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NASA NTRS1992

Modeling the global positioning system signal propagation through the ionosphere

Based on realistic modeling of the electron density of the ionosphere and using a dipole moment approximation for the Earth's magnetic field, one is able to estimate the effect of the ionosphere on the Global Positioning System (GPS) signal for a ground user. The lowest order effect, which is on the order of 0.1-100 m of group delay, is subtracted out by forming a linear combination of the dual frequencies of the GPS signal. One is left with second- and third-order effects that are estimated typically to be approximately 0-2 cm and approximately 0-2 mm at zenith, respectively, depending on the geographical location, the time of day, the time of year, the solar cycle, and the relative geometry of the magnetic field and the line of sight. Given the total electron content along a line of sight, the authors derive an approximation to the second-order term which is accurate to approximately 90 percent within the magnetic dipole moment model; this approximation can be used to reduce the second-order term to the millimeter level, thus potentially improving precise positioning in space and on the ground. The induced group delay, or phase advance, due to second- and third-order effects is examined for two ground receivers located at equatorial and mid-latitude regions tracking several GPS satellites.

Bassiri, S.↗

NASA NTRS1992

Upflowing ionospheric ions in the auroral region

Observations of upflowing ionospheric ions are obtained nearly simultaneously by DE 1 and DE 2 over the nightside auroral regions. At low altitudes, the mean value of the net upward ion number flux is of the order of 10 exp 9/sq cm per sec. The ionosphere is predominantly O(+), and the flux of ions with energy greater than 5 eV is a very small fraction (less than 1 percent) of the total ion flux. At high altitudes, the upflowing ions are accelerated by a parallel electric field and heated (with characteristic energies of hundreds of electron volts). Comparing upflowing fluxes at high and low altitudes yields an estimated height of the bottom of the auroral acceleration region of 1400-1700 km for the region of peak potential drop. This low-altitude acceleration could either be from a parallel electric field or from perpendicular acceleration. The fluxes at the edges of the arc are mostly H(+) thus implying a higher-altitude base of the acceleration region at the edges where the potential drop is lower.

Lu, G.↗

NASA NTRS1993

International Reference Ionosphere: Past, present, and future. I - Electron density. II - Plasma temperatures, ion composition and ion drift

The most important investigations leading to the International Reference Ionosphere 1990 (IRI-90) are overviewed, and the latest version of the model is described. The shortcomings and limitations of the IRI-90 are pointed out, together with the ways of overcoming them. The list of studies that the IRI group has yet to carry out includes the investigations of magnetic storm effects as the highest priority. This paper discusses determinations of and the available data on the electron density, plasma temperatures, ion composition, and ion drift in the ionosphere, together with future improvements needed on these parameters.

Bilitza, D.↗

NASA NTRS1993

Plasma transport in the equatorial ionosphere during the great magnetic storm of March 1989

We have modeled plasma transport in the low-latitude and equatorial ionosphere during the great magnetic storm of March 1989. Our goal was to provide a consistent explanation for the DMSP (Defense Meteorological Satellite Program) observations of dramatic decreases in ion density and rapid ion drifts in the low latitude ionosphere over South America during the storm. The modeling effort supports the hypothesis that abnormally large upward drifts lifted F region plasma above the satellite's altitude and created the density depletions observed by DMSP. Modeled O(+) densities at the satellite's altitude have a strong qualitative resemblance to DMSP observations. Both the model and the observations indicate a deep density trough with extremely sharp boundaries surrounding the equator. The widths of both the modeled and the observed equatorial troughs increase with time. Vertical ion drifts predicted by the model also have been compared with DMSP measurements. Like the observed vertical drifts, the modeled drifts reversed sign near the trough boundaries. The modeled vertical drifts are of the same order and direction as the vertical component of E x B convection near the equator, but of opposite direction (downward) near the trough boundaries and outside of the trough.

Rasmussen, C. E.↗

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 NTRS1993

Model of superthermal ions in the Venus ionosphere

A model is presented that simulates the behavior of superthermal ions previously reported in the dayside ionosphere of Venus. The model considers effects of ExB and gradient drifts, charge exchange and collisions with the ambient neutral atmosphere and the possible effects of a wave-particle (anomalous) scattering process. Results indicate that scattering processes are required if superthermal ions are the explanation for the observed 'missing pressure' component in the day-side Venus ionosphere. The scattering scale length required to match the 'missing pressure' distribution is similar to the scale length previously predicted for growth of a lower hybrid beam instability.

