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At least 235 records · Page 13
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NASA NTRS2003

Modeling Variability of Plasma Conditions in the Io Torus

Telescopic observations an in situ measurements of the Io plasma torus show the density, temperature and composition to vary over time, sometimes up to a factor of 2. While previous models of the physical and chemical processes in the Io plasma torus have reasonably modeled the conditions of the Voyager 1 era, their authors have not addressed the observed variability nor explored the sensitivity of torus conditions to input parameters. In this paper we present a homogeneous torus model parameterized by five variables (transport timescale, neutral source strength, ratio of oxygen sulfur to atoms in the source, fraction of superthermal electrons, temperature of these hot electrons). The model incorporates the most recent data for ionization, recombination, charge exchange and radiative energy losses for the major torus species (S, S(sup +), S(sup ++), S(sup +++), O, O(sup +), O(sup ++). We solve equations of conservation of mass and energy to find equilibrium conditions for a set of input parameters. We compare model plasma conditions with those observed by Voyager 1 Voyager 2, and Cassini. Furthermore, we explore the sensitivity of torus conditions to each parameter. We find that (1) torus conditions are distinctly different for the Voyager 1, Voyager 2 and Cassini eras, (2) unique torus input parameters for any given era are poorly constrained given the wide range of solution space that is consistent with the range of observed torus conditions, (3) ion composition is highly sensitive to the specification of a non-thermal electron distribution, (4) neutral O/S source ratio is highly variable with model values ranging between 1.7 for Cassini to 4.0 for Voyager 1 conditions, (5) transport times range between 23 days for Voyager 2 to 50 days for Voyager 1 and Cassini, (6) neutral source strengths range between 7 to 30 x 10(sup -4) cm (sup -3) s(sup -1) which corresponds to a net production of 0.4 to 1.3 tons/s for a torus volume of 1.4 x 10(sup 31) cm(sup 3), or 38 R(sub j)(sup 3).

Delamere, P. A.↗
NASA NTRS2006

Self-Consistent Ring Current Modeling with Propagating Electromagnetic Ion Cyclotron Waves in the Presence of Heavy Ions

The self-consistent treatment of the RC ion dynamics and EMlC waves, which are thought to exert important influences on the ion dynamical evolution, is an important missing element in our understanding of the storm-and recovery-time ring current evolution. Under certain conditions, relativistic electrons, with energies greater than or equal to 1 MeV, can be removed from the outer radiation belt by EMlC wave scattering during a magnetic storm (Summers and Thorne, 2003; Albert, 2003). That is why the modeling of EMlC waves is critical and timely issue in magnetospheric physics. This study will generalize the self-consistent theoretical description of RC ions and EMlC waves that has been developed by Khazanov et al. [2002, 2003] and include the heavy ions and propagation effects of EMlC waves in the global dynamic of self-consistent RC - EMlC waves coupling. The results of our newly developed model that will be presented at Huntsville 2006 meeting, focusing mainly on the dynamic of EMlC waves and comparison of these results with the previous global RC modeling studies devoted to EMlC waves formation. We also discuss RC ion precipitations and wave induced thermal electron fluxes into the ionosphere.

Khazanov, G. V.↗
NASA NTRS2006

Self-Consistent Ring Current Modeling with Propagating Electromagnetic Ion Cyclotron Waves in the Presence of Heavy Ions

The self-consistent treatment of the RC ion dynamics and EMIC waves, which are thought to exert important influences on the ion dynamical evolution, is an important missing element in our understanding of the storm-and recovery-time ring current evolution. Under certain conditions, relativistic electrons, with energies 21 MeV, can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm. That is why the modeling of EMIC waves is critical and timely issue in magnetospheric physics. To describe the RC evolution itself this study uses the ring current-atmosphere interaction model (RAM). RAM solves the gyration and bounce-averaged Boltzmann-Landau equation inside of geosynchronous orbit. Originally developed at the University of Michigan, there are now several branches of this model currently in use as describe by Liemohn namely those at NASA Goddard Space Flight Center This study will generalize the self-consistent theoretical description of RC ions and EMIC waves that has been developed by Khazanov and include the heavy ions and propagation effects of EMIC waves in the global dynamic of self-consistent RC - EMIC waves coupling. The results of our newly developed model that will be presented at GEM meeting, focusing mainly on the dynamic of EMIC waves and comparison of these results with the previous global RC modeling studies devoted to EMIC waves formation. We also discuss RC ion precipitations and wave induced thermal electron fluxes into the ionosphere.

