MHD Shock Waves Research Articles

The structure of MHD shock waves in a thermally-radiating gas at high mach numbers

The structure of a strong MHD shock wave which radiates thermally downstream of the shock is studied by asymptotic expansion. The exact integral equation for radiation is adopted for the study. Hence, the optically thick (and thin), the general differential approximate and the exact integral equation solutions may now be compared.

MagnetoHydrodynamic shock waves in molecular clouds

The fluid dynamics of MHD shock waves in magnetized molecular gas is reviewed. The different types of shock solutions, and the circumstances under which the different types occur, are delineated. Current theoretical work on C∗- and J-type shocks, and on the stability of C-type shocks, is briefly described. Observations of the line emission from MHD shocks in different regions appear to be in conflict with theoretical expectations for single, plane-parallel shocks. Replacement of plane-parallel shocks by bow shocks may help reconcile theory and observation, but it is also possible that the observed shocks may not be “steady”, or that theoretical models have omitted some important physics.

MagnetoHydrodynamic shock waves in molecular clouds

The fluid dynamics of MHD shock waves in magnetized molecular gas is reviewed. The different types of shock solutions, and the circumstances under which the different types occur, are delineated. Current theoretical work on C∗- and J-type shocks, and on the stability of C-type shocks, is briefly described. Observations of the line emission from MHD shocks in different regions appear to be in conflict with theoretical expectations for single, plane-parallel shocks. Replacement of plane-parallel shocks by bow shocks may help reconcile theory and observation, but it is also possible that the observed shocks may not be “steady”, or that theoretical models have omitted some important physics.

A new class of solutions for interstellar magnetohydrodynamic shock waves

view Abstract Citations (48) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A New Class of Solutions for Interstellar Magnetohydrodynamic Shock Waves Roberge, W. G. ; Draine, B. T. Abstract An analysis is presented of the equations of motion for steady MHD shock waves proopagating in interstellar clouds, for boundary conditions that preclude C shocks. In addition to J shocks, in which the neutral fluid component becomes subsonic at an adiabatic jump front, the equations admit a new class of solutions, called C-asterisk shocks, in which the transition to subsonic flow occurs continuously at a sonic point. Numerical methods are developed for computing the structure of J and C-asterisk shocks propagating in diffuse interstellar clouds. The effects of chemical, ionization, and recombination processes are included in this treatment. An alternative numerical method, which uses artificial viscosity to facilitate integration through sonic points, is analyzed and shown to be invalid. A set of exemplary solutions, computed for realistic shock parameters, shows that C-asterisk shocks occur for a broad range of conditions relevant to diffuse interstellar clouds. Publication: The Astrophysical Journal Pub Date: February 1990 DOI: 10.1086/168421 Bibcode: 1990ApJ...350..700R Keywords: Computational Astrophysics; Equations Of Motion; Interstellar Gas; Magnetohydrodynamic Waves; Shock Wave Propagation; Interstellar Chemistry; Magnetoacoustics; Molecular Clouds; Astrophysics; HYDROMAGNETICS; INTERSTELLAR: MATTER; SHOCK WAVES full text sources ADS |

Corrugation instability in MHD shock waves

Corrugation instability in MHD shock waves

Propagation of spherical MHD shock-waves in self-gravitating gas

A theoretical model of shock-wave propagation has been studied in a heat-conducting and a self-gravitating medium. The effects of magnetic field has been taken into consideration. The shock is strong enough to neglect the ambient gas pressure. The variation of flow variables behind the shock have been investigated numerically.

A study of self-similar cylindrical MHD shock waves in monochromatic radiation

Theoretical model of cylindrical MHD shock waves are presented under the action of monochromatic radiation into non-uniform stellar atmosphere with a constant intensity on unit area. Gas is assumed to be grey and opaque and shock to be transparent.

