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Results for “Collision frequency”

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NASA NTRS1977

An experiment to measure variations in nighttime E-region neutral concentration

An experimental procedure is demonstrated which holds promise for the nighttime measurement of ion-neutral collision frequencies or net neutral concentration in the E-region by combining incoherent scatter data with appropriate photometric measurements. Data illustrating the technique are applied to comparing relative variations in atomic oxygen concentrations (from the E-region component of the O I 5577 A emission) near 100 km to those in the net neutral concentration obtained from combined incoherent scatter data and neutral temperature data extracted from Fabry-Perot measurement of the O I 5577 A line profile.

Cogger, L. L.↗

NASA NTRS1978

On the relation between phase path, group path and attenuation in a cold absorbing plasma

Consideration is given to a cold absorbing plasma in which the collision frequency is zero. Expressions are developed which relate the attenuation and the group and phase refractive indices. It is found that because the expressions for the group and phase refractive indices and the imaginary part of the refractive index are closely related in form, the attenuation is related to the difference between the group and phase paths. Numerical calculations have derived approximations which significantly increase the range of known approximations of this type.

Bennett, J. A.↗

NASA NTRS1978

D-region differential-phase measurements and ionization variability studies

Measurements of electron densities in the D region are made by the partial-reflection differential-absorption and differential-phase techniques. The differential-phase data are obtained by a hard-wired phase-measuring system. Electron-sensity profiles obtained by the two techniques on six occasions are plotted and compared. Electron-density profiles obtained at the same time on 30 occasions during the years 1975 through 1977 are averaged to form a single profile for each technique. The effect of varying the assumed collision-frequency profile on these averaged profiles is studied. Time series of D-region electron-sensity data obtained by 3.4 minute intervals on six days during the summer of 1977 are examined for wave-like disturbances and tidal oscillations.

Weiland, R. M.↗

NASA NTRS1981

Rocket experiments

Payloads designed to study the ionization and the ionizing sources (energetic particles) during periods of auroral activity are described. These include a probe experiment to measure the variation in electron concentration, to measure electron temperature, and to measure the vehicle potential; a propagation experiment to obtain the electron concentration and the electron collision frequency; and an energetic particle experiment to measure particle energy spectra.

Source record↗

NASA NTRS1981

Fluid model of plasma outside a hollow cathode neutralizer

The present study analyzes the capability of a fluid model of electron transport to explain observed properties of the external plasma of a hollow cathode neutralizer used to neutralize beams emerging from ion thrusters. Calculations reported here show that when the effective collision frequency in such a model is near the plasma frequency, the resulting electric potential and electron temperature variations are in qualitative agreement with values measured in the plume mode of the hollow cathode. Both theory and experiment show strong variations of temperature and potential within a few centimeters of the cathode orifice.

Parks, D. E.↗

NASA NTRS1981

Nonthermal line profiles from O/1S/ in the thermospheric nightglow

Generalized composite nonthermal line profiles are calculated for the thermospheric nighttime emission from O(1S) metastable atoms based on an extension to the simple relaxation model of Hays and Walker (1966). It is shown that the Dynamics Explorer Fabry-Perot interferometer will enable accurate values to be obtained for the branching ratio of the O(1S) production reaction and for the excitation exchange collision frequency.

Killeen, T. L.↗

NASA NTRS1982

Dynamics of magnetosphere-ionosphere coupling including turbulent transport

A two dimensional two-fluid MHD model including anomalous resistivity was used to investigate the dynamics of magnetosphere-ionosphere coupling. When a field-aligned current is generated on auroral field lines, the disturbance propagates towards the ionosphere in the form of a kinetic Alfven wave. When the current exceeds a critical value, microscopic turbulence is produced, which modifies the propagation of the Alfven wave. This process is modeled by a nonlinear collision frequency, which increases with the excess of the drift velocity over the critical value. Turbulence leads to absorption and reflection of the Alfven wave, partially decoupling the generator from the ionosphere. The approach to a steady-state is strongly dependent on the presence or absence of the turbulence. The current is self-limiting, since a current in excess of critical causes a diffusion of the magnetic field perturbation and a reduction of current.

Lysak, R. L.↗

NASA NTRS1982

Orbital interactions - A new geometrical formalism

The geometry of encounters between two bodies on independent Keplerian orbits around a third body is considered by a novel analysis, which avoids approximations made in previous studies. For the case of most of the applications considered, the formulas for collision frequencies and orbital element rates of change due to close approaches which comprise the method agree with past results. It is shown that the method can be extended to such other applications as the consideration of oscillations in orbital elements due to secular perturbations, and the computation of probabilities of escape from the system.

