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NASA NTRS1990

Dispersive and viscous scale lengths in the two-stage ion heating at quasi-parallel collisionless shocks

A simulation of quasi-parallel (QP) collisionless shock waves is used to deduce the dispersive and viscous scale lengths of QP shocks. The findings were found to be consistent with the observational results obtained by Scudder et al. (1984) and Kennel et al. (1984). It is shown that the upstream wavelength is the dispersive scale length which governs the magnetic-field structure of the shock. It is also shown that the two-stage ion heating obtained by Mandt and Kan (1988) is an intrinsic characteristic of the ion heating process in QP shocks.

Mandt, M. E.↗
NASA NTRS1985

Numerical simulations of quasi-perpendicular collisionless shocks

Numerical simulations of collisionless quasi-perpendicular shock waves are reviewed. The strengths and limitations of these simulations are discussed and their experimental (laboratory and spacecraft) context is given. Recent simulation results are emphasized that, with ISEE bow shock observations, are responsible for recent progress in understanding quasi-steady shock structure.

Goodrich, C. C.↗
NASA NTRS1986

Plasma wave signatures of collisionless shocks and the role of plasma wave turbulence in shock formation

Observations of the plasma waves associated with collisionless shocks are reviewed, and the understanding of their generation mechanisms and their importance to shock physics are summarized. The emphasis is on waves generated directly at the shock, especially ion acoustic and lower-hybrid-like modes. The observations are discussed in the context of shock structure, with attention given to the distinctions between waves generated in the shock foot and ramp. The behavior of resistive, dispersive, and supercritical quasi-perpendicular shocks is contrasted. Evidence for the operation of various generation mechanisms, including interactions with cross-field currents, gyrating reflected ions, and field-aligned electron beams, are summarized. The various forms of plasma heating which are actually observed are outlined, and the role of the various wave modes in this heating is discussed. Conclusion, it is argued that, while plasma wave turbulence may play a vital role in plasma heating for some special shocks, it is of second-order importance in most cases.

Mellott, M. M.↗
NASA NTRS1991

Electrostatic potential jump across fast-mode collisionless shocks

The electrostatic potential jump across fast-mode collisionless shocks is examined by comparing published observations, hybrid simulations, and a simple model, in order to better characterize its dependence on the various shock parameters. In all three, it is assumed that the electrons can be described by an isotropic power-law equation of state. The observations show that the cross-shock potential jump correlates well with the shock strength but shows very little correlation with other shock parameters. Assuming that the electrons obey an isotropic power law equation of state, the correlation of the potential jump with the shock strength follows naturally from the increased shock compression and an apparent dependence of the power law exponent on the Mach number which the observations indicate. It is found that including a Mach number dependence for the power law exponent in the electron equation of state in the simple model produces a potential jump which better fits the observations. On the basis of the simulation results and theoretical estimates of the cross-shock potential, it is discussed how the cross-shock potential might be expected to depend on the other shock parameters.

Mandt, M. E.↗
NASA NTRS1985

Plasma heating at collisionless shocks due to the kinetic cross-field streaming instability

Heating at collisionless shocks due to the kinetic cross-field streaming instability, which is the finite beta (ratio of plasma to magnetic pressure) extension of the modified two stream instability, is studied. Heating rates are derived from quasi-linear theory and compared with results from particle simulations to show that electron heating relative to ion heating and heating parallel to the magnetic field relative to perpendicular heating for both the electrons and ions increase with beta. The simulations suggest that electron dynamics determine the saturation level of the instability, which is manifested by the formation of a flattop electron distribution parallel to the magnetic field. As a result, both the saturation levels of the fluctuations and the heating rates decrease sharply with beta. Applications of these results to plasma heating in simulations of shocks and the earth's bow shock are described.