Solar irradiance variations and magnetic field emergence

We have studied the association between different aspects of the magnetic field behaviour in large active regions and the solar irradiance variations. It was found that irradiance decreases were accompanied by enhanced flare and coronal mass ejection activity. A still better association seems to exist with broadband, long-lasting noise storm activity. This indicates that the enhanced generation of weak MHD shock waves causing noise storm in large interconnecting coronal loops is related to the appearance of irradiance dips.

Instability of the structure of strong oblique MHD cosmic-ray shocks

A criterion of the instability of a flow of a thermal plasma and cosmic rays in front of an oblique MHD shock wave with respect to short-wavelength magnetosonic disturbances is derived. The dependence of a cosmic-ray diffusion tensor on a plasma density and a large-scale magnetic field is taken into account. The most unstable disturbances propagate at an angle to the magnetic field if diffusion is strongly anisotropic. In some cases the most strong instability connects with the off-diagonal terms of the diffusion tensor.

Magnetohydrodynamic shocks in diffuse clouds. II - Production of CH(+), OH, CH, and other species

Molecule production and excitation of H<SUB>2</SUB> are studied for MHD shock waves propagating into diffuse molecular gas. Extensive calculations are carried out for a preshock density n<SUB>H</SUB> = 50 cm<SUP>-3</SUP> and for 0.5 ≤ H<SUB>2</SUB>/H ≤ 5. CH<SUP>+</SUP> column densities in excess of 10<SUP>13</SUP>cm<SUP>-2</SUP> can be produced by shocks with v<SUB>s</SUB> &gt; 9 km s<SUP>-1</SUP>; OH and CH are also produced in these shocks. The production of other species - in particular, CO, HCO<SUP>+</SUP>, CH<SUB>2</SUB><SUP>+</SUP>, and CH<SUB>3</SUB><SUP>+</SUP> - is discussed, and column densities of rotationally excited H<SUB>2</SUB> are computed. Specific shock models are proposed for the lines of sight to ζ Per, ο Per, and χ Oph, and the models are compared to available observations.

Magnetohydrodynamic shocks in diffuse clouds. I - Chemical processes

view Abstract Citations (95) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetohydrodynamic Shocks in Diffuse Clouds. I. Chemical Processes Draine, B. T. ; Katz, N. Abstract The chemistry of carbon- and oxygen-bearing species in MHD shock waves in diffuse interstellar clouds is examined, and the heating and cooling mechanisms in the shocked gas are discussed. Rate coefficients for endothermic and exothermic ion-neutral reactions are computed for conditions where the mean velocities of the ions and neutrals differ, resulting in a non-Maxwellian distribution of relative velocities, and energy-dependent cross sections are adopted for the principal reactions. Confirmation of the important effect on the ion abundance resulting from the endothermic reaction of C(+) with H2, followed by fast dissociative recombination, is obtained. Results for an illustrative model shock demonstrate that significant quantities of CH, OH, CH(+), CH2(+), CH3(+), and HCO(+) can be produced in MHD shocks in diffuse clouds, and that MHD shocks may play an important role in determining the overall abundance of CH(+). Publication: The Astrophysical Journal Pub Date: July 1986 DOI: 10.1086/164375 Bibcode: 1986ApJ...306..655D Keywords: Interstellar Chemistry; Magnetohydrodynamics; Molecular Clouds; Shock Waves; Abundance; Cooling; Cosmic Rays; Dissociation; Ionic Reactions; Photochemical Reactions; Reaction Kinetics; Astrophysics; HYDROMAGNETICS; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES; SHOCK WAVES full text sources ADS | Related Materials (2) Part 2: 1986ApJ...310..392D Part 3: 1986ApJ...310..408D

