SEARCH · Search NASA

Results for “collisionless”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3

All sources NASA NTRS DOE OSTI

DOE OSTI2022

Collaborative Research: Particle Simulation of Collisionless Magnetic Reconnection Under Finite Guide Field

Magnetic reconnection is believed to be one of the most fundamental physical processes in laboratory and space plasmas. Fundamental physics of collisionless reconnection under general conditions of finite guide magnetic field, realistic electron-to-ion mass ratio, and in 3D geometries is still not adequately understood. The major goal of this project is to study the physics of collisionless magnetic reconnection in a current sheet under a finite guide field and with a realistic ion-to-electron mass ratio mi/me. Under the support of this DoE grant, we have made significant progress in (1) the investigation of linear and nonlinear current sheet instabilities under a wide range of guide field using the 3-D gyro kinetic electron and fully kinetic ion (GeFi) particle simulation model and (2) the investigation of fully nonlinear reconnection process with a strong guide field with the GeFi simulations. Furthermore, (3) we have extended the study to the realistic geometry of the magnetopause using 3D global hybrid simulations. This study, from local to global scales and for 3-D physics at plasma boundaries, has provides advanced theoretical understanding of the fundamental physics of magnetic reconnection in laboratory and space plasma regimes on multi-scales.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1971

Collisionless solar wind protons: A comparison of kinetic and hydrodynamic descriptions

Kinetic and hydrodynamic descriptions of a collisionless solar wind proton gas are compared. Heat conduction and viscosity are neglected in the hydrodynamic formulation but automatically included in the kinetic formulation. The results of the two models are very nearly the same, indicating that heat conduction and viscosity are not important in the solar wind proton gas beyond about 0.1 AU. It is concluded that the hydrodynamic equations provide a valid description of the collisionless solar wind protons, and hence that future models of the quiet solar wind should be based on a hydrodynamic formulation.

Leer, E.↗

NASA NTRS1972

Stationary collector in a collisionless plasma

The effect of a neutral collecting body in a collisionless plasma on the plasma distribution in its immediate neighborhood is discussed. When a magnetic field is present an electron plasma with a neutralizing background charge has empty velocity space regions at points near a collector even though the distribution is Maxwellian far from the collector. For a thin cylinder, the collected collisionless plasma current is a function of the angle between the cylinder axis and the magnetic field with the minimum current collected when the cylinder and field lines are parallel.

Grebowsky, J. M.↗

NASA NTRS1980

The role of microturbulence on collisionless reconnection

The linear, non-linear and anomalous transport properties associated with various microinstabilities driven by cross field currents in reconnecting geometries are reviewed. An assessment of their role in collisionless tearing based on analytic theory, computer simulations and experimental evidence, supports the dominant role of lower hybrid waves. The relevance of microturbulence on macroscopic stationary and time dependent models of merging is presented. It is concluded that a fluid-numerical simulation approach that includes (at each space and time step) the effects of anomalous transport in a self consistent manner, similar to the one used for laboratory collisionless shocks, represents the best method for studying and modeling the details of the reconnection process.

Papadopoulos, K.↗

NASA NTRS1983

Plasma and energetic particle structure of a collisionless quasi-parallel shock

The quasi-parallel interplanetary shock of November 11-12, 1978 from both the collisionless shock and energetic particle points of view were studied using measurements of the interplanetary magnetic and electric fields, solar wind electrons, plasma and MHD waves, and intermediate and high energy ions obtained on ISEE-1, -2, and -3. The interplanetary environment through which the shock was propagating when it encountered the three spacecraft was characterized; the observations of this shock are documented and current theories of quasi-parallel shock structure and particle acceleration are tested. These observations tend to confirm present self consistent theories of first order Fermi acceleration by shocks and of collisionless shock dissipation involving firehouse instability.

Kennel, C. F.↗

NASA NTRS1983

Large-amplitude hydromagnetic waves in collisionless relativistic plasma - Exact solution for the fast-mode magnetoacoustic wave

An exact nonlinear solution is found to the relativistic kinetic and electrodynamic equations (in their hydromagnetic limit) that describes the large-amplitude fast-mode magnetoacoustic wave propagating normal to the magnetic field in a collisionless, previously uniform plasma. It is pointed out that a wave of this kind will be generated by transverse compression of any collisionless plasma. The solution is in essence independent of the detailed form of the particle momentum distribution functions. The solution is obtained, in part, through the method of characteristics; the wave exhibits the familiar properties of steepening and shock formation. A detailed analysis is given of the ultrarelativistic limit of this wave.

