Engineering topics

Winglee, R. M.

Publications and source records attributed to Winglee, R. M..

At least 37 records · Page 2

NASA NTRS1991

A model for the frequency fine structure of auroral kilometric radiation

A model for AKR frequency fine structure in which the fine structure is a natural consequence of the emission of electron cyclotron maser radiation in a nonuniform magnetic field is presented. The model is examined by using 1D electromagnetic particle-in-cell simulations. It is found that maser radiation is emitted as individual wave packets combine to form the drifting emission features that make up the fine structure. Rising frequency drifts are associated with wave packets emitted independently of one another. Wave packets that interact with one another produce features with falling frequency drift. The emission features produced have a large spread in both positive and negative frequency drift rates. The average positive drift rate of rapidly drifting features is found to be approximately proportional to the maser growth rate.

Mckean, M. E.↗

NASA NTRS1991

Interrelation of soft and hard X-ray emissions during solar flares. I - Observations

The interrelation between the acceleration and heating of electrons and ions during impulsive solar flares is determined on the basis of simulataneous observations of hard and soft X-ray emission from the Solar Maximum Mission at high time resolution (6 s). For all the flares, the hard X-rays are found to have a power-law spectrum which breaks down during the rise phase and beginning of the decay phase. After that, the spectrum changes to either a single power law or a power law that breaks up at high energies. The characteristics of the soft X-ray are found to depend on the flare position. It is suggested that small-scale quasi-static electric fields are important for determining the acceleration of the X-ray-producing electrons and the outflowing chromospheric ions.

Winglee, R. M.↗

NASA NTRS1991

Interrelation of soft and hard X-ray emissions during solar flares. II - Simulation model

Two-dimensional electrostatic particle simulations are presented which incorporate the effect of quasi-static electric fields on particle dynamics as well as effects associated with wave-particle interactions induced by the accelerated particles. The properties of the soft and hard X-ray and microwave emissions from such systems are examined. In particular, it is shown that acceleration by quasi-static electric fields and heating via wave-particle interactions produces electron distributions with a broken-power law, similar to those inferred from hard X-ray spectra. Also, heating of the ambient plasma gives rise to a region of hot plasma propagating down to the chromosphere at about the ion sound speed.

Winglee, R. M.↗

NASA NTRS1991

Simulations of pulsed electron beam injection during active experiments

Two-dimensional electromagnetic particle simulations are used to investigate the characteristics of the spacecraft charging, particle acceleration, and wave emissions during beam injection, turnoff and subsequent pulsing of the beam. It is shown that, during beam injection, the beam current is neutralized by a spatially separate return current region extending several tens of meters from the beam region, with the currents being closed across the field lines by the perpendicular acceleration of ambient plasma ions into the beam region. After beam turnoff, this current system reverses after a time lag of about an ion plasma period. The current reversal is accompanied by prolonged electron collection by the spacecraft near the beam region, preferential ion collection by sections of the spacecraft magnetically connected to the initial return current regions, and the creation of hot plasma extending well into the return current regions. Because of the time lag, the currents induced in the plasma during periodic beam injection can be strongly modified from the imposed beam current.

Winglee, R. M.↗

NASA NTRS1990

Effects of neutral gas releases on electron beam injection from electrically tethered spacecraft

The presence of high neutral densities at low altitudes and/or during thruster firings is known to modify the spacecraft potential during active electron beam injection. Two-dimensional (three velocity) particle simulations are used to investigate the ionization processes including the neutral density required, the modification of the spacecraft potential, beam profile and spatial distribution of the return current into the spacecraft. Three processes are identified: (1) beam-induced ionization, (2) vehicle-induced ionization, and (3) beam plasma discharge. Only in the first two cases does the beam propagate away with little distortion.

Winglee, R. M.↗

NASA NTRS1990

Modeling solar flare conduction fronts. I - Homogeneous plasmas and ion-acoustic turbulence. II - Inhomogeneous plasmas and ambipolar electric fields

A one-dimensional, electrostatic, particle-in-cell simulation is used here to model the expansion of a heated electron population in a coronal loop during a solar flare and the characteristics of the associated X-ray emissions. The hot electrons expand outward from the localized region, creating an ambipolar electric field which accelerates a return current of cooler, ambient electrons. Ion-acoustic waves are generated by the return currents as proposed by Brown et al. (1979), but they play little or no role in containing energetic electrons and the conduction front proposed by Brown et al. does not form. The X-ray emission efficiency of the electrons is too low in the corona for them to be the source of hard X-ray bursts. The particle dynamics changes dramatically if the heated plasma is at low altitudes and expands upward into the more tenuous plasma at higher altitudes. Two important applications of this finding are the radio-frequency heating of the corona and the collisional heating of the chromosphere by precipitating energetic electrons. In both cases, the overlying plasma has a density that is too low to supply a balancing return current to the expanding hot electrons. As a result, an ambipolar electric field develops that tends to confine the energetic electrons behind a front that propagate outward at about the speed of sound.