Numerical studies of particle acceleration at turbulent, oblique shocks with an application to prompt ion acceleration during solar flares

view Abstract Citations (117) References (52) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Numerical Studies of Particle Acceleration at Turbulent, Oblique Shocks with an Application to Prompt Ion Acceleration during Solar Flares Decker, R. B. ; Vlahos, L. Abstract In this paper we address the problem of charged particle acceleration at oblique, fast-mode collisionless MHD shock waves when magnetic turbulence exists in the regions upstream and downstream of the shock. Specifically, we consider how the acceleration rate depends upon the angle θ1 between the shock normal and the mean upstream magnetic field. To handle the general situation where θ1, the turbulence level, the shock strength, and the energy of injected particles can assume a range of values, we perform fully relativistic, test particle simulations that involve integrating along particle phase space orbits in the shock turbulence system. As an application of the numerical code, we study proton acceleration at shocks under conditions appropriate to the lower solar corona to simulate prompt ion acceleration during solar flares. Particles undergo shock acceleration through a combination of the shock drift and first-order Fermi processes. For protons injected at 100 keV and left in the system for 500 gyroperiods (∼ 7 ms in a 50 G magnetic field) we obtain the following results: (1) the percentage of protons accelerated above 10 MeV within 7 ms increases with increasing θ1, from 0% at θ1 = 0° to a maximum of 9% at θ1= 60° (2) the case θ1 = 75° produces the largest, most rapid energy gains, with ∼1% of the protons accelerated above 50 MeV; (3) for 45° < θ1 < 75° , a separate proton population with energies between 100 keV and 10 MeV is produced during a superfast acceleration phase lasting only ∼ 10 gyroperiods (∼ 100 μs) after injection; (4) we compare the peak energy reached at O = 0 and energy spectrum produced at θ1 = 75° with predictions from theoretical models, and find reasonable agreement, although discrepancies do exist. We discuss the implications of the numerical results as they pertain to time constraints and collisional loss processes during shock acceleration in the solar corona. Publication: The Astrophysical Journal Pub Date: July 1986 DOI: 10.1086/164381 Bibcode: 1986ApJ...306..710D Keywords: Magnetohydrodynamic Turbulence; Oblique Shock Waves; Particle Acceleration; Solar Flares; Computational Astrophysics; Energy Spectra; Power Spectra; Solar Protons; Solar Physics; HYDROMAGNETICS; PARTICLE ACCELERATION; SHOCK WAVES; SUN: FLARES; TURBULENCE full text sources ADS |

Development of a real-time algorithm for detection of solar wind discontinuities

A study is presented of an algorithm, based on the statistical analysis of interplanetary magnetic field data, for the real-time detection of discontinuities in the solar wind. The analysis is based on the application of a ‘sliding’ or ‘searching’ algorithm together with a minimum variance treatment. A demonstration is given for a Pioneer data set from 29 August, 1966. This particular data set was chosen because it has been extensively studied (in terms of the fast forward MHD shock wave) by previous groups who utilized the three-dimensional Rankine-Hugoniot shock equations after identifying the shock on a subjective basis. Our procedure differs in that it is perfectly objective because of its ‘searching’ technique. For this particular data set we identify the original shock with excellent agreement with the earlier studies of its normal vector, as well as an additional shock and tangential discontinuity, neither of which had been identified previously.

A parametric study of slow shock Rankine‐Hugoniot solutions and critical Mach numbers

We present a parametric survey of the Rankine‐Hugoniot solutions for slow MHD shock waves. We also calculate the parametric dependences of the critical Mach numbers above which resistivity or thermal conduction alone can no longer provide all the shock dissipation, as well as the Mach number above which the flux of shock‐catching ions can be significant. Slow shocks rival fast shocks in the degree to which their properties might depend upon upstream parameters.

