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At least 235 records · Page 13
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NASA NTRS1994

Electric currents in the subsolar region of the Venus lower ionosphere

The ion and electron momentum equations, along with Ampere's law, are solved for the ion and electron drift velocities and the electric field in the subsolar Venus ionosphere, assuming a partially ionized gas and a single ion species having the ion mean mass. All collision terms among the ions, electrons and neutral particles are retained in the equations. A general expression for the evolution of the magnetic field is derived and compared with earlier expressions. Subsolar region data in the altitude range 150-300 km from the Pioneer Venus Orbiter are used to calculate altitude profiles of the components of the current due to the electric field, gradients of pressure, and gravity. Altitude profiles of the ion and electron velocities as well as the electric field, electrodynamic heating, and the energy density are determined. Only orbits having a complete set of measured plasma temperatures and densities, neutral densities, and magnetic field were considered for analysis; the results are shown only for orbit 202. The vertical velocity at altitudes above 220 km is upgoing for orbit 202. This result is consistent with observations of molecular ions at high altitudes and of plasma flow to the nightside, both of which require upward velocity of ions from the dayside ionosphere. Above about 230 km the momentum equations are extremely sensitive to the altitude profiles of density, temperature, and magnetic field.

Cole, K. D.↗
NASA NTRS1986

Parallel acceleration and transport of ions from polar ionosphere to plasma sheet

The effect of the convection electric field in accelerating ions that escape from the polar ionosphere is investigated. It is shown that at high altitudes the velocity component of the ions along the magnetic field may be increased by more than an order of magnitude. The highest velocities are acquired by ions that escape from the region of the ionosphere that is connected along magnetic field lines to the dayside cusps. During disturbed times, ions from that region intercept the center plane of the magnetotail in one to two hours at radial distances exceeding about 6 earth radii. Investigation of the resulting O(+) properties in the center plane, viz., their locations, number densities, and energies, indicates that the polar ionosphere near the cusps is the principal source of the O(+) observed in the plasma sheet. Moreover, a study of the ion motion at quiet and disturbed times indicates that the increase of O(+) in the plasma sheet with increasing AE values (Lennartsson and Shelley, 1986) is due mainly to an increase in the source of O(+) rather than alteration of its transport path.

Cladis, J. B.↗
NASA NTRS1976

Model ionospheres of Jupiter

The principal concepts presently involved in modeling the Jovian ionosphere are reviewed. A model ionosphere is developed on the basis of our present knowledge of atmospheric composition, relevant chemical and ion-molecule reactions, with their associated rate constants. The shortcomings of this model are discussed when it is compared with the electron density profile obtained from the Pioneer 10 radio occultation data. It is demonstrated that the apparent great extent of the observed topside ionosphere may imply a hot thermosphere, as if Jupiter sustained a corona. Some of the layers observed in the electron density profile may be due to sporadic-E like clustering of protons and other ions.

Atreya, S. K.↗
NASA NTRS1980

Modulation of terrestrial ion escape flux composition /by low-altitude acceleration and charge exchange chemistry/

Motivated by recent observations of highly variable hot plasma composition in the magnetosphere, control of the ionospheric escape flux composition by low-altitude particle dynamics and ion chemistry has been investigated for an e(-), H(+), O(+) ionosphere. It is found that the fraction of the steady state escape flux which is O(+) can be controlled very sensitively by the occurrence of parallel or transverse ion acceleration at altitudes below the altitude where the neutral oxygen density falls rapidly below the neutral hydrogen density and the ionospheric source of O(+) tends to be rapidly converted by charge exchange to H(+). The acceleration is required both to overcome the gravitational confinement of O(+) and to violate charge exchange equilibrium so that the neutral hydrogen atmosphere appears 'optically' thin to escaping O(+). Constraints are placed on the acceleration processes, and it is shown that O(+) escape is facilitated by observed ionospheric responses to magnetic activity.