Winske, D.↗
NASA NTRS1985

Subcritical collisionless shock waves

The development history of theoretical accounts of low Mach number collisionless shock waves is related to recent observational advancements, with attention to weaker shocks in which shock steepening is limited by dispersion and/or anomalous resistivity and whose character is primarily determined by the dispersive properties of the ambient plasma. Attention has focused on nearly perpendicular shocks where dispersive scale lengths become small and the associated cross-field currents become strong enough to generate significant plasma wave turbulence. A number of oblique, low Mach number bow shocks have been studied on the basis of data from the ISEE dual spacecraft pair, allowing an accurate determination of shock scale lengths.

Mellott, M. M.↗
NASA NTRS1970

Dispersion discontinuities of strong collisionless shocks

Linear fluid equations are used to estimate wave train properties of strong collisionless shocks. Fast shocks exhibit several dispersion changes with increasing Mach number. For perpendicular propagation into a finite-beta plasma, an ion cyclotron radius trailing wave train exists only for (M sub F)2 is smaller than 2. Oblique fast shocks have a leading ion inertia wave train if M sub A is smaller than root of M(+)/M(-) cos theta/2 and a trailing electron inertia train if M sub A is greater than root of M(+)/M(-) cos theta/2. If the downstream sound speed exceeds the flow speed, linear wave theory predicts a trailing ion acoustic structure which probably resides within the magnetic shock. For a turbulent shock model in which an effective electron-ion collision frequency exceeds the lower hybrid frequency, ions decouple from the magnetic field; the shock wave train now trails with electron inertia and electron gyroradius lengths. Comparisons of this turbulent model and observations on the earth's bow shock are made.

Coroniti, F. V.↗
NASA NTRS1994

On the cross-field diffusion of ions in one- and two-dimensional hybrid simulations of collisionless shocks

It can be demonstrated analytically that under certain geometries used in numerical simulations of collisionless shocks in which there is at least one ignorable spatial coordinate, the transport of particles across the magnetic field is essentially zero. This notion is tested using one- and two-dimensional hybrid simulations (kinetic ions/fluid electrons). We find, as the theorem predicts, the particles treated kinetically are tied to the same field line on which they start.

Giacalone, Joe↗
NASA NTRS1985

A quarter century of collisionless shock research

This review highlights conceptual issues that have both governed and reflected the direction of collisionless shock research in the past quarter century. These include MHD waves and their steepening, the MHD Rankine-Hugoniot relations, the super-critical shock transition, nonlinear oscillatory wave trains, ion sound anomalous resistivity and the resistive-dispersive transition for subcritical shocks, ion reflection and the structure of supercritical quasi-perpendicular shocks, the earth's foreshock, quasi-parallel shocks, and finally, shock acceleration processes.

Kennel, C. F.↗
NASA NTRS1987

On the noncoplanarity of the magnetic field within a fast collisionless shock

Within the magnetic ramp of fast collisionless plasma shocks observed with spacecraft instruments and simulated numerically, the magnetic field undergoes an excursion out of the plane of coplanarity. This rotation is consistently in the direction such that the electrostatic potential jump across the shock, as measured in the de Hoffman-Teller frame of the reference (HTF), is about 2-6 times smaller than the electrostatic potential jump measured in the normal incidence frame. The preferred direction is consistent with a basic whistler mode transition between the upstream and downstream orientations. The potential jump in the HTF is considerably smaller than the change in bulk flow energy across the shock, confirming the recent suggestion that magnetic forces contribute importantly to the slowing of the plasma in that frame. A further consequence is that suprathermal particles leaking back into the upstream region across the shock do not gain much energy from the cross-shock electric field.

Thomsen, M. F.↗
NASA NTRS1988

Multi-spacecraft observations of collisionless shocks

The ability to make simultaneous, multipoint measurements of field and plasma properties has substantially increased knowledge of the structure and physics of collisionless shocks in space. In addition to allowing the determination of the large-scale macroscopic structure of such shocks as well as reliable estimates of shock normal direction, multipoint measurements have made it possible to measure shock thicknesses and speeds under a variety of upstream conditions. This ability has enabled a number of quantitative analyses of shock structure, including the confirmation of the dispersive whistler-mode character at low Mach numbers, the assessment of the relative importance of magnetic and electrostatic forces within the shock, and a detailed examination of possible dissipation mechanisms.

