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The paper studies wave propagation in 1 D bimodular media. Four cases are investigated: two cases with an external step load applied to a prestressed bimodular rod and two cases with a sign-alternating pulse applied to an unloaded bimodular rod. Shock wave formation and energy dissipation are observed in two cases. Analytical solutions showing the distortion of a wave profile due to shock wave formation for all cases are derived. A method of suppressing an arbitrary tensile pulse in a bimodular rod by creating a compressive pulse is proposed. Validation by numerical modeling using a Godunov-type explicit difference scheme is performed. [ABSTRACT FROM AUTHOR]

It is well known that the speed of elastic waves depends on the elasticity modulus, therefore in bimodular media the compressive and tensile waves run with different velocities. Due to this phenomenon, the parts of a one period sin excitation can either move away from each other or interact with each other depending on which part, compression or tension, was the first. The case when the parts of the wave are moving away is trivial because each part can be analysed separately. On the other hand, the case when the compressive wave overtakes the tensile one interacting with it leads to a nonlinear solution characterised by the formation of a shock front which is accompanied by energy dissipation and wave front distortion.In this study, we analyse a possible mechanism of energy dissipation during wave propagation in bimodular media in the absence of damping. A theoretical method of the evaluation of shock wave speed and energy dissipation is developed and compared with a numerical Godunov-type finite difference method. [ABSTRACT FROM AUTHOR]

Collisionless magnetized plasmas have the tendency to isotropize their velocity distribution function around the local magnetic field direction, i.e., to be gyrotropic, unless some spatial and/or temporal fluctuations develop at the particle gyroscales. Electron gyroscale inhomogeneities are well known to develop during the magnetic reconnection process. Nongyrotropic electron velocity distribution functions have been observed to play a key role in the dissipative process breaking the field line connectivity. In this paper, we present a new method to quantify the deviation of a particle population from gyrotropy. The method accounts for the full 3D shape of the distribution and its analytical formulation allows fast numerical computation. Regions associated with a significant degree of nongyrotropy are shown, as well as the kinetic origin of the nongyrotropy and the fluid signature it is associated with. Using the result of 2.5D Particle-In-Cell simulations of magnetic reconnection in symmetric and asymmetric configurations, it is found that neither the reconnection site nor the topological boundaries are generally associated with a maximized degree of nongyrotropy. Nongyrotropic regions do not correspond to a specific fluid behavior as equivalent nongyrotropy is found to extend over the electron dissipation region as well as in non-dissipative diamagnetic drift layers. The localization of highly nongyrotropic regions in numerical models and their correlation with other observable quantities can, however, improve the characterization of spatial structures explored by spacecraft missions. [ABSTRACT FROM AUTHOR]

Numerical studies implementing different versions of the collisionless Ohm's law have shown a reconnection rate insensitive to the nature of the non-ideal mechanism occurring at the X line, as soon as the Hall effect is operating. Consequently, the dissipation mechanism occurring in the vicinity of the reconnection site in collisionless systems is usually thought not to have a dynamical role beyond the violation of the frozen-in condition. The interpretation of recent studies has, however, led to the opposite conclusion that the electron scale dissipative processes play an important dynamical role in preventing an elongation of the electron layer from throttling the reconnection rate. This work re-visits this topic with a new approach. Instead of focusing on the extensively studied symmetric configuration, we aim to investigate whether the macroscopic properties of collisionless reconnection are affected by the dissipation physics in asymmetric configurations, for which the effect of the Hall physics is substantially modified. Because it includes all the physical scales a priori important for collisionless reconnection (Hall and ion kinetic physics) and also because it allows one to change the nature of the non-ideal electron scale physics, we use a (two dimensional) hybrid model. The effects of numerical, resistive, and hyper-resistive dissipation are studied. In a first part, we perform simulations of symmetric reconnection with different non-ideal electron physics. We show that the model captures the already known properties of collisionless reconnection. In a second part, we focus on an asymmetric configuration where the magnetic field strength and the density are both asymmetric. Our results show that contrary to symmetric reconnection, the asymmetric model evolution strongly depends on the nature of the mechanism which breaks the field line connectivity. The dissipation occurring at the X line plays an important role in preventing the electron current layer from elongating and forming plasmoids. [ABSTRACT FROM AUTHOR]