Moore, T. E.↗
NASA NTRS1988

A one-dimensional multispecies magnetohydrodynamic model of the dayside ionosphere of Venus

Using a modification of the one-dimensional multispecies 'one-major-ion' MHD model of Shinagawa et al. (1987), the behaviors of plasma and magnetic field in the dayside ionosphere of Venus was studied for both time-dependent and steady-state conditions. The present model is more complete than the one-major-ion model of Shinagawa et al., although a comparison of the results indicated that the one-major-ion treatment was a fairly good approximation. Two new cases are presented, including steady-state conditions for the magnetized ionosphere, and the inclusion of ion loss due to horizontal transport in the magnetized region. The resulting calculated profiles of the magnetic field and the electron density agree much better with the observations at high altitudes than those without the ion loss terms, indicating the importance of the horizontal transport processes in the ionosphere of Venus at high altitudes.

Shinagawa, H.↗
NASA NTRS1981

The ionospheric peak on the Venus dayside

The behavior of the ionospheric peak on the dayside of Venus is described and interpreted by combining radio occultation measurements with theoretical calculations. The theoretical models are shown to be able to reproduce the measured electron densities very accurately when careful consideration is given to such parameters as the level of solar activity, the electron temperature, and the neutral density. What is more, the models are able to provide a check on the accuracy of neutral atmospheric models in the vicinity of 140 km. Chemical equilibrium is assumed for the calculation of ion and electron densities. A table giving Pioneer Venus radio occultation measurements of the Venus dayside ionosphere is included.

Cravens, T. E.↗
NASA NTRS1982

Coordinated airborne and satellite measurements of equatorial plasma depletions

A series of experiments conducted in December 1979 to investigate the structure of plasma depletions in the low latitude, nighttime ionosphere is discussed. Density biteouts of about one order of magnitude in the dominant ion, O(+), are mapped to lower altitudes along magnetic field lines for comparison with 6300-A and 7774-A O I airglow depletions. Owing to the different airglow production mechanisms (dissociative recombination of O2(+) for 6300 A and radiative recombination of O(+) for 7774 A), the 6300-A depletions reflect plasma depletions near the bottomside of the F layer, while those at 7774 A are located near the peak of the layer. The O(+) biteouts map directly into the 7774-A airglow depletions in the same hemisphere and also when traced into the opposite hemisphere, which suggests magnetic flux tube alignment over north-south distances of approximately 2220 km. The 6300-A (bottomside) depletions are found to be wider in longitude than the 7774-A (F-peak) depeletions near the equatorward edge of the Appleton anomaly.

Weber, E. J.↗
NASA NTRS1992

Chemistry of the nightside ionosphere of Venus

The present 1D model of the nightside ionosphere of Venus assumes that ionization is maintained by day-to-night transport of atomic ions. Attention is given to the sources and sinks for molecular ions. If the ionosphere is primarily maintained by transport, the ratio of the peak densities of O(+) and O2(+) indicates the downward flux of O(+), independent of the absolute magnitudes of the densities. An examination is conducted of the inbound and outbound portions of six early nightside orbits with low periapsis.