Kramer, Leonard↗

NASA NTRS1993

Coupling of magnetopause-boundary layer to the polar ionosphere

The plasma dynamics in the low-latitude boundary layer and its coupling to the polar ionosphere under boundary conditions at the magnetopause are investigated. In the presence of a driven plasma flow along the magnetopause, the Kelvin-Helmholtz instability can develop, leading to the formation and growth of plasma vortices in the boundary layer. The finite ionospheric conductivity leads to the decay of these vortices. The competing effect of the formation and decay of vortices leads to the formation of strong vortices only in a limited region. Several enhanced field-aligned power density regions associated with the boundary layer vortices and the upward field-aligned current (FAC) filaments can be found along the postnoon auroral oval. These enhanced field-aligned power density regions may account for the observed auroral bright spots.

Wei, C. Q.↗

NASA NTRS1993

Plasma dynamics driven by finite-width current filament and KV potential drops in ionosphere-magnetosphere coupling

We have simulated the plasma dynamics of a quasi-two-dimensional current filament in ionosphere-magnetosphere coupling. The simulation consists of a set of one-dimensional flux tube simulations with different imposed time-dependent, field-aligned currents. The dynamical interactions of the individual current filaments with the ionospheric and magnetospheric plasma generate kV field-aligned potential drops along the field lines, and the side-by-side display exhibits the evolution of the implied potential structure in the horizontal direction. The primary cross-field motion produced by the horizontal E-field (ExB drift) is perpendicular to both of the significant spatial directions, and is thus ignorable in this geometry. The effects of other cross-field drict processes are discussed. The simulation thus provides insight into the dynamical evolution of 2D potential structures driven by an imposed finite-width field-aligned current profile. Spatial and temporal variation of other plasma parameters (temperatures, density, etc.) that are determined primarily by parallel transport effects are also displayed.

Ganguli, Supriya B.↗

NASA NTRS1993

Long-lived artificial ion clouds in the earth's ionosphere

In two barium releases made over the Caribbean during the summer of 1991, the ion cloud was observed to last more than 10 hours. Observations were made with a low light level TV imager and a TV spectrograph operated onboard a Russian research vessel conducting ionospheric research and participating in the NASA-CRRES mission. The two clouds were observed near the release region for 15 to 25 minutes during the twilight following the initial release. One of the releases was made during evening twilight and the ion cloud reappeared the following morning, while the other was a morning release and the late-time observations made the next evening. The observations are highly surprising since, generally, it has been assumed that the ion cloud would dissipate within a few hours. However, the long life time appears consistent with current knowledge about dissipative processes. The long lifetime of barium ion clouds may provide for the most sensitive experimental testing of ionospheric convection models, a possibility with clear implications for future barium release experiments.

Milinevskij, G. P.↗

NASA NTRS1993

Interaction with the lower ionosphere of electromagnetic pulses from lightning - Heating, attachment, and ionization

A Boltzmann formulation of the electron distribution function and Maxwell's equations for the EM fields are used to simulate the interaction of lightning radiated EM pulses with the lower ionosphere. Ionization and dissociative attachment induced by the heated electrons cause significant changes in the local electron density, N(e). Due to 'slow' field changes of typical lightning EM pulses over time scales of tens of microsec, the distribution function follows the quasi-equilibrium solution of the Boltzmann equation in the altitude range of interest (70 to 100 km). The EM pulse is simulated as a planar 100 microsec long single period oscillation of a 10 kHz wave injected at 70 km. Under nighttime conditions, individual pulses of intensity 10-20 V/m (normalized to 100 km horizontal distance) produce changes in N(e) of 1-30 percent while a sequence of pulses leads to strong modification of N(e) at altitudes less than 95 km. The N(e) changes produce a 'sharpening' of the lower ionospheric boundary by causing a reduction in electron density at 75-85 km (due to attachment) and a substantial increase at 85-95 km (due to ionization) (e.g., the scale height decreases by a factor of about 2 at about 85 km for a single 20 V/m EM pulse). No substantial N(e) changes occur during daytime.