Barnes, A.↗

NASA NTRS1984

Dielectric and permeability effects in collisionless plasmas

Using the unabridged Maxwell equations (including vectors D, E and H) new effects in collisionless plasmas are uncovered. In a steady state, it is found that spatially varying energy density of the electric field (E perpendicular) orthogonal to B produces electric current leading, under certain conditions, to the relationship P perpendicular + B(2)/8 pi-epsilon E perpendicular(2)/8 pi = constant, where epsilon is the dielectric constant of the plasma for fields orthogonal to B. In steady state quasi-two-dimensional flows in plasmas, a general relationship between the components of electric field parallel and perpendicular to B is found. These effects are significant in geophysical and astrophysical plasmas. The general conditions for a steady state in collisionless plasma are deduced. With time variations in a plasma, slow compared to ion-gyroperiod, there is a general current, (j-asterisk), which includes the well-known polarization current, given by J-asterisk = d/dt (E x M) + (P x B) x B B(-2) where M and P are the magnetization and polarization vectors respectively.

Cole, K. D.↗

NASA NTRS1984

The physical mechanisms of subcritical collisionless shock-wave formation

The key process in shock wave formation is related to energy dissipation, and the nature of the operative dissipation mechanism determines the basic character of the resulting shock. In the case of collisionless plasmas, the primary problem consists in the identification of the processes which can provide the necessary dissipation in relatively short spatial scale lengths. The present investigation is concerned with the various collisionless dissipation mechanisms which can operate in weak shocks, taking into account the effects of different mechanisms on shock structure. Particular attention is given to a restricted class of quasi-perpendicular low beta low Mach number shocks. Such shocks are traditionally called 'laminar shocks'. Resistive shocks are considered along with subcritical shocks observed with the aid of the ISEE spacecraft.

Mellott, M. M.↗

NASA NTRS1985

Collisionless shocks in the heliosphere: A tutorial review

An update is presented on current knowledge of collisionless shocks in the heliosphere. The individual papers address: a quarter century of collisionless shock research, some macroscopic properties of shock waves in the heliosphere, microinstabilities and anomalous transport, and acceleration of energetic particles.

Stone, Robert G.↗

NASA NTRS1987

The interaction of quasiperpendicular shock waves in a collisionless plasma

The interaction between collisionless shocks is examined from a kinetic viewpoint using computer simulations of shock collisions spanning the range of ambient plasma and shock parameters typical of the interplanetary medium. It is found that the interaction between two subcritical, perpendicular shocks is largely fluidlike with no evidence for accelerated particles. The electromagnetic fields associated with each shock, but not the plasmas, pass through each other during the collision. This interaction is due to self-consistently generated electric fields which adjust to the value needed to bring to rest the two incoming streams. The interaction between two supercritical perpendicular collisionless shocks is similar to the subcritical case, in that the electromagnetic fields associated with each shock are transmitted whereas particles are not. However, the interaction is also highly dynamic with substantial acceleration of ions. Increasing the Mach number gives rise to an increasing number of energetic particles.

Cargill, P. J.↗

NASA NTRS1988

Collisionless shock formation and the prompt acceleration of solar flare ions

The formation mechanisms of collisionless shocks in solar flare plasmas are investigated. The priamry flare energy release is assumed to arise in the coronal portion of a flare loop as many small regions or 'hot spots' where the plasma beta locally exceeds unity. One dimensional hybrid numerical simulations show that the expansion of these 'hot spots' in a direction either perpendicular or oblique to the ambient magnetic field gives rise to collisionless shocks in a few Omega(i), where Omega(i) is the local ion cyclotron frequency. For solar parameters, this is less than 1 second. The local shocks are then subsequently able to accelerate particles to 10 MeV in less than 1 second by a combined drift-diffusive process. The formation mechanism may also give rise to energetic ions of 100 keV in the shock vicinity. The presence of these energetic ions is due either to ion heating or ion beam instabilities and they may act as a seed population for further acceleration. The prompt acceleration of ions inferred from the Gamma Ray Spectrometer on the Solar Maximum Mission can thus be explained by this mechanism.

Cargill, P. J.↗

NASA NTRS1988

A quadratic-form analysis of the collisionless tearing mode

A method for studying collisionless tearing mode properties in cases where complicated orbit integrals must be evaluated is presented. The method is based on a quadratic form of a self-adjoint integrodifferential operator. The collisionless tearing instability in a non-Maxwellian neutral sheet is used to illustrate the advantages of the present method.