Fox, J. L.↗
NASA NTRS1995

The Thermal Ion Dynamics Experiment and Plasma Source Instrument

The Thermal Ion Dynamics Experiment (TIDE) and the Plasma Source Instrument (PSI) have been developed in response to the requirements of the ISTP Program for three-dimensional (3D) plasma composition measurements capable of tracking the circulation of low-energy (0-500 eV) plasma through the polar magnetosphere. This plasma is composed of penetrating magnetosheath and escaping ionospheric components. It is in part lost to the downstream solar wind and in part recirculated within the magnetosphere, participating in the formation of the diamagnetic hot plasma sheet and ring current plasma populations. Significant obstacles which have previously made this task impossible include the low density and energy of the outflowing ionospheric plasma plume and the positive spacecraft floating potentials which exclude the lowest-energy plasma from detection on ordinary spacecraft. Based on a unique combination of focusing electrostatic ion optics and time of flight detection and mass analysis, TIDE provides the sensitivity (seven apertures of about 1 cm squared effective area each) and angular resolution (6 x 18 degrees) required for this purpose. PSI produces a low energy plasma locally at the POLAR spacecraft that provides the ion current required to balance the photoelectron current, along with a low temperature electron population, regulating the spacecraft potential slightly positive relative to the space plasma. TIDE/PSI will: (a) measure the density and flow fields of the solar and terrestrial plasmas within the high polar cap and magnetospheric lobes; (b) quantify the extent to which ionospheric and solar ions are recirculated within the distant magnetotail neutral sheet or lost to the distant tail and solar wind; (c) investigate the mass-dependent degree energization of these plasmas by measuring their thermodynamic properties; (d) investigate the relative roles of ionosphere and solar wind as sources of plasma to the plasma sheet and ring current.

Moore, T. E.↗
NASA NTRS1994

Centrifugal acceleration of the polar wind

The effect of parallel ion acceleration associated with convection was first applied to energization of test particle polar ions by Cladis (1986). However, this effect is typically neglected in 'self-consistent' models of polar plasma outflow, apart from the fluid simulation by Swift (1990). Here we include approximations for this acceleration, which we broadly characterize as centrifugal in nature, in our time-dependent, semikinetic model of polar plasma outflow and describe the effects on the bulk parameter profiles and distribution functions of H+ and O+. For meridional convection across the pole the approximate parallel force along a polar magnetic field line may be written as F(sub cent, pole) = 1.5m(E(sub i))/B(sub i))squared (r(squared)/r(sup 3)(sub i)) where m is ion mass, r is geometric distance; and E(sub i), B(sub i) and r(sub i) refer to the electric and magnetic field magnitudes and geocentric distance at the ionosphere, respectively. For purely longitudinal convection along a constant L shell the parallel force is F(cent. long) = F(sub cent, pole)(1 - (r/(r(sub i)L))(sup 3/2)/(1 - 3r/(4 r(sub i)L))(sup 5/2). For high latitudes the difference between these two cases is relatively unimportant below approximately 5 R(sub E). We find that the steady state O+ bulk velocities and parallel temperatures strongly increase and decrease, respectively, with convection strength. In particular, the bulk velocities increase from near 0 km/s at 4000 km altitude to approximately 10 km/s at 5 R(sub E) geocentric distance for 50-mV/m ionospheric convection electric field. However, the centrifugal effect on the steady O+ density profiles depends on the exobase ion and electron temperatures: for low-base temperatures (T(sub i) = T(sub e) = 3000 K) the O+ density at high altitudes increases greatly with convection, while for higher base temperatures (T(sub i) = 5000 K, T(sub e) = 9000 K), the high-altitude O+ density decreases somewhat as convection is enhanced. The centrifugal force further has a pronounced effect on the escaping O+ flux, especially for cool exobase conditions; as referenced to the 4000-km altitude, the steady state O+ flux increases from 10(exp 5) ions/sq cm/s when the ionospheric convection field E(sub i) = 0 mV/m to approximately 10(exp 7) ions/sq cm/s when E(sub i) = 100 mV/m. The centrifugal effect also decreases the time scale for approach to steady-state. For example, in the plasma expansion for T(sub i) = T(sub e) = 3000 K, the O+ density at 7 R(sub E) reaches only 10(exp -7) of it final value approximately 1.5 hours after expansion onset for E(sub i) = 0. For meridional convection driven by E(sub i) = 50 mV/m, the density at the same time after initial injection is 30-50% of its asymptotic level. The centrifugal acceleration described here is a possible explanation for the large (up to approximately 10 km/s or more) o+ outflow velocities observed in the midlatitude polar magnetosphere with the Dynamics Explorer 1 and Akebono spacecraft.