Taranenko, Y. N.↗

NASA NTRS1993

Effects of the equatorial ionosphere on L-band Earth-space transmissions

Ionosphere scintillation can effect satellite telecommunication up to Ku-band. Nighttime scintillation can be attributed to large-scale inhomogeneity in the F-region of the ionosphere predominantly between heights of 200 and 600 km. Daytime scintillation has been attributed to sporadic E. It can be thought of as occurring in three belts: equatorial, high-latitude, and mid-latitude, in order of severity. Equatorial scintillation occurs between magnetic latitudes +/- 25 degrees, peaking near +/- 10 degrees. It commonly starts abruptly near 2000 local time and dies out shortly after midnight. There is a strong solar cycle dependence and a seasonal preference for the equinoxes, particularly the vernal one. Equatorial scintillation occurs more frequently on magnetically quiet than on magnetically disturbed days in most longitudes. At the peak of the sunspot cycle scintillation depths as great as 20 dB were observed at L-band.

Smith, Ernest K.↗

NASA NTRS1995

Differenced Range Versus Integrated Doppler (DRVID) ionospheric analysis of metric tracking in the Tracking and Data Relay Satellite System (TDRSS)

The Differenced Range (DR) Versus Integrated Doppler (ID) (DRVID) method exploits the opposition of high-frequency signal versus phase retardation by plasma media to obtain information about the plasma's corruption of simultaneous range and Doppler spacecraft tracking measurements. Thus, DR Plus ID (DRPID) is an observable independent of plasma refraction, while actual DRVID (DR minus ID) measures the time variation of the path electron content independently of spacecraft motion. The DRVID principle has been known since 1961. It has been used to observe interplanetary plasmas, is implemented in Deep Space Network tracking hardware, and has recently been applied to single-frequency Global Positioning System user navigation This paper discusses exploration at the Goddard Space Flight Center (GSFC) Flight Dynamics Division (FDD) of DRVID synthesized from simultaneous two-way range and Doppler tracking for low Earth-orbiting missions supported by the Tracking and Data Relay Satellite System (TDRSS) The paper presents comparisons of actual DR and ID residuals and relates those comparisons to predictions of the Bent model. The complications due to the pilot tone influence on relayed Doppler measurements are considered. Further use of DRVID to evaluate ionospheric models is discussed, as is use of DRPID in reducing dependence on ionospheric modeling in orbit determination.

Radomski, M. S.↗

NASA NTRS1994

Global observations and modeling of the ionosphere, thermosphere and mesosphere

The Imaging Spectrometric Observatory (ISO) flown on the Atmospheric Laboratory for Applications and Science (ATLAS) 1 mission between 24 Mar 1992 and 2 Apr 1992, acquired a database designed to study several outstanding problems in the ionosphere, thermosphere and mesosphere. In this paper we discuss the goals and preliminary results from three of these studies. To support these studies, the ISO acquired a database of: (1) emissions for the retrieval of neutral and ion densities to test global models of the ionosphere and thermosphere; (2) emissions for the retrieval of mesospheric composition of major and minor constituents needed to test models of the oxygen-hydrogen photochemistry, (3) emissions of the bands of the metastable states of O2, and O(S-1) produced by three-body recombination of O in the mesosphere.

Torr, D. G.↗

NASA NTRS1994

Ion transport and loss in the Earth's quiet ring current. 2: Diffusion and magnetosphere-ionosphere coupling

We have studied the transport and loss of H(+), He(+), and He(++) ions in the Earth's quiet time ring current (1 to 300 keV/e, 3 to 7 R(sub E), Kp less than 2+, absolute value of Dst less than 11, 70 to 110 degs pitchangles, all LT) comparing the standard radial diffusion model developed for the higher-energy radiation belt particles with measurements of the lower energy ring current ions in a previous paper. Large deviations of that model, which fit only 50% of the data to within a factor of 10, suggested that another transport mechanism is operating in the ring current. Here we derive a modified diffusion coefficient corrected for electric field effects on ring current energy ions that fit nearly 80% of the data to within a factor of 2. Thus we infer that electric field fluctuations from the low-latitude to midlatitude ionosphere (ionospheric dynamo) dominated the ring current transport, rather than high-latitude or solar wind fluctuations. Much of the remaining deviation may arise from convective electric field transport of the E less than 30 keV particles. Since convection effects cannot be correctly treated with this azimuthally symmetric model, we defer treatment of the lowest-energy ions to a another paper. We give chi(exp 2) contours for the best fit, showing the dependence of the fit upon the internal/external spectral power of the predicted electric and magnetic field fluctuations.