Greenberg, R.↗

NASA NTRS1983

Dynamics of magnetosphere-ionosphere coupling including turbulent transport

The dynamics of magnetosphere-ionosphere coupling has been investigated by means of a two-dimensional two-fluid MHD model including anomalous resistivity. When field-aligned current is generated on auroral field lines, the disturbance propagates toward the ionosphere in the form of a kinetic Alfven wave. When the current exceeds a critical value, microscopic turbulence is produced, which modifies the propagation of the Alfven wave. This process is modeled by a nonlinear collision frequency, which increases with the excess of the drift velocity over the critical value. The system evolves toward an electrostatic structure, with the perpendicular electric field having a shorter scale than the field-aligned current. The approach to a steady state is strongly dependent on the presence or absence of the turbulence and on the boundary conditions imposed in the generator. As current is increased or scale size is decreased, the turbulent region reflects and absorbs most of the Alfven wave energy, decoupling the generator from the ionosphere.

Lysak, R. L.↗

NASA NTRS1983

A thermal oscillating two-stream instability

A theory for the oscillating two-stream instability, in which the Ohmic heating of the electrons constitutes the nonlinearity, is developed for an inhomogeneous and magnetized plasma. Its possible role in explaining short-scale, field-aligned irregularities observed in ionospheric heating experiments is emphasized. The theory predicts that the initial growth of such irregularities is centered around the level of upper hybrid resonance. Furthermore, plane disturbances nearly parallel to the magnetic meridian plane have the largest growth rates. Expressions for threshold, growth rate, and transverse scale of maximum growth are obtained. Special attention is paid to the transport theory, since the physical picture depends heavily on the kind of electron collisions which dominate. This is due to the velocity dependence of collision frequencies, which gives rise to the thermal forces

Dysthe, K. B.↗

NASA NTRS1983

Ion-beam-driven electrostatic ion cyclotron instabilities

Results are presented of a particle simulation study of the electrostatic ion-cyclotron (EIC) instability driven by a parallel ion beam. The results of this simulation study demonstrate the nonlinear consequences of nonresonant EIC waves destabilized by an ion beam parallel to the magnetic field for the case of a large beam velocity. As a consequence of the instability, it is shown that the beam ions are heated strongly in the perpendicular direction and suffer a strong anomalous friction via EIC waves which leads to the beam slowing down. Simulation results indicate that the anomalous slowing down of beam ions by EIC waves is much larger than that from the classical electron-drag, and the perpendicular collision frequency measured from perpendicular beam heating is as large as that from Bohm diffusion. It is concluded that the ion beam driven EIC wave is a viable mechanism for the transfer of ion parallel beam energy to the ion perpendicular energy.

Okuda, H.↗

NASA NTRS1983

Beam-plasma interactions in a positive ion-negative ion plasma

An electron-free plasma consisting of negative ions /SF6(-)/ and positive ions /Ar(+)/, and negligible neutral-ion collision frequencies has been created in the laboratory. This plasma has a mass ratio of approximately 3.5-similar to many computer particle-in-cell simulated systems. A fluid description of this positive and negative ion confinement (PANIC) plasma is given and compared to experimental measurements of a beam-plasma instability for both beam species and a wide range of beam energies. The fluid dispersion relation and most growing modes are predicted to be insensitive to many parameters of the PANIC beam-plasma system, and found to the consistent with the data.

Intrator, T.↗

NASA NTRS1984

Theory of plasma waves in the auroral E region

A general theory is developed for both electrojet waves and ion cyclotron and current convective waves observed above 120 km altitude. Previously defined electrojet instability theories are extended to encompass the effects of the magnetic field on ions and the presence of field-aligned currents. The ion-cyclotron (E) waves are assumed produced by the two-stream instability in regions dominated by ion magnetization effects. Field-aligned and cross-field currents drive the E waves, which have displayed threshold drift velocities (TDV) sensitive to conditions at altitudes with effective electron/ion and anomalous electron collision frequencies. The electron density gradients in the region affect the magnitude of the TDV for waves on scales of tens of meters. Recombinational damping increases the TDV for marginal damping of two-stream E waves and establishes a TDV for excitation of large-scale gradient drift waves which propagate nearly perpendicularly to the magnetic field and may have only 10-20 m wavelengths.