Mckean, M. E.↗

NASA NTRS1990

Simulation of the collapse and dissipation of Langmuir wave packets

Particle-in-cell (PIC) simulations and Zakharov's partial differential equations (PDEs) are used to investigate the collapse of isolated Langmuir wave packets in two dimensions. Collapse thresholds are determined numerically, and the roles of enhanced Langmuir wave damping and nonlinearities not included in the standard Zakharov equations are discussed. The Langmuir wave and ion dynamics in PIC simulations are compared with the predictions of PDE simulations incorporating enhanced Langmuir damping. Electron heating and coherent acceleration in the PIC simulations are discussed and compared with predictions of the transit-time theory.

Newman, D. L.↗

NASA NTRS1990

Electron beam injection during active experiments. I - Electromagnetic wave emissions

The wave emissions produced in Echo 7 experiment by active injections of electron beams were investigated to determine the properties of the electromagnetic and electrostatic fields for both the field-aligned and cross-field injection in such experiments and to evaluate the sources of free energy and relative efficiencies for the generation of the VLF and HF emissions. It is shown that, for typical beam energies in active experiments, electromagnetic effects do not substantially change the bulk properties of the beam, spacecraft charging, and plasma particle acceleration. Through simulations, beam-generated whistlers; fundamental z-mode and harmonic x-mode radiation; and electrostatic electron-cyclotron, upper-hybrid, Langmuir, and lower-hybrid waves were identified. The characteristics of the observed wave spectra were found to be sensitive to both the ratio of the electron plasma frequency to the cyclotron frequency and the angle of injection relative to the magnetic field.

Winglee, R. M.↗

NASA NTRS1990

Electron beam injection during active experiments. II - Collisional effects

During active beam experiments, the presence of high neutral densities at low altitudes and/or during thruster firings has been observed to modify the spacecraft charging and the properties of the beam. Two-dimensional (three-velocity) electromagnetic particle simulations with ionizing collisions incorporated are used to investigate the modification of the beam-plasma interaction as the neutral density is increased. It is shown that when the spacecraft is uniformly immersed in a neutral cloud, most of the ionization is produced by direct ionization by the beam and its secondaries, rather than via vehicle-induced or wave-induced ionization for the neutral densities considered.

Winglee, R. M.↗

NASA NTRS1989

Heating and acceleration of heavy ions during solar flares

The heating of the various ion species during solar flares is examined in an attempt to develop a unified model incorporating the enrichments in abundances seen by ISEE 3 and some of the characteristics of the soft X-ray line emissions. Problems associated with identifying the initial conditions and the driving mechanisms for the particle acceleration are addressed. The acceleration of heated plasma ions as they propagate upward into the corona is studied via particle simulations.

Winglee, R. M.↗

NASA NTRS1989

Simulations and observations of heating of auroral ion beams

Two-dimensional three-velocity electrostatic particle simulations were used to determine the nonlinear evolution of the distributions of auroral ion beams and thereby to determine quantitatively signatures in the ion distributions produced by the ion-ion instability for a variety of plasma conditions in the auroral zone. The signatures determined from these simulations were compared with observations from DE 1, making it possible to characterize semiquantitatively the heating of the ionospheric ions, and to investigate the causes of variability seen in the observations of Reiff et al. (1988). A comparison of the simulation with observations showed features consistent with heating via the ion-ion instability including perpendicular heating in the supersonic regime and parallel heating in the subsonic regime, and a change in the heating between these regimes as the ratio of the H(+) beam speed to the local sound speed decreases.

Winglee, R. M.↗

NASA NTRS1989

Simulations and Observations of Heating of Auroral Ion Beams

In the auroral zone, quasi-static parallel electric fields produce beams of ionospheric ions (e.g., H(+), He(+) and 0(+)), which flow outward into the magnetosphere, providing a significant source of ions for the ring current and plasma sheet. Because the velocities to which these beams are accelerated is dependent on the mass of the ions, differential flows between the various ion species can develop which are unstable to an ion-ion streaming instability. Particle simulations and observations from DE 1 are used to investigate the heating of the ion beams produced by this instability. It is shown that there is net transfer of energy from the light ions to the heavy ions, with the heavy ions reaching maximum velocities near the beam velocity of the light ions. Bulk heating of the heavy ions occurs when their relative density is low while high-energy tails are produced when their relative density is high. The heating is primarily parallel to the magnetic field if the difference in the heavy and light ion beam velocities is subsonic while both perpendicular and parallel heating can occur if it is supersonic. In the latter case, very strong heating of an intermediate ion's species such as He(+) can also occur. Comparison with observations shows features consistent with heating via the ion-ion instability including perpendicular heating in the supersonic regime and parallel heating in the subsonic regime and a change in the heating between these regimes as the ratio of the H(+) beam speed to the local sound speed is observed to decrease. This heating is, however, not always observed in association with enhanced wave emissions. This lack of waves is attributed to reabsorption of the waves as the ions become heated.