Horwitz, J. L.↗
NASA NTRS1990

The ionosphere of Triton

A model of the atmospheric temperature structure and composition inferred from the Voyager (UVS) solar occultations was used together with a one-dimensional chemical diffusive model to interpret the Voyager Radio Science Spectrometer (RSS) ingress measurements of Triton's electron density. Although N2(+) is the major ion created, N(+) produced by dissociative ionization is the dominant ion. Reaction of thermospheric H2, produced by Lyman-alpha dissociation of CH4 in the lower atmosphere, is the major loss for N(+) ions and maintains these ions in PCSS below 600 km. Solar EUV ionization cannot generate electron densities at the magnitude measured by the RSS experiment and an additional ionization source about 3 x 10 to the 8th ions/sq cm per sec is required. The ionosphere may undergo a transition from PCSS to diffusive control if the N(+) ion production rates were greater than the H2 flux derived from CH4. In this case, the upward flowing H2 is totally converted to H by reaction with N(+) and the remaining N(+) ions recombine radiatively to create an ionosphere under diffusive control above the peak.

Majeed, T.↗
NASA NTRS1979

Observations of warm plasma in the dayside plasma trough at geosynchronous orbit

Positive ions in the energy range E = 3-50 eV have been measured on the dayside at geosynchronous orbit with the University of California at San Diego auroral particles experiment on ATS 6. It is found that the near-equatorial plasma trough contains a warm plasma component with a relatively stable temperature 11 eV and a low 'isotropic component' of density typically in the range 0.1-0.4 ion/cc. The pitch angle distributions of these warm ions are typically field-aligned, indicating an ionospheric origin, and have pitch angle half widths in the range 20-40 deg. Pancake and conical distributions are also observed. Abrupt enhancements of the plasma density to 1-10 ions/cc, or more, are seen in the late afternoon and last from several minutes to a few hours.

Horwitz, J. L.↗
NASA NTRS1987

The ionosphere as a fully adequate source of plasma for the earth's magnetosphere

The ionospheric contribution of the polar wind and cleft ion fountain at energies less than 10 eV has been added to previously measured sources; this total ion outflow has then been used to calculate the resulting ion density in the different internal regions of the earth's magnetosphere: plasmasphere, plasma trough, plasma sheet, and magnetotail lobes. Using estimated volumes for these regions and an ion residence time characteristic of each region, it is found that the observed magnetospheric densities can be attained in all cases with no contribution from the solar wind plasma. In the case of the plasma sheet the ionospherically supplied density is more than enough to match the observations and even suggests an invisible component of low-energy plasma (less than 10 eV) which has never been observed. A detailed comparison between the calculated ionospheric source effects in the plasma sheet and those recently measured by ISEE shows excellent agreement and suggests a direct polar low-energy ion source for the plasma sheet which has remained unmeasured because of spacecraft potential effects. Although the solar wind is clearly the earth's magnetospheric energy source and energetic solar wind ions are observed in the magnetosphere, these calculations suggest the possibility that the ionospheric source alone is sufficient to supply the entire magnetospheric plasma content under all geomagnetic conditions.

Chappell, C. R.↗
NASA NTRS1981

Studies of the role of metastables and doubly ionized species in the chemical and thermal structure of the Venusian and Martian ionospheres

Models of the upper atmospheres of Mars and Venus were constructed using Viking and Pioneer Venus data. The neutral densities, with the exception of NO, N(4S), N(2D) and N(2P) were taken from the measured values, along with the neutral, ion, and electron temperatures. Using solar fluxes and relevant cross sections, the production rates of ions and neutral fragments by photo and electron impact processes were computed. These production rates were combined with chemical production rates and loss along with one dimensional transport eddy diffusion, molecular and ambi polar diffusion, and thermal diffusion, to determine the densities of ions and odd nitrogen species. Preliminary calculations show that the chemistry of metastables and doubly ionized species is important in the ionospheres of Mars and Venus. Production of N(+) in metastable reactions is particularly important, and it explains the discrepancy between the measurements of earlier models. Production of CO(+) is also affected. Reactions of O(++) and O(+)(2D) with N2 have important consequences for the escape rate of atomic nitrogen from the Martian atmosphere.