Khazanov, George V.↗
NASA NTRS1973

Ion streaming instabilities with application to collisionless shock wave structure

The electromagnetic dispersion relation for two counterstreaming ion beams of arbitrary relative strength flowing parallel to a dc magnetic field is derived. The beams flow through a stationary electron background and the dispersion relation in the fluid approximation is unaffected by the electron thermal pressure. The dispersion relation is solved with a zero net current condition applied and the regions of instability in the k-U space (U is the relative velocity between the two ion beams) are presented. The parameters are then chosen to be applicable for parallel shocks. It was found that unstable waves with zero group velocity in the shock frame can exist near the leading edge of the shock for upstream Alfven Mach numbers greater than 5.5. It is suggested that this mechanism could generate sufficient turbulence within the shock layer to scatter the incoming ions and create the required dissipation for intermediate strength shocks.

Golden, K. I.↗
NASA NTRS1972

Multiple soliton production and the Korteweg-de Vries equation.

Compressive square-wave pulses are launched in a double-plasma device. Their evolution is interpreted according to the Korteweg-de Vries description of Washimi and Taniuti. Square-wave pulses are an excitation for which an explicit solution of the Schrodinger equation permits an analytical prediction of the number and amplitude of emergent solitons. Bursts of energetic particles (pseudowaves) appear above excitation voltages greater than an electron thermal energy, and may be mistaken for solitons.

Hershkowitz, N.↗
NASA NTRS1973

Propagation of hydromagnetic waves in the upper F2-region

The theory of hydromagnetic-wave in the upper F2-region, in which electrons are in a transitional regime from collisional to collisionless conditions and ions are in a collisionless state, is examined. Derivation of the governing equations is based on the fact that the isotropic electrons are fluid-like, and the anisotropic ions follow kinetic equations modified by ion-electron collisions. Magneto-acoustic waves of a period of from about 0.2 to 10 sec are dissipated by ion Landau damping and electron thermal conduction and viscosity. Numerical solutions under ionospheric conditions show that the dissipation of hydromagnetic waves is insufficient to modify the large scale heating of the ionosphere.

Hung, R. J.↗
NASA NTRS1974

Interstellar scattering of pulsar radiation. 1: Scintillation

An investigation of the intensity fluctuations of 28 pulsars near 0.4 GHz indicates that scintillation spectra have a Gaussian shape, scintillation indices are near unity, and the scintillation bandwidth depends linearly on dispersion measure. Observations near 2.5 GHz suggest a strong dependence of the frequency at which scintillation indices fall below unity on dispersion measure. Multistation measurements of scintillation provide values or limits for the scale size of the scattering diffraction pattern. The dependences of scattering parameters on dispersion measure is discussed in terms of the current models. It is suggested that any line of sight through the galaxy encounters increasingly rare, increasingly large deviations of thermal electron density on the scale of 10 to the 11th power cm.

Backer, D. C.↗
NASA NTRS1975

Coulomb collisions of ring current particles: Indirect source of heat for the ionosphere

The additional energy requirements of the topside ionosphere during a magnetic storm are less than one quarter of the ring current energy. This energy is supplied largely by Coulomb collisions of ring current protons of energy less than about 20 keV with background thermal electrons which conduct the heat to the ionosphere. Past criticisms are discussed of this mechanism for the supply of energy to the SAR-arc and neighboring regions of the ionosphere.

Cole, K. D.↗
NASA NTRS1975

Structure of the quasi-perpendicular laminar bow shock

It was found that low solar wind parameters M (less than or around 2.5) and beta (much less than 1) and high angles to the local shock normal, theta (greater than or around 65 deg), produced oblique laminar shock profiles as expected from theory, with marginal or vanishing upstream standing whistlers probably damped by acoustic or other plasma wave instabilities. The whistler mode appeared to dominate the electromagnetic spectrum. The laminar shock ramp thickness was several hundred kilometers and equal to (2-4)c/omega-pi. Composition of the shock as an accumulation of near-standing waves and an evidently reproducible varying flux pattern was discernible. Electron thermalization occurred early in, or just before, the magnetic ramp, while proton thermalization appeared to occur later in the ramp. Instantaneous shock velocities derived from the standing whistler wavelength were consistent with average velocities derived from the elapsed time estimates and were as high as 200 km/sec.