Sheldon, R. B.↗

NASA NTRS1994

Langmuir turbulence in the auroral ionosphere 1: Linear theory

Intense bursts of Langmuir waves with electric fields of 50 to 500 mV / m have been frequently observed at altitudes greater than 500 km in the auroral ionosphere. These bursts are driven by 20 eV to 4 keV field-aligned electrons, which are embedded in an approximately isotropic nonthermal tail of scattered electrons. The Langmuir bursts are often observed at altitudes where the ionosphere is moderately magnetized (OMEGA (sub e) approximately equals omega (sub pe)). Both the moderate magnetization and the scattered electrons have a major influence on the linear dispersion and damping of Langmuir waves. In particular, the linear dispersion is topologically different depending on whether the magnetic field is subcritical (OMEGA (sub e) less than omega (sub pe)) or supercritical (OMEGA (sub e) greater than omega (sub pe)). The correct dispersion and damping can account for the observed polarization of the Langmuir waves, which is very nearly parallel to the geomagnetic field. Inferred properties of the linear instability driven by the field-aligned electrons are discussed. The linear dispersion and damping derived here provide the basis for a nonlinear turbulence study described in a companion paper (Newman et al., this issue).

Newman, D. L.↗

NASA NTRS1994

Interhemispheric asymmetry of the high-latitude ionospheric convection pattern

The assimilative mapping of ionospheric electrodynamics technique has been used to derive the large-scale high-latitude ionospheric convection patterns simultaneously in both northern and southern hemispheres during the period of January 27 to 29, 1992. When the interplanetary magnetic field (IMF) B(sub z) component is negative, the convection patterns in the southern hemisphere are basically the mirror images of those in the northern hemisphere. The total cross-polar cap potential drops in the two hemispheres are similar. When B(sub z) is positive and absolute value of B(sub y) greater than B(sub z), the convection configurations are mainly determined by B(sub y) and they may appear as normal 'two-cell' patterns in both hemispheres much as one would expect under southward IMF conditions. However, there is a significant difference in the cross-polar-cap potential drop between the two hemispheres, with the potential drop in the southern (summer) hemisphere over 50% larger than that in the northern (winter) hemisphere. As the ratio of absolute value of B(sub y)/B(sub z) decreases (less than one), the convection configuration in the two hemispheres may be significantly different, with reverse convection in the southern hemisphere and weak but disturbed convection in the northern hemisphere. By comparing the convection patterns with the corresponding spectrograms of precipitating particles, we interpret the convection patterns in terms of the concept of merging cells, lobe cells, and viscous cells. Estimates of the ' merging cell' potential drops, that is, the potential ascribed to the opening of the dayside field lines, are usually comparable between the two hemispheres, as they should be. The 'lobe cell' provides a potential between 8.5 and 26 kV and can differ greatly between hemispheres, as predicted. Lobe cells can be significant even for southward IMF, if absolute value of B(sub y) greater than the absolute value of B(sub z). To estimate the potential drop of the 'viscous cells,' we assume that the low-latitude boundary layer is on closed field lines. We find that this potential drop varies from case to case, with a typical value of 10 kV. If the source of these cells is truly a viscous interaction at the flank of the magnetopause, the process is likely spatially and temporally varying rather than steady state.

Lu, G.↗

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 NTRS1994

A thermosphere-ionosphere-mesosphere-electrodynamic general circulation model (time-GCM): Equinox solar cycle minimum simulations (30-500 km)

A new simulation model of the mesosphere, thermosphere, and ionosphere with coupled electrodynamics has been developed and used to calculate the global circulation, temperature and compositional structure between 30-500 km for equinox, solar cycle minimum, geomagnetic quiet conditions. The model incorporates all of the features of the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere- electrodynamics general circulation model (TIE-GCM) but the lower boundary has been extended downward from 97 to 30 km (10 mb) and it includes the physical and chemical processes appropriate for the mesosphere and upper stratosphere. The first simulation used Rayleigh friction to represent gravity wave drag in the middle atmosphere and although it was able to close the mesospheric jets it severely damped the diurnal tide. Reduced Rayleigh friction allowed the tide to penetrate to thermospheric heights but did not close the jets. A gravity wave parameterization developed by Fritts and Lu (1993) allows both features to exist simultaneously with the structure of tides and mean flow dependent upon the strength of the gravity wave source. The model calculates a changing dynamic structure with the mean flow and diurnal tide dominant in the mesosphere, the in-situ generated semi-diurnal tide dominating the lower thermosphere and an in-situ generated diurnal tide in the upper thermosphere. The results also show considerable interaction between dynamics and composition, especially atomic oxygen between 85 and 120 km.

Roble, R. G.↗