Chen, J.↗

NASA NTRS1990

Collisionless tearing instability in magnetotail plasmas

The problem of the linear stability of collisionless tearing modes in the earth's magnetotail is revisited. It is found that the collisionless tearing mode is linearly unstable with wavelengths of the order of 10 R(E). It is shown that an important feature neglected in earlier theories is a nonzero equilibrium B(y)-field. The physics of the instability is elucidated in the context of a simple slab model and a sheared parabolic model which is representative of the magnetotail in which all three components of the magnetic field are nonzero. The dispersion equation for the instability is obtained by a boundary-layer analysis. The implications of the theory for recent observations on current disruption and diversion during substorms is discussed.

Wang, Xiaogang↗

NASA NTRS1990

Magnetic reconnection in collisionless plasmas - Prescribed fields

The structure of the dissipation region during magnetic reconnection in collisionless plasma is investigated by examining a prescribed two-dimensional magnetic x line configuration with an imposed inductive electric field E(y). The calculations represent an extension of recent MHD simulations of steady state reconnection (Biskamp, 1986; Lee and Fu, 1986) to the collisionless kinetic regime. It is shown that the structure of the x line reconnection configuration depends on only two parameters: a normalized inductive field and a parameter R which represents the opening angle of the magnetic x lines.

Burkhart, G. R.↗

NASA NTRS1991

Particle simulations of collisionless magnetic reconnection in magnetospheric current sheets

The driven collisionless magnetic reconnection processes at the dayside magnetopause and in the magnetotail are investigated through 2.5 D particle simulations. During reconnection, magnetic field configurations with both a single X line and multiple X lines are obtained. Magnetic reconnection enhancements, which are indicated by the peaks in the inductive electric field, are observed in the simulations. Coincident with the magnetic reconnection enhancements, bursts of energetic particles, electrons with an energy spectrum of about E exp -3.8 and ions with an energy spectrum of about E exp -4.4, are generated during the reconnection processes. Field-aligned particle heat fluxes are also generated. Intense plasma waves associated with the reconnection processes are observed. It is found that the fluctuating magnetic field has a power spectrum of about f exp -3.8 and the fluctuating electric field has a power spectrum of about f exp -1.8, respectively. A review of the simulation studies of driven collisionless magnetic reconnection and slow shocks in the magnetospheric current sheets is presented.

Ding, D. Q.↗

NASA NTRS1992

The beta dependence of the collisionless tearing instability at the dayside magnetopause

The beta dependence (where beta is the ratio between plasma pressure to magnetic pressure) of the collisionless tearing instability at the dayside magnetopause current sheet was investigated analytically for several current sheet models. The results show that, when the asymmetry of the dayside magnetopause current sheet is taken into account, the beta dependence of the collisionless tearing instability at the dayside magnetopause is controlled by the magnetic field profile across the magnetopause current sheet.

Ding, D. Q.↗

NASA NTRS1992

Particle simulations of driven collisionless magnetic reconnection at the dayside magnetopause

Particle simulations (including both ions and electrons) of the driven collisionless magnetic reconnection process at the earth's dayside magnetopause were carried out using a specially developed two-and-one-half-dimensional electromagnetic particle code with a driven inflow boundary and an open outflow boundary. The simulations were used to examine many features associated with the dayside magnetic reconnection process, including the acceleration and heating of particles and the generation of energetic particles and particle heat flux. In addition, the fluctuating electromagnetic field associated with a driven collisionless magnetic reconnection was examined and compared with the satellite observations of flux transfer events at the day-side magnetopause.

Ding, D. Q.↗

NASA NTRS1992

The coalescence instability in collisionless plasmas

The process whereby magnetic island structures tend to merge into larger units is known as magnetic island coalescence. This coalescence process is investigated for the case of a collisionless plasma by means of 2D particle simulations. The pairwise merging of a preformed island chain leads to a gentle redistribution of the density and current density profiles. The nonlinear evolution of the collisionless tearing instability in a long system is found to lead to a much more violent coalescence process resulting in the formation of a vacuum X region. This stage is characterized by enhanced growth rates for the long-wavelength modes that exceed the maximum linear tearing growth rate, super-Alfvenic flows away from the reconnection X line, substantial bulk heating in the parallel direction, and the formation of energetic particle tails in the direction of the inductive electric field.

Pritchett, P. L.↗