discontinuity. Using the Laplace transform, we obtain the solution to the initial value problem describing the evolution of a perturbation of the Discontinuity. We use this solution to study the leaky modes that determine the asymptotic behaviour of the solution for large time. We find the approximate expressions describing the leaky modes for a small inclination angle of the magnetic field. We also discuss the transition to the Tangential Discontinuity as the inclination angle tends to zero. We show that there is no continuous transition from the leaky modes on a contact Discontinuity to the surface modes on a Tangential Discontinuity. However, such a transition exists if we take the average quantities describing the leaky modes.We study the wave propagation on a magnetohydrodynamic contact Discontinuity. Using the Laplace transform, we obtain the solution to the initial value problem describing the evolution of a perturbation of the Discontinuity. We use this solution to study the leaky modes that determine the asymptotic behaviour of the solution for large time. We find the approximate expressions describing the leaky modes for a small inclination angle of the magnetic field. We also discuss the transition to the Tangential Discontinuity as the inclination angle tends to zero. We show that there is no continuous transition from the leaky modes on a contact Discontinuity to the surface modes on a Tangential Discontinuity. However, such a transition exists if we take the average quantities describing the leaky modes.

  • Rayleigh-Taylor instability of a magnetic Tangential Discontinuity in the presence of flow
    Astronomy & Astrophysics, 2015
    Co-Authors: Michael S. Ruderman
    Abstract:

    We studied the magnetic Rayleigh-Taylor (MRT) instability of a magnetohydrodynamic Tangential Discontinuity in an infinitely conducting incompressible plasma in the presence of flow. We assumed that the flow magnitude is small enough to guarantee that there is no Kelvin-Helmholtz (KH) instability. In addition, we assumed that there is the magnetic shear, that is, the magnetic field has different directions at the two side of the Discontinuity. In this case, only perturbations whose wavelength is greater than the critical one are unstable. As a consequence, the perturbation growth rate is bounded, and the initial-value problem describing their evolution is well posed. We also studied the absolute and convective nature of the MRT instability using the Briggs method. We obtained the necessary and sufficient condition for a perturbation propagating in a given direction to be only convectively unstable but absolutely stable. We also obtained the condition for perturbations propagating in any direction to be only convectively unstable, but absolutely stable. The results of the general analysis were applied to the MRT instability of prominence threads and the heliopause. Similar to previous research, we assumed that the thread disappearance is related to the MRT instability and the thread lifetime is equal to the inverse instability increment. Using this assumption we estimated the angle between the magnetic field inside the thread and in the surrounding plasma and studied how this estimate depends on the magnitude of the flow inside the thread. We found that this dependence is very weak. To apply this to the heliopause stability, we carried out the local analysis and restricted it to the near flanks of the heliopause only where the plasma flow can be considered incompressible. We showed that, for values of the magnetic field magnitude observed by Voyager 1, there is no KH instability. We then studied the MRT instability that can occur when the heliosheath is accelerated in the antisolar direction due to the increase in the solar wind dynamic pressure. We showed that, for typical values of the plasma flow and magnetic field parameters, there are directions such that perturbations propagating in this directions are absolutely unstable.

  • Absolute and Convective Instability of Tangential Discontinuities in Viscous Fluids: Application to Heliopause
    Astrophysics and Space Science, 2000
    Co-Authors: Michael S. Ruderman
    Abstract:

    The stability of the heliopause, which is a Tangential Discontinuity separating the flow of the solar wind plasma compressed at the termination shock, from the flow of the insterstellar plasma compressed at the bow shock, is discussed. A brief review of the normal mode analysis is given. The recent results of the study of the absolute and convective instability of a Tangential Discontinuity in an incompressible plasma, viscous at one side of the Discontinuity, and ideal at the other side, are presented. This equilibrium configuration can be considered as a crude model of the flow near the heliopause in its near-flank regions, where the flow is essentially subsonic. The obtained results suggest that the near flanks of the heliopause are only convectively unstable. The relation of these results with results of recent numerical investigations of the absolute and convective instability of the heliopause are discussed.

  • Dissipative instability of the MHD Tangential Discontinuity in magnetized plasmas with anisotropic viscosity and thermal conductivity
    Journal of Plasma Physics, 1996
    Co-Authors: Michael S. Ruderman, Erwin Verwichte, Robert Erdélyi, Marcel Goossens
    Abstract:

    The stability of the MHD Tangential Discontinuity is studied in compressible plasmas in the presence of anisotropic viscosity and thermal conductivity. The general dispersion equation is derived, and solutions to this dispersion equation and stability criteria are obtained for the limiting cases of incompressible and cold plasmas. In these two limiting cases the effect of thermal conductivity vanishes, and the solutions are only influenced by viscosity. The stability criteria for viscous plasmas are compared with those for ideal plasmas, where stability is determined by the Kelvin—Helmholtz velocity V KH as a threshold for the difference in the equilibrium velocities. Viscosity turns out to have a destabilizing influence when the viscosity coefficient takes different values at the two sides of the Discontinuity. Viscosity lowers the threshold velocity V below the ideal Kelvin—Helmholtz velocity VKH, so that there is a range of velocities between V and V KH where the overstability is of a dissipative nature.

Thorsten Koch - One of the best experts on this subject based on the ideXlab platform.

  • instability of a Tangential Discontinuity surface in a three dimensional compressible medium
    Physics of Fluids, 2021
    Co-Authors: Thorsten Koch
    Abstract:

    Compressible flows appear in many natural and technological processes, for instance, the flow of natural gases in a pipe system. Thus, a detailed study of the stability of Tangential velocity Discontinuity in compressible media is relevant and necessary. The first early investigation in two-dimensional (2D) media was given more than 70 years ago. In this article, we continue investigating the stability in three-dimensional (3D) media. The idealized statement of this problem in an infinite spatial space was studied by Syrovatskii in 1954. However, the omission of the absolute sign of cos θ with θ being the angle between vectors of velocity and wave number in a certain inequality produced the inaccurate conclusion that the flow is always unstable for entire values of the Mach number M. First, we revisit this case to arrive at the correct conclusion, namely that the Discontinuity surface is stabilized for a large Mach number with a given value of the angle θ. Next, we introduce a real finite spatial system such that it is bounded by solid walls along the flow direction. We show that the Discontinuity surface is stable if and only if the dispersion relation equation has only real roots, with a large value of the Mach number; otherwise, the surface is always unstable. In particular, we show that a smaller critical value of the Mach number is required to make the flow in a narrow channel stable.