92 research outputs found
The determination of shock ramp width using the noncoplanar magnetic field component
We determine a simple expression for the ramp width of a collisionless fast
shock, based upon the relationship between the noncoplanar and main magnetic
field components. By comparing this predicted width with that measured during
an observation of a shock, the shock velocity can be determined from a single
spacecraft. For a range of low-Mach, low-beta bow shock observations made by
the ISEE-1 and -2 spacecraft, ramp widths determined from two-spacecraft
comparison and from this noncoplanar component relationship agree within 30%.
When two-spacecraft measurements are not available or are inefficient, this
technique provides a reasonable estimation of scale size for low-Mach shocks.Comment: 6 pages, LaTeX (aguplus + agutex);
packages:amsmath,times,graphicx,float, psfrag,verbatim; 3 postscript figures
called by the file; submitted to Geophys. Res. Let
Efficient electron heating in relativistic shocks and gamma ray burst afterglow
Electrons in shocks are efficiently energized due to the cross-shock
potential, which develops because of differential deflection of electrons and
ions by the magnetic field in the shock front. The electron energization is
necessarily accompanied by scattering and thermalization. The mechanism is
efficient in both magnetized and non-magnetized relativistic electron-ion
shocks. It is proposed that the synchrotron emission from the heated electrons
in a layer of strongly enhanced magnetic field is responsible for gamma ray
burst afterglows.Comment: revtex
Kinetic description of avalanching systems
Avalanching systems are treated analytically using the renormalization group
(in the self-organized-criticality regime) or mean-field approximation,
respectively. The latter describes the state in terms of the mean number of
active and passive sites, without addressing the inhomogeneity in their
distribution. This paper goes one step further by proposing a kinetic
description of avalanching systems making use of the distribution function for
clusters of active sites. We illustrate application of the kinetic formalism to
a model proposed for the description of the avalanching processes in the
reconnecting current sheet of the Earth magnetosphere.Comment: 9 page
Relativistic filamentary equilibria
Plasma filamentation is often encountered in collisionless shocks and inertial confinement fusion. We develop a general analytical description of the two-dimensional relativistic filamentary equilibrium and derive the conditions for existence of potential-free equilibria. A pseudopotential equation for the vector-potential is constructed for cold and relativistic Maxwellian distributions. The role of counter-streaming is explained. We present single current sheet and periodic current sheet solutions, and analyze the equilibria with electric potential. These solutions can be used to study linear and nonlinear evolution of the relativistic filamentation instabilit
Width dependent collisionless electron dynamics in the static fields of the shock ramp, 1, Single particle behavior and implications for downstream distribution
International audienceWe study the collisionless dynamics of electrons in the shock ramp using the numerical trajectory analysis in the model electric and magnetic fields of the shock. Even with very modest assumptions about the cross-shock potential the electron trajectories are very sensitive to the width of the ramp. The character of electron motion changes from the fully adiabatic (with conservation of v2 /B) when the ramp is wide, to the nonadiabatic one, when the ramp becomes sufficiently narrow. The downstream electron distribution also changes drastically, although this change depends on the initial electron temperature
Determination of the dispersion of low frequency waves downstream of a quasiperpendicular collisionless shock
International audienceA method of wave mode determination, which was announced in Balikhin and Gedalin, is applied to AMPTE UKS and AMPTE IRM magnetic field measurements downstream of supercritical quasiperpendicular shock. The method is based on the fact that the relation between phase difference of the waves measured by two satellites, Doppler shift equation, the direction of the wave propagation are enough to obtain the dispersion equation of the observed waves. It is shown that the low frequency turbulence mainly consists of waves observed below 1 Hz with a linear dependence between the absolute value of wave vector |k| and the plasma frame wave frequency. The phase velocity of these waves is close to the phase velocity of intermediate waves Vint = Vacos(?)
Determination of the dispersion of low frequency waves downstream of a quasiperpendicular collisionless shock
Optical Solitary Waves in the Higher Order Nonlinear Schrodinger Equation
We study solitary wave solutions of the higher order nonlinear Schrodinger
equation for the propagation of short light pulses in an optical fiber. Using a
scaling transformation we reduce the equation to a two-parameter canonical
form. Solitary wave (1-soliton) solutions exist provided easily met inequality
constraints on the parameters in the equation are satisfied. Conditions for the
existence of N-soliton solutions (N>1) are determined; when these conditions
are met the equation becomes the modified KdV equation. A proper subset of
these conditions meet the Painleve plausibility conditions for integrability.Comment: REVTeX, 4 pages, no figures. To appear in Phys. Rev. Let
The first direct observational confirmation of kinematic collisionless relaxation in very low mach number shocks near the Earth
Collisionless shocks are ubiquitous throughout the known Universe. They mainly convert the energy of the directed ion flow into heating. Upon crossing the shock front, the ion distribution becomes nonâgyrotropic. Relaxation to gyrotropy then occurs mainly via kinematic collisionless gyrophase mixing and interaction with waves. The theory of collisionless relaxation predicts that the downstream pressure of each ion species varies quasiâperiodically with the distance from the shock transition layer and the amplitude of the variations gradually decrease. The oscillations due to each species has its own spatial period and damping scale. Pressure balance requires that the variations in the total plasma pressure should cause antiâcorrelating variations in the magnetic pressure. This process should occur at all Mach numbers, but its observation is difficult at moderate/high Mach numbers. In contrast, such magnetic oscillations have been observed at low Mach number cases of the Venusian bow shock and interplanetary shocks. In this paper, simultaneous inâsitu magnetic field and plasma measurements from the THEMISâB and C spacecraft are used to study, for the first time, the antiâcorrelated total ion and magnetic pressure spatial variations at lowâMach number shocks. It is found that kinematic collisionless relaxation is the dominant process in the formation of the downstream ion distribution and in shaping the downstream magnetic profile of the observed shocks, confirming fundamental theoretical results. Comparison with the results from numerical models allows the role of the different ion species to be investigated and confirms the role heavy ions play in forming the downstream magnetic profile
Pseudoscalar Meson Mixing in Effective Field Theory
We show that for any effective field theory of colorless meson fields, the
mixing schemes of particle states and decay constants are not only related but
also determined exclusively by the kinetic and mass Lagrangian densities. In
the general case, these are bilinear in terms of the intrinsic fields and
involve non-diagonal kinetic and mass matrices. By applying three consecutive
steps this Lagrangian can be reduced into the standard quadratic form in terms
of the physical fields. These steps are : (i) the diagonalization of the
kinetic matrix, (ii) rescaling of the fields, and (iii) the diagonalization of
the mass matrix. In case, where the dimensions of the non-diagonal kinetic and
mass sub-matrices are respectively, and , this procedure
leads to mixing schemes which involve angles and
field rescaling parameters. This observation holds true irrespective with the
type of particle interactions presumed. The commonly used mixing schemes,
correspond to a proper choice of the kinetic and mass matrices, and are derived
as special cases. In particular, - mixing, requires one angle, if
and only if, the kinetic term with the intrinsic fields has a quadratic form.Comment: REVTeX, 6 page
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