Fejer, B. G.↗

NASA NTRS1984

A Model of the Influence of Neutral Air Dynamics on the Seasonal Variation in the Low Ionosphere

Recently it has become clear that the phenomena in the ionospheric D-region are determined to a great extent by dynamical processes in the strato-mesosphere D-region. In this respect much attention is paid to the study of the winter anomaly (WA) phenomenon on medium and short radiowaves, in which the meteorological character of the lower ionosphere is most prominent. Significant experimental data about the variations of the electron concentration, N, ion composition, temperature and dynamic regime during WA permit a better understanding of the character of the physical processes in the middle atmosphere. The influence of the neutral wind on the seasonal variation of the electron concentration N for the altitude interval 90 or = z or = 120 km, where the ratio upsilon sub in/omega sub i, of the ion-neutral collision frequency, upsilon sub in and the ion gyrofrequency, omega sub i decreases from 40 to 1 was evaluated. CIRA-72 is used as a model of the zonal wind.

Nestorov, G.↗

NASA NTRS1984

The measured motions inside equatorial plasma bubbles

A preliminary study of the vertical and north-south horizontal ion motions in plasma bubbles in the near-equatorial ionosphere utilizing drift meter data from Atmosphere Explorer E is presented. High-resolution data show that the vertical ion velocity in some bubbles increases approximately linearly with (N(0)-N)/N, where N(0) is the background ion concentration and N is the bubble ion concentration. At sufficiently large N(0)/N the vertical ion velocity saturates, but often at a value substantially larger than the ratio of the gravitational acceleration to the ion neutral collision frequency. These larger than nominal velocities may result from background eastward electric fields and/or from a vertically elongated bubble cross section. The unanticipated observation that large poleward horizontal drifts accompany these vertical drifts seems to follow naturally from a redistribution of plasma along flux tubes as the plasma convects from the bottomside of the F region to high altitudes.

Hanson, W. B.↗

NASA NTRS1985

The collisional dynamics of particulate disks

It is shown that the use of the Krook equation greatly simplifies the discussion of the collisional dynamics of particulate disks. Collisions are assumed to be inelastic from the outset. A simple heuristic argument is used to compute the effective collision rate in a disk of spherical particles with a power-law distribution of sizes. For Saturn's rings, the effective collision rate for momentum transport is substantially lower than that conventionally estimated on the basis of an observed optical depth at visual wavelengths. The closure of the vertically integrated set of moment equations without discarding the third-order moments at the outset is discussed, allowing for the possibility of a bent disk. In the limit that the collision frequency is much larger than the orbit frequency, the usual Navier-Stokes equations of viscous hydrodynamics for a thin disk are recovered with an explicit expression for the shear viscosity. The method is generalized to include the effects of gravitational scattering.

Shu, F. H.↗

NASA NTRS1985

A Simulation of High Latitude F-Layer Instabilities in the Presence of Magnetosphere-Ionosphere Coupling

A magnetic-field-line-integrated model of plasma interchange instabilities is developed for the high latitude ionosphere including magnetospheric coupling effects. We show that primary magnetosphere-ionosphere coupling effect is to incorporate the inertia of the magnetospheric plasma in the analysis. As a specific example, we present the first simulation of the E x B instability in the inertial regime, i.e., nu sub i omega where nu sub i is the ion-neutral collision frequency and omega is the wave frequency. We find that the inertial E x B instability develops in a fundamentally different manner than in the collisional case ni sub i omega. Our results show that striations produced in the inertial regime are spread and retarded by ion inertial effects, and result in more isotropic irregularities than those seen in the collisional case.

Mitchell, H. G., Jr.↗

NASA NTRS1986

Theory of plasma contractors for electrodynamic tethered satellite systems

Recent data from ground and space experiments indicate that plasma releases from an object dramatically reduce the sheath impedance between the object and the ambient plasma surrounding it. Available data is in qualitative accord with the theory developed to quantify the flow of current in the sheath. Electron transport in the theory is based on a fluid model of a collisionless plasma with an effective collision frequency comparable to frequencies of plasma oscillations. The theory leads to low effective impedances varying inversely with the square root of the injected plasma density. To support such a low impedance mode of operation using an argon plasma source for example requires that only one argon ion be injected for each thirty electrons extracted from the ambient plasma. The required plasma flow rates are quite low; to extract one ampere of electron current requires a mass flow rate of about one gram of argon per day.

Parks, D. E.↗