Shock wave

About: Shock wave is a research topic. Over the lifetime, 36184 publications have been published within this topic receiving 635848 citations. The topic is also known as: Shock waves & shockwave.

Collisionless dissipation in quasi‐perpendicular shocks

Abstract: Microscopic dissipation processes in quasi-perpendicular shocks are studied by two-dimensional plasma simulations in which electrons and ions are treated as particles moving in self-consistent electric and magnetic fields. Cross-field currents induce substantial turbulence at the shock front reducing the reflected ion fraction, increasing the bulk ion temperature behind the shock, doubling the average magnetic ramp thickness, and enhancing the upstream field aligned electron heat flow. The short scale length magnetic fluctuations observed in the bow shock are probably associated with this turbulence.

Waves in the Solar Atmosphere

Abstract: The study of waves in the solar atmosphere started with an attempt to explain the observed Doppler shifts (wiggly line spectra) and broadening of photospheric spectral lines. Then Biermann (1946) and Schwarzschild (1948) suggested that acoustic waves, generated in the hydrogen convection zone, supplied the non­ radiative energy needed to heat the chromosphere and corona. In 1959 Leighton (1960) discovered that most of the solar surface was covered with regions that oscillated with periods near five minutes. Although it was assumed that these observed motions and theoretically postulated waves were related, development of these two aspects has been rather independent. The heating models have concen­ trated on shock wave propagation and dissipation, without much consideration of the observed motions, while the oscillation models have concentrated on explaining the oscillations without regard for their implications for energy and momentum transfer. The early observations have been summarized by Noyes (1967), and theories of wave generation, propagation, and heating have been reviewed by Lighthill (1967), Schatzman & Souffrin (1967), and Kuperus (1969). Michalitsanos (1974) has reviewed the recent literature on the "five-minute" oscillation. Research on solar motions has recently progressed in three areas. 1. Computers have made possible the numerical integration of the nonlinear equations of motion. This has shown that the upper chromosphere and corona can be heated by the "five-minute" oscillation (and so brought observations and heating models closer together), but short-period waves are needed to heat the low chromosphere. 2. A driving mechanism, which utilizes thermal overstability of trapped subphotospheric modes, has been suggested for the oscillations. This complements penetrative convection and turbulent motions in the convection zone, which generate short­ wavelength waves. 3. Observations of oscillations and transients in active regions

Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments

Abstract: The intense magnetic field produced by the 20 MA Z accelerator is used as an impulsive pressure source to accelerate metal flyer plates to high velocity for the purpose of performing plate impact, shock wave experiments. This capability has been significantly enhanced by the recently developed pulse shaping capability of Z, which enables tailoring the rise time to peak current for a specific material and drive pressure to avoid shock formation within the flyer plate during acceleration. Consequently, full advantage can be taken of the available current to achieve the maximum possible magnetic drive pressure. In this way, peak magnetic drive pressures up to 490 GPa have been produced, which shocklessly accelerated 850μm aluminum (6061-T6) flyer plates to peak velocities of 34km∕s. We discuss magnetohydrodynamic (MHD) simulations that are used to optimize the magnetic pressure for a given flyer load and to determine the shape of the current rise time that precludes shock formation within the flyer during acceleration to peak velocity. In addition, we present results pertaining to plate impact, shock wave experiments in which the aluminum flyer plates were magnetically accelerated across a vacuum gap and impacted z-cut, α-quartz targets. Accurate measurements of resulting quartz shock velocities are presented and analyzed through high-fidelity MHD simulations enhanced using optimization techniques. Results show that a fraction of the flyer remains at solid density at impact, that the fraction of material at solid density decreases with increasing magnetic pressure, and that the observed abrupt decrease in the quartz shock velocity is well correlated with the melt transition in the aluminum flyer.

Diffusive Acceleration of Particles at Oblique, Relativistic, Magnetohydrodynamic Shocks

Abstract: Diffusive shock acceleration (DSA) at relativistic shocks is expected to be an important acceleration mechanism in a variety of astrophysical objects including extragalactic jets in active galactic nuclei and gamma-ray bursts. These sources remain good candidate sites for the generation of ultrahigh energy cosmic rays. In this paper, key predictions of DSA at relativistic shocks that are germane to the production of relativistic electrons and ions are outlined. The technique employed to identify these characteristics is a Monte Carlo simulation of such diffusive acceleration in test-particle, relativistic, oblique, magnetohydrodynamic (MHD) shocks. Using a compact prescription for diffusion of charges in MHD turbulence, this approach generates particle angular and momentum distributions at any position upstream or downstream of the shock. Simulation output is presented for both small angle and large angle scattering scenarios, and a variety of shock obliquities including superluminal regimes when the de Hoffmann-Teller frame does not exist. The distribution function power-law indices compare favorably with results from other techniques. They are found to depend sensitively on the mean magnetic field orientation in the shock, and the nature of MHD turbulence that propagates along fields in shock environs. An interesting regime of flat-spectrum generation is addressed; we provide evidence for it being due to shock drift acceleration, a phenomenon well known in heliospheric shock studies. The impact of these theoretical results on blazar science is outlined. Specifically, Fermi Large Area Telescope gamma-ray observations of these relativistic jet sources are providing significant constraints on important environmental quantities for relativistic shocks, namely, the field obliquity, the frequency of scattering, and the level of field turbulence.