Solar wind observations associated with a slow forward shock wave at 0.31 AU

A slow forward shock‐type discontinuity observed by Helios 1 on day 147, 1980, at 0.31 AU is analyzed in detail, employing high‐resolution (40.5 s) plasma and magnetic field measurements. It is shown within the bounds of uncertainties in the various parameters (1) that this event is not an MHD discontinuity, as the total pressure balance is violated and as the entropy increases across this structure even after inclusion of the alpha particles, (2) that the Hada‐Kennel relations, i.e., Ti/Te &lt; 1 and β &lt; 1, are satisfied upstream of this event even after inclusion of the alpha particles, indicating that steepening of MHD slow waves dominates Landau damping, (3) that the Rankine‐Hugoniot relations are fulfilled to an absolute difference of &lt;10−5 by employing any possible combination of an upstream with a downstream time‐averaged data set, (4) that the MHD evolutionary conditions are satisfied for any solution to the jump conditions, and (5) that the shock speed, the amplitudes, and the relative amplitudes associated with this event are absolutely comparable to those derived earlier for fast mode shock waves within 1 AU. It is shown that this event is an almost quasi‐perpendicular slow forward MHD shock wave.

SOME COMMENTS ON THE STABILITY THEORY OF MHD SHOCK WAVES

The stability (evolutionarity) problem for a kind of MHD shock waves is discussed in this paper. That is to solve the interaction problem of MHD shock waves with (2-dimensional) oblique incident disturbances. In other words, the result of gasdynamic shocks is generalized to the case of MHD shocks. The previous conclusion of stability theory of MHD shock waves obtained from the solution of interaction problem of MHD shock wave with (one-dimensional) normal shock wave is that only fast and slow shocks are stable, and intermediate shocks are unstable. However, the results of this paper show that when the small disturbances are the Alfven waves a new stability condition which is related to the parameters in front of and behind the shock wave is derived. When the disturbances are entropy wave and fast and slow magneto acoustic waves the stability condition is related to the frequency of small disturbances. As the limiting ease, i. e. when a normal incident (reflection, refraction) is considered, the fast and slow shocks are unstable. The results also show that the conclusion drawn by Kontorovich is invalid for the stability theory of shock waves.

Dimensionless jump conditions for MHD shock waves

In this paper, jump conditions for MHD shock waves are expressed in a simple form in terms of dimensionless variables, and a generalized analytical solution valid for both MHD and gas-dynamical shocks is given. The cases of oblique, perpendicular and parallel MHD shocks are discussed in detail.

The structure of MHD shock waves

The MHD equations are used to discuss the formation and structure of shock waves in channel flow of an ionized gas with finite viscosity, thermal and electrical conductivity. The fundamental resulting complicated nonlinear differential equations are solved exactly, and the shock structure is determined completely. A necessary compatibility condition for the evolution of shock waves is obtained in the form:wherePrandPrmare ordinary and magnetic Prandtl numbers and γ the ratio of specific heats. The flow quantities and parameters are found to depend, in a non-analytical manner, onPrandPrm, so that special cases of interest are to be treated separately. Numerical results are given and discussed.

Magnetohydrodynamic shock waves in molecular clouds

The fluid dynamics of MHD shock waves in magnetized molecular gas is reviewed. The different types of shock solutions, and the circumstances under which the different types occur, are delineated. Current theoretical work on C*- and J-type shocks, and on the stability of C-type shocks, is briefly described. Observations of the line emission from MHD shocks in different regions appear to be in conflict with theoretical expectations for single, plane-parallel shocks. Replacement of plane-parallel shocks by bow shocks may help reconcile theory and observation, but it is also possible that the observed shocks may not be “steady”, or that theoretical models have omitted some important physics

On the evolution of normal ionizing shock waves in helium

The generation, structure and propagation of one-dimensional ionizing MHD shock waves in helium under a pressure of 100 mTorr are investigated with the help of numerical simulation. The normal magnetic field varies within 3 to 10 kG and the longitudinal magnetic field varies up to 2·5 kG. The model includes the kinetics of ionization and photo-processes. If a solid conducting piston is a source of perturbation, it may give rise to generation and further development of an MHD switch-on wave. Its evolution at an advanced stage depends weakly on the source. The curves for the dependence of the shock speed on time and the driving magnetic field as well as the profiles for the main quantities are presented. A possibility of comparison with real experiments is discussed. Algorithms based on Godunov's sliding meshes and the imbedding methods are used for numerical simulation.