Thomsen, M. F.↗
NASA NTRS1993

Collisionless shock structures of Earth and other planets

This report summarizes the closing segment of our multi-spacecraft, multi-instrument study of collisionless shock structure. In this last year of our study, we have necessarily concentrated on subjects that limited time and remaining resources could be expected to bring to reasonable stopping points, if not full conclusions. Our attention has been focused therefore on matters that were either well underway when the year began or that could be expected to yield rapidly completed reports publishable quickly in abbreviated versions. Contemporary publication delays prevent any new initiatives from reaching the literature within the year in the best of circumstances. The topics that fell into these categories were detailed plasma wave (pw) phenomenology in slow shocks in the Earth's distant geomagnetic tail, instantaneous orientations of theta(sub Bn) in quasiparallel (Q(sub parallel)) shock structure, and a comprehensive overview of the relationship between structural ULF waves in the Qll shock environment and waves in the magnetosphere, i.e. geomagnetic ULF pulsations. The remainder of this report describes our freshly completed results, discusses two related investigations of pw waves in the foreshock and magnetosheath, and appends the abstracts of published papers and the texts of papers in press.

Greenstadt, Eugene W.↗
NASA NTRS1989

Cyclic behavior at quasi-parallel collisionless shocks

Large scale one-dimensional hybrid simulations with resistive electrons have been carried out of a quasi-parallel high-Mach-number collisionless shock. The shock initially appears stable, but then exhibits cyclic behavior. For the magnetic field, the cycle consists of a period when the transition from upstream to downstream is steep and well defined, followed by a period when the shock transition is extended and perturbed. This cyclic shock solution results from upstream perturbations caused by backstreaming gyrating ions convecting into the shock. The cyclic reformation of a sharp shock transition can allow ions, at one time upstream because of reflection or leakage, to contribute to the shock thermalization.

Burgess, D.↗
NASA NTRS1988

Noncoplanar magnetic fields at collisionless shocks - A test of a new approach

The expressions derived by Jones and Ellison (1987) (JE) for the spatial integral of the noncoplanar field component in a fast-mode collisionless shock and for the magnitudes of the magnetic-field rotation and potential drop difference are evaluated on the basis of ISEE 1 and 2 observations in the earth bow shock and numerical simulations using the hybrid code of Leroy et al. (1981 and 1982). The data and simulation results are presented graphically and shown to be consistent with the JE hypothesis, that the rotation and drop difference are due to unequal electron and ion masses. The rotations and potential drop differences predicted by the JE equations, however, are found to be accurate only for subcritical shocks at low Mach numbers; the underestimation of the values for hypercritical shocks is attributed to neglect of the ion current from reflected gyrating ions.

Gosling, J. T.↗
NASA NTRS1982

ISEE work on collisionless shocks - CDAW-3, the meeting and the results July 23-26, 1979

At CDAW(Coordinated Data Analysis Workshop)-3, data from ISEE-1 and -2 were used, and 17 data sets were incorporated into a total of 44 megabytes of disk storage. The presence of ISEE-1 and -2 at variable separation distances has made it possible to settle the question of shock motion and to obtain shock thicknesses. A full range of wave instrumentation was available to study upstream waves and turbulence, and energetic particle investigations made it possible to obtain good observations of shock-induced particle acceleration. It is concluded that these improved observations have resulted in a revival of interest in the study of the collisionless shock, both as an experimental and theoretical research problem. It is expected that these investigations will lead to a better understanding of how to use the earth's bow shock to provide well-defined and theoretically useful examples for study.

Ogilvie, K. W.↗