Winglee, R. M.↗

NASA NTRS1989

Generation and propagation of kilometric radiation in the auroral plasma cavity

The two-dimensional particle simulations and three-dimensional ray-tracing calculations presently used to characterize the generation and propagation of EM radiation due to the auroral plasma cavity's electron cyclotron maser instability incorporate a continuous flow of primary energetic electrons along the magnetic field. The magnetic field gradient is found to be unimportant in accounting for various quasi-local properties of the maser instability. The maser instability can produce the observed amplification of auroral kilometric radiation above cosmic background levels within a distance smaller than the dimensions of the auroral cavity.

Pritchett, P. L.↗

NASA NTRS1989

PIC code modeling of spacecraft charging potential during electron beam injection into a background of neutral gas and plasma, part 1

Injections of nonrelativistic electron beams from an isolated equipotential conductor into a uniform background of plasma and neutral gas were simulated using a 2-D electrostatic particle code. The ionization effects on spacecraft charging are examined by including interactions of electrons with neutral gas. The simulations show that the conductor charging potential decreases with increasing neutral background density due to the production of secondary electrons near the conductor surface. In the spacecraft wake, the background electrons accelerated towards the charged spacecraft produce an enhancement of secondary electrons and ions. Simulations run for longer times indicate that the spacecraft potential is further reduced and short wavelength beam-plasma oscillations appear. The results are applied to explain the spacecraft charging potential measured during the SEPAC experiments from Spacelab 1.

Koga, J. K.↗

NASA NTRS1989

Propagation and absorption of electron-cyclotron maser radiation during solar flares

The propagation and absorption of the maser radiation during solar flares are examined through linear theory and electromagnetic particle simulations. It is shown using linear theory that strong absorption of the radiation should occur as it propagates toward the second harmonic layer, where the magnetic field is half as strong as in the emission region. Only radiation propagating nearly parallel to the magnetic field in a low-temperature plasma may be able to escape under certain limited conditions. Finite temperature effects can cause radiation propagating nearly perpendicular to the magnetic field to refract, causing enhanced absorption. Particle simulations are then used to evaluate the nonlinear response of the plasma as the maser radiation propagates through the absorption layer. It is shown that some of the maser radiation is able to escape through a process of absorption below the second harmonic of the local gyrofrequency and re-emission above it. The fraction able to escape is much higher than that predicted by linear theory, although the amount of escaping energy is only a small fraction of the incident energy.

Mckean, M. E.↗

NASA NTRS1988

Particle acceleration and wave emissions associated with the formation of auroral cavities and enhancements

A unified model is presented which interrelates the various processes involved in auroral particle acceleration and the associated wave emissions, and it is shown how energy is coupled from the magnetosphere into the ionosphere. The phenomena in the magnetotail which can provide the source of free energy for the perpendicular electrostatic shocks are identified along with the associated potential structures in the nightside auroral region. The characteristics of the particle acceleration produced by the shock are described, including efficiency and energy transport between magnetospheric and ionospheric plasmas, the properties of the particle distributions throughout the shock region and return current regions, and the associated wave emissions. The source of free energy for the generation of ion conics is identified, as are the signatures for the formation of plasma cavities and/or enhancements.

Winglee, R. M.↗

NASA NTRS1988

Ion-ion waves in the auroral region - Wave excitation and ion heating

The properties of the ion-ion mode which is excited in plasmas when two or more cold ion beams are streaming relative to one another are investigated assuming a warm electron distribution at rest (the model consistent with particle distributions observed in auroral plasma cavities). Numerical solutions are derived for the generalized electrostatic dispersion equation for parallel propagation and for oblique propagation of ion waves. It is shown that the relative ion temperature and concentration have significant effects on the stability of the accelerated plasma and the expected ion heating. Finally, a relationship between ion drift and thermal speed is derived using the marginal stability of ion-ion waves for nonzero ion temperature; the relationship was found to compare favorably with DE-1 ion observations at the high-altitude boundary of the auroral cavity.

Dusenbery, P. B.↗

NASA NTRS1988

Comparative study of cross-field and field-aligned electron beams in active experiments

Beam-plasma interactions associated with the cross-field and field-aligned injection of electron beams from spacecraft were investigated using a two-dimensional (three velocity component) electrostatic particle simulations. It is shown that the beam properties and plasma response can be characterized well by the ratio between the stagnation time and the plasma response time, which depends on the ratio of the ambient plasma density to the beam density, the beam width, the beam energy, and the spacecraft length. It was found that the beams injected across the field lines tend to lose their coherence after about one or two gyrations due to space-charge oscillations induced by the beam, irrespective of the spacecraft charging. These oscillations scatter the beam electrons into a hollow cylinder of a radius equal to a beam electron gyroradius and thickness of the order of two beam Debye lengths. Parallel injected beams are subjected to similar oscillations, which cause the beam to expand to fill a solid cylinder of a comparable thickness.

Winglee, R. M.↗