Fox, J. L.↗
NASA NTRS1974

Thermal proton flow in the plasmasphere - The morning sector

Vertical profiles of electron density obtained in the vicinity of the plasmapause using the Alouette-2 topside sounder have been analyzed to assess the presence of H(+) flow in the topside ionosphere. The observations in the midnight sector show clearly the presence of the plasmapause - i.e., there is a sharp boundary separating the poleward regions of polar wind H(+) flow and the more gentle conditions of the plasmasphere where light ions are present in abundance. In contrast, in the sunlit morning sector upward H(+) flow is deduced to be present to invariant latitudes as low as 48 deg (L = 2.2) in the regions normally known to be well inside the plasmasphere. The upward H(+) flux is so large 300,000,00 ions per sq cm per sec that the plasmapause cannot be seen in the latitudinal electron density contours of the topside ionosphere.

Banks, P. M.↗
NASA NTRS2007

Self-consistent Model of Magnetospheric Electric Field, RC and EMIC Waves

Electromagnetic ion cyclotron (EMIC) waves are an important magnetospheric emission, which is excited near the magnetic equator with frequencies below the proton gyro-frequency. The source of bee energy for wave growth is provided by temperature anisotropy of ring current (RC) ions, which develops naturally during inward convection from the plasma sheet These waves strongly affect the dynamic s of resonant RC ions, thermal electrons and ions, and the outer radiation belt relativistic electrons, leading to non-adiabatic particle heating and/or pitch-angle scattering and loss to the atmosphere. The rate of ion and electron scattering/heating is strongly controlled by the Wave power spectral and spatial distributions, but unfortunately, the currently available observational information regarding EMIC wave power spectral density is poor. So combinations of reliable data and theoretical models should be utilized in order to obtain the power spectral density of EMIC waves over the entire magnetosphere throughout the different storm phases. In this study, we present the simulation results, which are based on two coupled RC models that our group has developed. The first model deals with the large-scale magnetosphere-ionosphere electrodynamic coupling, and provides a self-consistent description of RC ions/electrons and the magnetospheric electric field. The second model is based on a coupled system of two kinetic equations, one equation describes the RC ion dynamics and another equation describes the power spectral density evolution of EMIC waves, and self-consistently treats a micro-scale electrodynamic coupling of RC and EMIC waves. So far, these two models have been applied independently. However, the large-scale magnetosphere-ionosphere electrodynamics controls the convective patterns of both the RC ions and plasmasphere altering conditions for EMIC wave-particle interaction. In turn, the wave induced RC precipitation Changes the local field-aligned current distributions and the ionospheric conductances, which are crucial for a large-scale electrodynamics. The initial results from this new self-consistent model of the magnetospheric electric field, RC and EMIC waves will be shown in this presentation.

Gamayunov, K. V.↗
NASA NTRS1986

The tail lobe ion spectrometer

A two-dimensional kinetic 'tail lobe ion spectrometer' model for the transport of ionospheric ions from polar cleft ionosphere into the tail lobes is developed to semiquantitatively simulate the behavior of the observed O(+) ion streams in the magnetotail lobes. The consequences of the present model include: (1) the increase in velocity of the O(+) streams away from the tail midplane; (2) the existence of 'tongues' of O(+) density from the cleft ionosphere into the tail lobes whose distribution configuration depends on the convection electric field and the source thermal energies; and (3) very low parallel thermal energies and speeds in the lobes which are correlated with the parallel ion bulk velocities. These consequences are reasonably consistent with the trends in lobe streams observed by recent spacecraft.

Horwitz, J. L.↗