Greenstadt, E. W.↗
NASA NTRS1975

Metastable 2P oxygen ions in the daytime thermosphere

Dayglow radiation at 7319 A has been measured by the visible airglow experiment on Atmosphere Explorer C. The overhead surface brightness measured at 15.4 hours local solar time on January 27, 1974, was 250 plus or minus 10 R. The volume emission rate had a peak value of 20 photons per cu cm per sec at a height of about 210 km. The data show clear evidence of quenching by collisons with thermal electrons at the higher altitudes and with neutral particles at lower altitudes. Quenching by neutral particles occurs at nearly the gas kinetic rate.

Walker, J. C. G.↗
NASA NTRS1975

Impulsive solar flare X-rays greater than 10 keV and some characteristics of cosmic gamma-ray bursts

Observations of impulsive solar flare X-rays greater than 10 keV are summarized and their interpretation in terms of nonthermal and thermal electron spectra is discussed. This is followed by a brief consideration of models of the hard X-ray source and the requirements of the electron acceleration process during the flash phase of solar flares. Finally, the characteristics of the recently discovered cosmic gamma-ray bursts are compared with those of the impulsive solar X-ray bursts. If both types of emissions are interpreted as bremsstrahlung from energetic electrons, then the electron spectra must be widely different in the two cases. For example, in case of solar flares, most of the energy is carried by electrons with energies of about 5 keV. On the other hand, electrons with kinetic energy of about 300 keV carry most of the energy in the cosmic source.

Kane, S. R.↗
NASA NTRS1975

Interstellar scattering of pulsar radiation. I - Scintillation

An investigation of the intensity fluctuations of 28 pulsars near 0.4 GHz indicates that spectra of interstellar scintillation are consistent with a Gaussian shape, that scintillation indices are near unity, and that the scintillation bandwidth depends linearly on dispersion measure. Observations at cm wavelengths show that the observer is in the near field of the scattering medium for objects with the lowest dispersion measures, and confirm the steep dependence of correlation bandwidth on dispersion measure found by Sutton (1971). The variations of scattering parameters with dispersion measure may indicate that the rms deviation of thermal electron density on the scale of 10 to the 11th cm grows with path length through the galaxy.

Backer, D. C.↗
NASA NTRS1976

Twilight helium 10,830-A calculations and observations

The 10,830-A column emission rates from metastable He(2 3S) atoms in the upper atmosphere are calculated and compared with other calculations and observations. It is shown that the calculated emission rates agree well with the observed emission rates provided that the cross sections for loss of He(2 3S) by Penning ionization on atomic oxygen is near 5 A 2 and not 44 A 2 as previously measured. It is assumed that the local and conjugate photoelectron fluxes above 300 km are about double those calculated. The dawn/dusk intensity ratio observed in New Mexico, Brazil and Peru is accounted for by changes in the ambient thermal electron concentration in Brazil and Peru and by changes in plasmasphere opacity to conjugate point electrons for New Mexico and to some extent for Brazil. The rate at which He-4 would escape from the earth if sufficient energy were imparted to the He-4 atoms during the Penning reaction was recalculated and found to be inadequate by an order of magnitude to balance the terrestrial production.

Tinsley, B. A.↗
NASA NTRS1977

Coordinated rocket and satellite measurements of an auroral event. I - Satellite observations and analysis

Results of a coordinated auroral experiment involving the Atmosphere Explorer C satellite and a sounding rocket are reported. Auroral primary electron fluxes and neutral gas densities measured by instruments on the satellite are used in a model calculation of the thermospheric manifestation of the aurora. There is encouraging agreement between the calculated and measured electron density, electron temperature, secondary electron flux, and O I emissions at 5577 and 6300 A. A discrepancy between the calculated and the rocket-measured 3914-A emission profile is discussed in terms of experiment geometry and auroral physics. The coordinated measurements are used to infer vertical fluxes of ionization and of electron thermal energy at high altitudes

Rees, M. H.↗